Harveys Knob Blog

Migration

2008-11-10T11:11:00.004-05:00

Our experience this year suggests there may not be a hawk migration at Harvey's Knob, but rather a random occurrence of hawks. In fact, thinking back, very few hawks actually use the ridge - most are crossing. Broadwings seem to use the ridge late in the evening when they are trying to get the last possible southward movement - otherwise they occur randomly all over the place. Redtails use the ridge in November when there are strong NW winds only - otherwise they are hunting or crossing. The most consistent hawks on the ridge are Ospreys in the Spring on SE winds.

It is my experience that all hawkwatching sites are experiencing a random occurance of migrating hawks both in the spring and the fall. A hawkwatching site with a purview of two to three miles cannot claim to be a site that is collecting data for a final determination of the whys and wherefores of hawk migration. Such a site is merely gathering data for appeasing those at that site in competition with those at another site. Ha Ha, we saw more than you!

The topography and weather ,on any given day, of the site within the two to three mile purview absolutely dictates what the hawkwatcher will see on any given day. If the weather is static the hawks will be moving on solar induced thermals and if the territory within the two to three mile purview of the site is not conducive to solar induced thermals then few migrating hawks will be seen. If the weather is dynamic such as after a cold front passage the weather within the two to three mile purview will be strong northwest winds. Such winds will produce a ridge lift soaring condition on which the migrating hawks will congregate.

Migrating hawks will only congregate in areas that are conducive to soaring flight. If your site is lacking of the terrain that produces thermals then your site is lacking in the count of hawks. If your site is level withot a bump in ita course then ridge lift flying hawks will not congregate. But, each of these condintions will exist at any site. And they will produce3 a congregation of migrating hawks. The only questio0n is: How much of a congregation?

2008-05-10T17:23:00.001-05:00

Hawk Migration Spring and Fall

Apr 30, 2008

Hawkwatchers everywhere should realize that their purview consists entirely of examining, primarily with the naked eye, a two to three mile patch of sky. Considering that there are less than one hundred consistently manned hawkwatching sites on the North American Continent this examination of the hawk migration is pathetically small. It is minuscule to the point of being laughable insofar as any studies of hawk migration with a desire for an accurate monitor of hawk populations is concerned.

We all flock to the lookouts in the Fall of the year to witness a sample of the breeding populations that are heading to a territory that is more conducive to the survival of those breeding hawks. And, we all dutifully record those numbers of southbound migrants with the thought in mind that we are indeed making a contribution to the preservation of the environment of migrating hawks and in a larger sense the ecology of the vertebrates of the earth including ourselves.

Excuse me if I am more convinced that we are only concerned with having a good time on watch and that we are collecting our numbers for the simple reason that we are highly competitive with those who dared to attend a lookout different from ours. Well every game, sandlot or not, needs a referee and a record.

It is the two mile purview that bothers me as that narrow view is severely lacking for the Fall migration. But, to completely ignore the Spring migration just blows my mind. If one is truly interested in hawk migration he should be willing to move about in the fall in order to intercept more migrants than the the nearest site. If one is truly interested in hawk migration one must participate in a hawkwatch during the spring. There is no reason to believe that every hawk on each day in the fall will pass by the selected site and there is no reason to believe that the breeding hawks will return by that selected site in the spring.

For those who are truly interested there is a great menu of selections for you to consider in order to truly contribute valuable data to the hawkwatching record. Take the Candler's Mountain record and the weather conditions that produced the flights there and compare them with what was found at Harvey's Knob. That should provide a clue to where the hawks are flying on their southbound journey.

Consider the spring migration movement. For some thirty years it has been considered at Harvey's Knob that the spring migration produced lower counts because of the mortality rate during the fall migration. Such a notion was based, in part, on the fact that the Hawk Mountain counts were extremely lower in the spring than in the fall. Permit me to submit one question: Where is it written that the migration is symmetrical? Those of us in the southeast know to concentrate our hawkwatches on NW winds in the fall. And, we found early, that NW winds produce no hawks in the spring. Our solution to this pattern was to abandon spring watches.

During the spring migration the multitude of breeding hawks that wintered in the tropics are indeed migrating symmetrically through Central America. They are exploding out of Mexico in late March and early April. They are drifting around northward in the southern and central plains. Then a cold front passes through that territory and spawns northwest winds across that territory. Those winds blow across the western front of the Appalachian Mountains. Playing those winds (there is no such thing as a wind blown hawk) the northbound migrants gather along the western slopes of the Appalachians in Tennessee, Kentucky, and West Virginia. There are two mountains in this purview that reach into Virginia: Pine Mountain on the border of Virginia and Kentucky that hosts the Breaks Interstate Park and Clinch Mountain a bit eastward hosting a VDGIF Wildlife Management Area. As a last resort for spring migration on NW winds there is Peter's Mountain near Bluefield, WVA.

It is obvious that hawkwatchers need to expand their scope if they are truly interested in learning the behavior of the hawks during migration. The above suggestions will go a long way toward such a conclusion.

Yes Virginia there is a viable spring hawkwatch.

Dave Holt

Broadwings 2007 at Selected Sites

2007-12-13T16:31:00.000-05:00

(Click on the chart)

I can’t do anything with the surface maps as there is too much info and I cannot get oriented.

But the tabular info is very interesting as it shows a high flight of Broadwings thru northern New Jersey where it is hard to distinguish a ridge from normal low level terrain. The flight at Scott Mt., New Jersey clearly indicates a flight that may or may not follow ridges but proceed down thru the Piedmont areas of the mid-Atlantic states, and it was a huge flight. The fact that the Hawk Mt., count was only very slightly increased and that Waggoner’s Gap and Hanging Rock did not show substantial increases indicates to me that the bulk of the Bws did soar down the Piedmont and not along the ridges.

We definitely need more data from Piedmont sites. I wonder if the Militia Hill site in Philadelphia reported in 2007. It is a pity that there are no sites situated in the Piedmont of Virginia as such a site or sites could provide a great deal of information about the behavior of Broad-winged hawks in migration.

In support of the Scott Mt. site in NJ I also wonder if the Montclair, NJ site is still reporting.

You see, we have a cluster of sites in the NE or northern tier of the U.S. but coverage is sparse to non-existent east of the Appalachian Mountains in the south starting at the Maryland State line.

I mention all this because we at the Harvey’s Knob site are always wondering where our hawks come from. We can assume almost any direction but we cannot offer a plausible explanation for any direction. A large bulk of hawks moving down through the Piedmont of Virginia that encounters several days of easterly winds may indeed show up at the Harvey’s Knob site in extraordinary numbers. That is precisely what we are beginning to surmise about the extraordinary flight of 2007. But we have absolutely no supporting data from Piedmont sites in our own state.

If there are any folks from around Richmond or Farmville or wherever who wish to go hawkwatching without the long drive to Rockfish or Snickers, may I suggest that you find a place close to home with a near 360 degree view of the sky and sit it out for a few days next fall. It isn’t written anywhere that a mountain is necessary for a hawkwatch. Ask the folks at Detroit, MI, and Windsor, Ont., and they will refer you to years of outstanding counts in some of the flattest country I have ever seen.

Excuse me for proselytizing, but in light of the fall 2007 counts at Harvey’s and Rockfish the need for Piedmont hawkwatches in Virginia has once again raised its ugly head. That is if we are ever to fully understand the behavior of hawks in migration.

Dave Holt
12/19/07

dh

Weather Data of September 18, 2007

2007-12-07T10:29:00.000-05:00

I trust Dr. Holt will comment on these data relative to weather and the Broadwing flight of that September 18, 2007.

If you click on any of these graphs, they enlarge into the visible range.

Here are data from Buchanan for the month of September, 2007:

Here is the graph from Buchanan for 9/18/07:

Here is Lynchburg for the same day:

And Hollins:

Finally, Blacksburg:

Here is the count for 9/18/07 with weather mountain observations:

I continue my search for a representation of the weather that may explain flight. To that end, below are other graphs for that day which I trust Dr. Holt will comment on below (or possibly delete).

Roanoke airport:
This is "Frontal Positions":

Surface Pressure:

Frontal Pressure Tendency:

Surface wind:

Surface relative humidity:

Sorry about the graphics.

Dr. Holt responds:

I am overwhelmed as I am one who prefers to reduce the complex to its simplest terms and then build the theory from those simple terms incorporating the complexities into the complex whole the theory may entail.

Having said that, I am convinced that most people reading the above answer will be convinced that I am full of...

I am doing my own research of some data that I have selected and I will try to incorporate the data that Bill James has unexhaustively extracted from the web. But it will take awhile.

Meanwhile permit me to offer my theory of migration and how it coincides with the latest findings at the Harvey's Knob site.

I believe that because the migrating hawks especially the migrating buteos are driven to seek the means of soaring flight that the means of soaring flight are the prime determination of their behavior during migration.

There are two meteorological/topographical events that will provide the means for soaring flight. They are solar induced thermals and ridge lift. As the migrants move south they are constantly seeking either of these two means of soaring flight.

Since the solar induced thermal is the primary means of soaring flight the southbound hawk is usually moving southward via the solar induced thermal. If the southbound hawk does encounter mountainous terrain and ridge crossing winds then the hawk will elect to continue its southbound journey via the ridge lift means of soaring flight.

Mountain ridges are not leading lines for the migrating buteo unless the ridges are under the influence of ridge crossing winds. When the winds cross ridges and the ridges are aligned NE to SW then the ridge lift condition exists and the southbound buteos will be seen along the ridges. But, we must remember that the solar induced thermal condition is the rule while the ridge lift condition is the exception to the rule.

Taking that last fact into consideration requires that we consider that the migrating buteos are meandering southward on thermals most of the time. When strong winds occur in a ridge crossing direction on the few occasions they occur, the migrants will then gravitate to the ridges. They will remain in the vicinity of the ridges as long as those ridge crossing winds occur with any degree of consistency.

Therefore it is not only important to know what the weather conditions were on the day when the big flight took place it is important to know what the weather conditions were for several days before the big flight occurred.

We all know that westerly winds at the Harvey's Knob site will produce large flights but do we know what weather preceding the day of the big flight will produce an even larger flight?

Dave Holt

December 7, 2007 9:19:00 PM EST

BJ adds:

I may be wrong in trying to analyze the day of the big flight as opposed to equal scrutiny for preceding days. However, my thinking is that the Broadwings go by Harvey's Knob every year and the question is whether we can see them or not. When we do see them, it seems it is likely related to the conditions on that day. To some extent, we can research a lot of those days and, of course, we have even more data characterizing when they do not come into view. The difference between seeing them and not seeing them is certainly related to the weather and should be quantifiable. I am looking for variables that will help us quantify the 'ideal' day and I don't believe we have that model yet. To me, the biggest problem with studying hawk migration relative to weather is that you do not know how many hawks are out there. In that small window in mid-September, we know they are out there in big numbers and can use that time to effectively study hawk flight and atmospheric conditions.

I also wonder if there are not more possibilities for flight strategies. Couldn't hawks use thermals that are present on the mountain as well as in the valley? Can't they fly in a good wind through the valley without using the mountain?

If we could create the perfect day for hawk flight by Harvey's Knob, what would characterize that day?

Dr. Holt responds:

Bill and I are in total agreement about the migration in the vicinity of the Harvey’s Knob site. In my opinion the BWs are constantly moving through the latitude of the HK site during the last two weeks of September. Whether they are seen at HK site is indeed a matter of the weather conditions that occur while the watch is manned.

We must remember that we are only sampling a two mile patch of sky. A BW seen by the naked eye must be at most 3/4 of a mile away. Most of the BWs seen at the HK site are less than 3/4 mile away even those dots high overhead. Why are they in view in the first place is the question Bill and I are striving to answer.

As I mentioned above the birds are soaring on thermals during most of their migration period.
But, the heavily forested slopes of the Blue Ridge are not conducive to thermal soaring. The sun does not directly strike the western slope of the Blue Ridge during the third week of September and after the autumnal equinox it does not strike the western slope in the vicinity of the HK site at all. That leaves the crest of the ridge to be exposed to direct sunlight between the hours of 10:00 AM and 2:00 PM. The asphalt of the BRP, overlooks, and various openings and rock outcrops. A brief survey of the valley on each side of the HK site reveals a terrain rich in thermal activity. These thermal soaring possibilities on the mountain will produce large numbers of migrating Broad-winged hawks. Numbers that will continue excite the veteran hawkwatcher.

But, the average number of migrating Bws at the HK site is between 6 and 7 thousand. On 18 Sept, 2007 this number was exceeded in a single day. Also on a day during the third week of September in 1996, 5000 hawks were recorded. These are extraordinary numbers and there must be an explanation for their occasional appearance at the HK site. This is what I am interested in finding out and where Bill and I part ways a bit.

I am convinced that a continuation of ridge crossing winds for several days before the extraordinary day will influence the thermal soaring hawks out in the valley to gravitate to the ridge in numbers that when the proper conditions occur at the HK site an extraordinary single day count will occur. It doesn’t happen often and for that reason that is what I am after. I simply want to explain the extraordinary as I believe the normal may already be known.

Consider the following scenario: The bulk of the migrating BWs are migrating southward via the near constant thermal soaring condition. However the thermal density is such that fewer thermals occur within a purview of two miles at the HK site. Thermal density beyond that purview is greater therefore many thermal soaring migrants escape detection from the HK site.

What will bring the soaring migrants into view is a stiff ridge crossing wind. When such a wind occurs the thermals are shut down at ground level and ridge lift soaring becomes the dominate means of soaring. A wind speed gradient that always occurs may produce light winds at the valley floor and allow thermals to take place but they will not achieve much altitude before being blown away downwind. However, a thermal tilted by the wind toward the mountain will put even more migrants in view.

I am convinced that extraordinary days in the Broad-winged hawk migration season at the Harvey’s Knob site is due to ridge crossing winds either on the day in question or for a few days before the day in question. The data that Bill James has presented above offers absolutely no evidence to support my conclusion. I shall now invoke that overused phrase so often heard in the soft science circles; More work is needed.

Dave Holt
Dec. 8, 2007

BJ:

I am ready for more work. Perhaps my research into weather conditions on that day is premature. I am assuming there are a lot of BWs around during this period in September, likely thermaling through the valleys on both sides of us or riding the ridges to the North, and that they come into our view when conditions on a specific day are conducive to movement in our direction. If I understand your hypothesis, you suggest that a necessary ingredient for their appearance in numbers is a series of days where conditions bring them closer to the ridges and, more specifically, ridge crossing winds of substance. This appears to be testable.

But where do we look for that data for the test? We don't look at Harvey's since in the days before the big flight, they are not there. We would have to look to the North. Should I look for ridge crossing winds at Rockfish and Snickers in the week before our big flight? Or should I look for the passage of low pressure systems in the mid-Atlantic during that period? If I do, what would you suspect I will find?

12/9/07

Thanks for your suggestion to click on the graphs to enlarge them. I thought this method was only used on porn sites. Anyway, after review I find that there are no conclusions about the weather conditions for the month of September 2007 that can point to why the extraordinary flight of 18 Sept. Certainly the winds immediately preceding that day were not conducive to bringing Great Valley bound hawks to the Blue Ridge. They may have been conducive to bringing Piedmont bound hawks to the east to the Blue Ridge but that is a stretch.

Before we get into a discussion of wind blown hawks let me say emphatically that there is no such a thing as a wind blown hawk. Hawks are a flying object that has nearly zero drag. They are not wind blown. If they were those massive migrations in the Detroit area would be blown into the Atlantic Ocean when caught by the NW winds following a cold front passage. It is my belief that those hawks migrating in or near mountainous country sense the winds when they rise from their perch in the AM. If the winds are of a direction to cause ridge crossing winds the hawks will look to and gravitate toward the highest object in their purview.

The absence of ridge crossing winds of a consistency and strength during the days preceding the extraordinary flight of 18 Sept., lead me to believe that my theory above is completely wrong. And you and I have long since agreed that the hawks seen on 18 Sept., were strictly soaring on thermals. While I can account for the good viewing conditions on 18 Sept., I cannot account for the exceptional numbers of that day.

My theories have all been derived from my experience at the WRNC site during the Red-tailed hawk migration season. At that point close to the valley, when the hawks were soaring on thermals we would see them rise and catch a thermal and soar SW without moving toward the Blue Ridge. But if the winds were strong NW we would see them rise and immediately move toward the Blue Ridge to soar SW on ridge lift.

I recap that experience because I am coming to the conclusion that it is grounded in a very narrow view. Perhaps, an examination of the more northerly lookouts and their wind conditions are the next step. So if the boss (Bill James) is eager to work I would say go to the Rockfish Gap site and if possible to the Candlers Mt., site to look for something that put an extraordinary number of Broad-winged hawks in the vicinity of the Harvey’s Knob site on 18 Sept, 2007.

Dave Holt
12/10/07

Great Observation 2 Nov 2007

2007-12-06T19:27:00.000-05:00

On 2 Nov. 2007, Bill James found at the Harvey’s Knob (HK) site that the Red-tailed hawks weren’t flying past the site during the morning hours while he was experiencing westerly winds at a velocity that should have provided the Red-tails with a ridge lift soaring opportunity.

He did find a substantial number of Red-tails after the noon hour and he wondered at this apparent contradiction in what we have come to believe is the motivation for Red-tails to pass the HK site. Bill did a little research, checking with two weather stations at lower altitudes and found that the lower altitudes did not experience the wind velocity that the higher altitudes did during the morning hours. Therefore the Red-tails that were perched on the valley floor the night before did not experience winds strong enough to influence them to seek out the ridge lift condition when they awoke that morning.

They sought and probably found a few thermals suited for soaring flight during the morning hours. These thermals were tilted by westerly winds toward the crest of the Blue Ridge where Bill was diligently perched. But the strong winds began to dissolve the thermals and left the Red-tails to seek out and find the ridge lift opportunity for soaring flight that they missed when they left the perch. It was at this time that Bill was able to start his count.

In my years of hawkwatching during the Red-tailed hawk migration season I have found the same characteristics at the HK site. No flight during the AM hours but a reasonable though not sensational flight in the PM hours. Wind velocity was the same all day. I have also watched the Red-tails migrate from a valley site at the Woodpecker Ridge Nature Center (WRNC) site.
Lo and behold at the WRNC site there is always a flight in the early morning hours.

When the winds are moderate on the mountain they are light in the valley. Light winds will not cancel thermal activity but strong winds will. Therefore in the morning hours when the winds are moderate to strong on the mountain and light in the valley the migrating Red-tails will choose thermals for there desired soaring flight southward. But when the Red-tails reach the top of the thermals they will find that the thermals are not only tilted toward the mountain when the winds are westerly the thermals will loose their strength for upward flight. The Red-tails will then gravitate toward the mountain. Quite a few will continue to proceed southward via thermals which are by their nature displaced too far from the mountain for good viewing from the mountain.

What Bill found on 2 Nov, 2007 is that the wind velocity varies and that where the birds perched the night before also varies. The Red-tails moving north to south the day before may not have perched on the mountain for the night. The next morning when they awoke the Red-tails sought some indication for where their opportunity for soaring flight would occur. Ordinarily one would assume that a westward wind sweeping across their bodies would have influenced them to seek ridge lift. But, if the winds at their perch were light they would seek their old standby: the solar induced thermal.

Thermal soaring Red-tails at the HK site are very difficult to find as thermals do not occur very close to the site. But ridge lift soaring opportunities will bring the hawks very close to the site.
If the hawks are flying far from the site on thermals in the morning and close to the site in the afternoon, the day on watch can be well described as a differential in wind speed between the AM and PM hours.

As an aside my observations on this type of day in the Red-tail migration season at the HK site are that many of the Red-tails seen in the PM hours have full crops indicating that they may have spent the morning hunting prey rather than soaring flight conditions. But, if that was so, the kudos should still be presented to Bill James for his inexhaustable research in reminding us all of the fact that we should have known all along. The wind is not as strong in the valley as it is at any altitude above the valley. And, that fact alone is very important in understanding the behavior of hawks in migration.

Dave Holt

Wind

2007-11-02T11:06:00.000-05:00

Yesterday, November 1, we had strong wind (B = 2,3) early in the morning (I was there at 9:00 with high hopes), continuing most of the day. I expected lots of RTs, but saw very little until the 12 to 1 period (EDT). I talked with KT at about 10:00 and she said there was no wind in the valley where she was in the morning. I tried to see leaves moving on the ridges, but there was little movement. My impression was that there was wind on the mountain and not in the valley - thus the sparse number of hawks early. But between 12 and 4, we had 33 RTs.

Today, I looked at the weather map from Buchanan for yesterday (November 1st). Here it is:

I had the wind as varying from W, WNW and NW. I had speed at a constant 2 to 3. You can see that the wind in Buchanan did not pick up until around noon.

Here is a map from Roanoke which is lower than the site in Buchanan, I believe:

You can see that the wind speed did not get above 10 MPH until after noon. I believe this data confirms my impression that there was more wind on the mountain than in the valley and that this might explain the lack of hawks until there was wind in the valley. Additionally, there were clouds most of the day with some clearing during the noon hour (10% from previous 70%). It might be instructive to have a similar day with few clouds to determine if hawks could get up using thermals.

This is an exercise in attempting to use data sources to confirm observations.

bj

Broadwing Musings

2007-10-21T21:00:00.001-05:00

Subj: [Va-bird] Raptor populations and migratory paths Date: Wednesday, September 19, 2007 10:02:14 AM From: brenda@birdsofvirginia.com To: va-bird@listserve.com, shenvalbirds@yahoogroups.com

It is a documented fact that in our part of the Piedmont here in Virginia, the majority of Broad-winged Hawks and other raptor species will follow along and over north to south ridges during fall migration. For a number of years Bill Minor, John Irvine, Jr., Paul Saunier and others conducted a linear study in the Piedmont that included Rockfish Gap and in each and every year the majority of the birds were concentrated along and over the north to south ridges at and to the north and south of Rockfish Gap and not spread out over the valleys to the east and west. Here in our section of the Piedmont, it is MY belief that when winds are light to moderate and out of the east, the majority of Broad-wings up the pipeline following along the ridges heading toward Rockfish Gap will steer more along a narrow strip of ridges that run southward toward Charlottesville, ESE of Rockfish Gap, and they will either make a wide swing and veer SW to pick up the Blue Ridge directly south of Rockfish Gap or continue southward to Lynchburg area. The data collected by Gene Sattler at Candler Mountain located along the southeast edge of Lynchburg, approximately 45 miles SSE of Rockfish Gap and 32 miles East of Harvey's Knob, would appear to support this theory. On days when winds are light and out of the east, numbers drop significantly at Rockfish Gap UNLESS we are fortunate to catch them streaming and/or kettling far off to the east out over the Piedmont. Oftentimes a spotting scope is required to get a sufficient visual for counting purposes. Data collected at participating hawk watch sites is an important component of migration study especially now with the increasing threat of wind turbine development on top of the ridges along the eastern flyway. It it critical that migration patterns along our mountain ridges continue to be monitored, not only for raptors but bats and other avian species. As to population indicators for certain species such as Broad-winged Hawk wherein the bulk of the population migrates into South America, I would recommend individuals check totals collected at Corpus Christi, Texas and Veracruz, Mexico which provides a better picture of year-to-year population trends. Collected data by participating hawk watches is readily available by going to www.hawkcount.org. One additional note: Yesterday morning at Rockfish Gap, shortly after dawn, there was a "massive" liftoff of Broad-winged Hawk along the western slopes near Waynesboro and to the east along the base. These birds were very close to the ridge and could very well have been some of the birds that flew over Harvey's Knob to our south. The wind was light to moderate out of the NW to N. I later received reports that the wind shifted to variable and east with significant drop in migrating raptors over Rockfish Gap later in the day. Based on reports by Condon and others, it appears the Broad-wing steered along that narrow eastern ridge to head southward over Charlottesville area. Another report has come in that there may have been another late afternoon push of birds over Rockfish Gap. Shift in the winds? Totals to follow.

Subj: RE: [Va-bird] Raptor populations and migratory paths Date: Wednesday, September 19, 2007 7:32:16 PM From: MARLENECONDON@aol.com To: va-bird@listserve.com cc: shenvalbirds@yahoogroups.com

Two comments in regard to this statement posted earlier today: "It is a documented fact that in our part of the Piedmont here in Virginia, the majority of Broad-winged Hawks and other raptor species will follow along and over north to south ridges during fall migration. For a number of years Bill Minor, John Irvine, Jr., Paul Saunier and others conducted a linear study in the Piedmont that included Rockfish Gap and in each and every year the majority of the birds were concentrated along and over the north to south ridges at and to the north and south of Rockfish Gap and not spread out over the valleys to the east and west." (1) Conducting a one-day linear study each year for a few years in a row does not scientifically prove that Broadwings are concentrated along the mountains. It only proves that they were flying along the mountains on that particular day (if, indeed, they were). A linear study would need to be conducted on MANY days in September and for more than a few years, especially on a big day at the mountains, in order to make a definitive statement regarding where Broadwings migrate through this area. I personally know that on many of the high-count days at Afton, there were also big numbers of Broadwings spread out on both sides of the Blue Ridge. I am not familiar with any study that has truly proved that "the majority of Broad-winged Hawks and other raptor species will follow along and over north to south ridges during fall migration." Unless people are down in the valleys to count when hawks are known to be going over Rockfish Gap, you just can't make such a statement. This isn't to say that more hawks aren't counted on mountain tops--it makes sense that they would be because that's where people are looking for them! And, of course, the elevation assists people in spotting them because you may be closer to the birds and with more eyes scanning the skies together, the higher the likelihood of spotting birds that could otherwise be missed. (2) From the September 16 report of this year in which over 6000 Broadwings were counted : "At times, large kettles would flicker in and out in the distance and only with spotting scopes could counters get a clear visual on swirling masses, with hundreds kettling and then streaming out. One impressive flight was over 1,000 that formed several large kettles." Sure sounds like a fair percentage of Broadwings were over the Piedmont rather than above or along the ridges!
Sincerely, Marlene

Subj: Re: Fwd: [Va-bird] 2000+ Broad-wingeds at Warrenton, Fauquier 19 September Date: Saturday, September 29, 2007 7:38:47 PM From: doubtindave@comcast.net To: MikeLPurdy@aol.com --------------

Dave Holt 's reply: The author of this letter could not have been more correct. Broad-winged hawks indeed migrate on a broad front throughout all regions of the eastern United States. A very broad front at most sites but there are a few where the migratory flight is concentrated. The area around Detroit, Hawk Ridge, and Montclair N.J. are sites that come to mind. Sites where natural barriers such as large expanses of water cause the hawks to converge over small areas of terrain over which they prefer to migrate. And other sites along the Atlantic Coast such as Cape May and Kiptopeke. Would that there was a concentrated watch on the outer banks of North Carolina. Alas! Now all of us hawkwatchers who are bound to our sites inland along mountain ridges and maybe in the Piedmont regions are not making any contribution whatsoever to the monitoring of hawk populations. If anything comes from our hobby it is maybe the study of the behavior of hawks in migration. The only scientifically legitimate place for monitoring the population of migrating hawks is in the Isthmus of Panama. Back in the 1970s Dr. Neal Smith spent ten years trying to count them there and found it impossible to do so. Over the thirty years or so that I have been hawkwatching and scanning the data from other sites I have concluded that massive (in the thousands) counts of hawks can be seen from anywhere during the migration season. I have also been satisfied that my hawkwatching efforts have nothing to do with monitoring the populations of hawks and has no contribution to the preservation of hawks. Thousands of people in that enterprise came to the conclusion that the Osprey and Bald Eagle no longer needed protection long before my puny efforts on the hawkwatch came to the same conclusion. But, I still enjoy hawkwatching as it provides me with a question to answer: Why was the hawk I saw in my view in the first place? I am examining, at best, a patch of sky that has a two mile radius. That is an extremely small sampling area when one considers that the hawk one has seen could be anywhere in a billion mile radius. When I have dutifully attended a mountaintop site in the hope of finding thousands of hawks only to be advised by another watcher who was to attend the site that day that he counted 3000 hawks from his home while loading the car to attend the site, I began to wonder why attend the mountaintop site indeed. It is all about that question of : Why was that hawk in my view in the first place? One must place oneself in a place where one can see the most hawks and in our region it is on a ridge top or some hill in the Piedmont that affords one with a near 360 degree view. And it must be a place where we can easily attend as many days as possible during the migration season. Then we must collect and store three aspects of data: weather, hawks, and topography. One of these aspects appears to be a constant and that would be the topography but that can change also. A new clear cut in the nearby forest would have an effect on the thermal density within the purview of the site. So the data collected must be strict and comprehensive. Weather, hawks, and topography strictly recorded will answer the question of "Why was that hawk in my view in the first place?" Anything else is simply scorekeeping among sporting birders . Dave Holt >

Subj: Re: Fwd: [Va-bird] Raptor populations and migratory paths Date: Saturday, September 29, 2007 8:00:42 PM From: doubtindave@comcast.net To: MikeLPurdy@aol.com --------------

Dave Holt replies: I was privy to the linear studies that John Irvine conducted in the vicinity of Rockfish Gap and I saw no control evidence of searching the Valley or Piedmont areas alongside of the Rockfish Gap site during those linear studies. There are plenty of documented sitings from valley and piedmont of massive movements of Broadwings over many of the years that the Rockfish Gap and Harvey's Knob sites were manned. I highlighted the June Crutchfield / Alice Davis finding of 3500 hawks from a point off the east side of the ridge at Rockfish Gap well within the Piedmont when the Rockfish Gap site was clouded in which it often is. Myriam Moore once reported that counting 3000+ Broadwings prevented her from leaving home to attend the watch when no hawks were seen on the mountaintop site at Harvey's Knob. I personally have seen massive Broadwing flights over the city of Roanoke while there were massive flights over Harvey's Knob. And in the past ten years or so there have been several massive flights over Candlers Mountain near Lynchburg that have surpassed the flights over Harvey's Knob. The only time that the Broadwinged flights stick to the ridges is when the prevailing winds are in a ridge crossing direction, either NW or SE . Otherwise the hawks are soaring on thermals and their only direction is southward and the ridges be damned. Dave Holt 9/29/07

Subj: Re: Fwd: [Va-bird] hawks going by to the East Date: Saturday, September 29, 2007 8:46:11 PM From: doubtindave@comcast.net To: MikeLPurdy@aol.com -------------- Original message

Marlene Condon has hit the nail on the head. She asks what value our counts can be to researchers? Absolutely none. No professional researcher would touch our data as it isn't in the least gathered in a professional manner. The counts themselves are fairly accurate I believe but the conditions under which the counts are gathered leave much to be desired. Wind speed and direction are extremely important but most observers only pay attention at the turn of the hour. Visibility is important but there is no standard by which everyone can agree. Cloud cover is useless without knowing the cloud types. We are simply playing games with other sites by keeping score. We get burned and we wonder why but we make no effort to find out why we get burned. We fail to answer the most pertinent question: Why was that hawk in my view in the first place? After all, I am only examining a two mile patch of sky. Am I really interested in why I am seeing the hawk or am I just interested in scorekeeping versus some other site? Dave Holt

Subj: Hawk migration musings Date: Saturday, September 29, 2007 9:23:46 PM From: doubtindave@comcast.net To: MikeLPurdy@aol.com

I am not interested in arguing with birders at large. But I am interested in letting those who are interested know where I stand. The data has to be maintained locally at each site. HMANA does not analyze the data. It never has and it never will as the analysis of the data is not its mission. Its mission is simply to collect and store the data. The so-called HMANA Journal is merely a score keeping publication. Any analysis in the Journal is simply what the local site personnel come up with about the seasons records. The data from each site is stored in file cabinets at the Hawk Mountain Sanctuary to be released to any legitimate researcher who requests it. As you know any researcher would be pressed by time. Would he or she be willing to sacrifice time to look at the data at an obscure site on the Blue Ridge called Harvey's Knob? A site where no one can agree on the cloud cover, visibility, wind speed, or wind direction. I don't think so. We are quite capable of answering the question: Why was that hawk in my view in the first place? At least at Harvey's Knob we are capable of answering that question. We have the data to do so and the inquisitiveness to do so. Dave Holt

Subj: Re: [Va-bird] Raptor populations and migratory paths Date: Sunday, September 30, 2007 12:06:34 PM From: MARLENECONDON To: MikeLPurdy

Gosh Mike, I can't tell you how very much this correspondence means to me. I have more than once seen many thousands of Broadwings in the Piedmont as well as in the Valley. I would report them to the Rockfish Gap hawkwatch site and no one ever bothered to add them to the count. This mystified me since I thought the point was to document how many were migrating, not "prove" that they only go in large numbers over the mountain sites. Thank you so much and please thank Dave Holt as well. I deeply appreciate the time that both of you have taken to answer my musings on the list-serve. Ever so gratefully, Marlene

Subj: Re: Fwd: [Va-bird] Raptor populations and migratory paths Date: Monday, October 1, 2007 6:34:54 PM From: doubtindave@comcast.net To: MikeLPurdy@aol.com -------------- Original message ---------------------- From: MikeLPurdy@aol.com > Hey Dave, > > I got this note from Marlene after I forwarded your reply regarding one of > her posts. > > Mike > >

Dear Mike, Marlene was victimized by John Irvine's notions about hawk migration. I was privy to his so-called linear study and I found that it had no value whatsoever and I think that Marlene suspected the same thing. I was glad to find a comrade in Marlene. Back in the late nineties I submitted a theory about hawkwatching to Myriam Moore. That theory tried to explain why the Harvey's Knob Hawkwatch bested the Rockfish Gap Hawkwatch for the first and only time in twenty years of concurrent coverage of the Broadwing flights. Myriam sent my suggestion to John Irvine and he proceded to criticize it in his publications for the Rockfish Gap Hawkwatch. When I offered rebuttals he dug up some part time hawkwatchers to rebut my rebuttals and so on ad infinitum. I grew weary of shooting down his rebuttals and even discussing the subject with him. It was at that time that I decided to quit HMANA, the VSO, and the RVBC. Hawkwatching requires people who are receptive to new ideas and who seek new frontiers to explain the phenomena that is taking place before our very eyes while on watch. I am absolutely convinced that there is a meteorological/topographical reason for each and every hawkwatching site to observe migrating hawks and that each and every site is limited by its meteorological/topographical orientation. If you are truly interested I will submit that theory and maybe more. Do with this whatever you please. Dave Holt

Subj: Re: [Va-bird] Raptor populations and migratory paths Date: Thursday, October 4, 2007 9:33:23 PM From: doubtindave@comcast.net To: MikeLPurdy@aol.com -------------- Original message ---------------------- From: MikeLPurdy@aol.com > In a message dated 10/1/2007 9:59:32 PM Eastern Daylight Time, > doubtindave@comcast.net writes: > > the 2007 counts at HK has far exceeded the 1995 counts and I do not know what > the prevaling winds were this year. > I think most of the big Broadwing days this year were on easterly winds, if > I'm not mistaken. We were discussing that if not for the East winds Candler Mt. > would probably have gotten the larger numbers. The biggest flights of BWs on > the Tuesday with 4,000+ were pretty much directly overhead, or slightly east > of the ridge, if I remember correctly. It was pretty awesome. It seemed like > Broadwings were coming thru all day. One of the Hollins Univ. groups came up > and got to see over a thousand hawks. Their instructor was telling them how > lucky they were, that this had never happened to one of the Hollins groups, etc. > I > wish you had been up there. > > Mike > We just returned from a three day trip to the Poconos in PA for a visit to Joyce's brother. My general theory has no problem with hawks soaring on ridge crossing winds but my specific theory does have a problem with the numbers you scored this year at Harvey's Knob on easterly winds. They should have been there but where did they come from. More about this later, as I need to recover from a bug-eyed traffic fighting trip down I-81. But, thanks for the confirmation info. I need to look a bit deeper this year into the prevailing winds for the season and what they have been for the past ten years. Just to tickle your interest the prevailing winds from 1975 thru 1995 at Woodrum Field were SE at 9 mph. Wind speed and direction is very important. Dave

Personally, I would like to track the BWs this year from Canada to here in > relation to local weather conditions to perhaps get some idea of how they came > to the Blue Ridge in such numbers. > > bj > > A few years ago Kerrie Kirkpatrick of northern Virginia tracked high number sighting reports of Broadwings via official and anecdotal reports. It was a flight of 10,000 to 15,000 BWs maybe 20,000. She tracked them down to the southern Pennsylvania border (Mason Dixon line). The Snickers gap site had a fairly large flight at the proper time along the track but Rockfish Gap and Harvey's Knob got zilch. If it was a wave of hawks steadily progressing in a wave latitude to latitude it veered eastward. I don't remember the reason I concluded that but it was probably due to the fact that no huge flight showed up at any site south of the Snicker's Gap site. From what she found it did look like a wave, at least, in the southern PA area. But, it did not maintain the appearance of a wave after Snicker's. Southern Pa is where she received the most reports. I think I concluded that a change in weather when the birds should have shown up at the Rockfish and Harvey's Knob sites caused them to disperse. In all the years I have been hawkwatching while privy to information outside of our immediate area I never saw any indication that a wave of migrating Broadwings ever existed other than the above. My own compilations showed peak flights for one season at Washington Mounument (near Hagerstown MD) Rockfish Gap, Harvey's Knob,and Pilot Mt., NC simultaneously. That is on the same date during the same part of the day. I do not believe that there is a wave of migrating BWs that will show up north of southern Texas. You are quite right in assessing the variables but the independent variable will show up at each and every lookout to a different degree. And it will only determine the number of migrants seen not the true number of migrants. For a population monitor a huge number of sites aligned east to west situated as close as possible along a designated latitude all manned at the same time would be necessary. Add to that problem that each and every site along that line, would have to be evaluated over time as to its ability to display to the human eye the maximum number of migrants. It is that last statement that I would like to assign a weight to the Harvey's Knob site. That is probably the best the amateur hawkwatcher can contribute to the art of hawkwatching. DJ

Red Tail - 9/22/07

2007-10-13T09:30:00.000-05:00

Occasionally, they fly into my focus setting!

More ID Posers From Tad

2007-10-04T06:07:00.000-05:00

Name That Bird!!!

2007-09-25T16:09:00.000-05:00

On 9/22/07, we had this hawk at Harvey's Knob and there has been discussion of what it is exactly:

Behavior of Hawks

2007-09-22T05:38:00.000-05:00

It perhaps goes without saying that in observing hawks through all angles, you are also observing the behavior of that hawk (Re: Commentary below). Visually challenged hawk-watchers such as myself rely as much on behavior as physical characteristics such as flapping, turning diameter in thermalling, etc. And, speaking of hawk behavior, I have noticed the past couple of years that Vultures will flap hurriedly at the approach of a Bald Eagle - my impression being that they are trying to get above the flight path of the Eagle. I also have noticed that in a thermal with Vultures the Eagle will be on the bottom. Just observations. This Spring I saw a Coopers turn into a Harrier. We generally have nesting Coopers in our neighborhood, but I did not find a nest this Spring. Nevertheless, we saw a lot of Coopers Hawks. Repeatedly, I saw Coopers begin to flap with ninety degree wing beats and circle like Harriers. It was perplexing. Katie found a reference in Liqouri's fine book about this behavior (page 33) so note carefully what you see in the Spring.

Speaking of Harriers, here's a photo I got this week:

bill james

Hawk Identification

2007-09-11T19:45:00.000-05:00

I have a note about identifying a migrating hawk that I would like to place on the blog. That is the importance of looking at the bird in question through all the angles it presents.

Many times, at great distances a migrating hawk will present a view that is impossible to discern between two or more species. It is for this reason that the hawkwatcher must discipline himself to make sure that he has viewed the bird through all possible angles before stating an identification of the bird.

The noted author and hawk identification expert, Peter Dunne, has derived an adage that well describes the quandary facing every hawkwatcher. That is “On any given day any species can look exactly like another species.” While the quote may not be exact I believe it is a very accurate paraphrase. If we accept the premise of Dunne’s Law then we must strive to look more closely at any bird in our purview that can be something other than what we came up with on the first sighting.

For instance, you have an eagle two to three miles out and the color markings are not so evident due to the distance. You want to note the length of the head in front of the body and wing in order to make the identification (ID). Once you see that the head length with reference to its prominence to the body is sort of short you will be inclined to call a Golden Eagle. But, did you see the short head length through all angles the bird presented?

Was the bird soaring toward you when you called it? If so, what did it show when it was directly overhead (full ventral view) or directly opposite of you? Had you taken down your binoculars after you called the bird out in front of you? The full ventral or directly opposite views will provide the only proper assessment of head length of the bird in question. A lack of these views will seriously jeopardize the acceptance of the identification. The dependence on the proportions of a migrating hawks body parts is essential, yet it requires viewing the bird through all possible angles.

Another for instance: Referring back to Peter Dunne he has stated that if the bird appears to be a buteo at first sight but becomes more like an accipiter, then the bird is probably a goshawk. A buteo at first sight must be looked upon through its complete flight past your observation point so that it will not be a goshawk when called a redtail. Ditto in converse for a Red-shoulder: If a bird appears at first to be a large accipiter and yet becomes a buteo then the “Through all angles” dictate has prevailed and the bird will probably exhibit some characteristic of a Red-shouldered hawk.

I have seen eager but inexperienced hawkwatchers repeatedly call falcons, accipiters. Accipiters, called harriers and broadwings and etc. Most of those false ID calls were made because the observer did not follow the bird through all the angles. It is watching the bird progress through all the angles it presents to you that will enable you to make a proper ID. If the bird does not present any other angle than the first one you saw then it should go on the report as unidentified (UN). Tail length, head length, wing length in proportion to the body will not be evident unless you follow the bird through all the angles it presents to you.

Of course, you must have determined by now that it is not necessary to be the first to make an ID. It is not even necessary to be the second to make an ID. In other words keep your mouth shut until you have viewed the bird through all angles. If you are alone it will be you who makes the record. Accuracy is just as important as being the one calling the hawks in a crowd. I submit this just to make it easier when in a crowd. Just make sure you use the prescribed methodology when alone.

Dave Holt

Renegade Harrier Spotted on Harvey's Knob

2007-09-11T16:56:00.000-05:00

Last week (9/7/07), Dillard and I were watching a BW very high, looking Eastward. We turned to the West and there was a Harrier coming right at us - I mean we were ducking. So was the Harrier and I got this photo:

It later took off to the South:

I report this in order to see if I can still publish on the blog.

bj

Harveys Knob Purpose

2007-01-10T19:27:00.000-05:00

The Essence of the Harvey’s Knob Hawkwatch

Back in the early to mid seventies when Myriam Moore was seeking help in intercepting the maximum of migrating hawks through southwest Virginia the primary purpose of our efforts was to find the optimum location that would be easy to access and that would provide an intercept of migrating hawks that would be at maximum. In short, lots of hawks at a location that was easy to acquire was the prime consideration.

During the first few years at Harvey’s Knob the site there satisfied both of the conditions for a hawkwatching site that would meet all the requirements for a major hawkwatching site. But, Myriam wasn’t satisfied as she continued to encourage hawkwatching at other locations from Harvey’s Knob. Rockfish Gap and Kiptopeke are just a couple of those locations that she strived to have covered even while she worked to maintain constant coverage of the Harvey’s Knob site.

And, she even encouraged us newcomers to hawkwatching to go experimenting. She encouraged us to find other locations where a maximum number of migrating hawks could be intercepted and yet would be an easily accessible site. In some cases she even wanted us to find sites where a maximum number of hawks could be found that didn’t provide easy access. It was important to know whether or not the hawks could be seen from places that were easily accessed. If the maximum number of migrating hawks could

not be seen from easily accessed sites then so be it. In those days all wheel drive vehicles with high ground clearance were not available to the average hawkwatcher.

The idea was to study hawk migration to its fullest extent because only the study of the migration will reveal the nuances of the migration that will eventually resolve why the hawk is to be seen at a lookout in the first place. Has anyone ever thought of that quandary? Why is that hawk in view in the first place? It is an interesting question and one that must be answered before any theories of hawk migration can be used to resolve the population density of the hawks.

During those early seventies watches with Harvey’s Knob covered for one or two weekends in September there were lots of hawkwatchers dispersed to other mountain tops and ridges in the area. Crawford Mt., Flat top Mt. of the Peaks of Otter, Thunder Ridge on the Blue Ridge, and Potts Mt in Craig Co., were covered. These are but a few that I can remember but we were all to meet at Myriam’s Green Valley retreat after our watches to tally up our counts and try to make heads or tails from them. The idea was to find the primary location to intercept the most migrating hawks possible. That went on for several years with very mixed results.

The control site for this statistical effort was, of course, Harvey’s Knob. Many of the sites investigated such as Crawford Mt., and Flat Top Mt., were only accessible by foot.

Thunder Ridge and Apple Orchard Mt., while accessible by motor vehicle did not provide the numbers referenced to Harvey’s Knob. These are just a sample of the efforts of those years and just the sample that created the Harvey’s Knob Hawkwatch that has continued down through the years. But, the primary question has never been answered.

That question was and still exists in this form: Where is the best location in southwest Virginia for a hawkwatcher to intercept the maximum number of migrating hawks?

With Myriam Moore’s encouragement my wife Joyce Holt and I spent our first six years of hawkwatching at several sites on a ridge called Pott’s Mountain primarily in Craig County, Virginia. We made every effort to cover Pott’s Mt., when we knew that Harvey’s Knob would be covered. Harvey’s Knob was the control for our statistical investigation. On one day in the early eighties I was able to score more hawks at Pott’s than the counters at Harvey’s. But, that did not reveal Pott’s as a better hawkwatching site than Harvey’s. However when one considers that on those days of concurrent coverage the Harvey’s Knob site, was covered by several experienced hawkwatchers, while the Pott’s Mt. site was covered by only one inexperienced hawkwatcher one has to wonder if the experiment was valid.

Okay, the point of this essay should have been laid out by now: The best hawkwatching site in southwest Virginia has yet to be found. And, it is obvious that it isn’t Harvey’s Knob. But, Harvey’s Knob has a history of full season coverage since 1978. You don’t get much better coverage than this. Especially, when volunteers have provided the coverage with no motive other than getting the most hawks. Neither students nor, anyone else that will be given some sort of credit for their efforts has participated in the watch at Harvey’s Knob. So one can conclude that Harvey’s Knob is the best hawkwatching site in the area around the Lynchburg/Roanoke, VA. But one must also conclude that it isn’t the best site for all conditions.

When the hawks are migrating upon the soaring conditions of thermals the Harvey’s Knob site is only good during the early weeks of September. When the hawks are migrating upon the ridge lift during the late September, October and, November then Harvey’s Knob is the place for the hawkwatcher. During the spring, and the spring migration is important, the Harvey’s Knob site stinks on NW winds, which it thrives on in the fall. The reason is simple, the hawks do not migrate symmetrically. They do not migrate symmetrically nor do they migrate on any condition that is not conducive to soaring flight.

But the fact remains and that fact is: The Harvey’s Knob site is not the best hawkwatching site in southwest Virginia but it is the site that provides us with the control numbers in southwest Virginia. To intercept the maximum hawks in southwest Virginia one must choose between several lookouts in the fall as well as several lookouts in the spring. Harvey’s Knob is indeed a control site for southwest Virginia hawkwatching but it is not the only site that should be manned. Those sites have yet to be determined.

Dave Holt

Jan 1, 2007

The Natural History of a Solar Induced Thermal

2006-12-21T21:25:00.000-05:00

A definition is in order. A solar induced thermal is a column of rising air that is created by the sun and it will henceforth be referred to as simply a thermal. Thermals are generated by the sun’s heating of the ground in an uneven manner. As the sun illuminates the earth’s surface certain areas of that surface are more susceptible to heating than other areas. Therefore certain portions of the ground are superheated when compared to other portions. The air above those superheated portions of the ground then becomes superheated by conduction. Now we have a parcel of air that is superheated relative to the surrounding air.

The parcel of superheated air contains molecules that are extremely agitated and vibrate more than those in the cooler surrounding air. The vibration of the molecules causes them to collide with one another thus knocking them about to the extent that the superheated parcel of air is less dense than the surrounding air. The less dense parcel of superheated air now begins to rise. Thus the thermal is born.

It must be stressed that it is the ground that is superheated first and the air above the superheated ground that is superheated second. The sun cannot superheat parcels of air as the air is uniform in color and texture while the ground isn’t. In fact the air is transparent to the suns rays. Therefore the ground with its variations in color, texture, and moisture content is of prime importance in the generation of a thermal. Just consider that the ground is heated first and the air above it second.

To illustrate the variations in color and texture consider the following. A freshly plowed area surrounded by fields of dried grasses. The plowed area is darker than the dried grasses thus allowing it to absorb heat rather than reflect it. The plowed area will become superheated. A strip mall with an asphalt parking lot situated in the median of a concrete highway will become superheated due to the dark asphalt absorbing heat while the light concrete roadway reflects heat. A grassy knoll surrounded by grassy fields will become superheated quicker than the area around it even though the surrounding area is of similar color and texture. This would be because the knoll has better drainage and will shed moisture quicker than the low-lying ground. Heavy moisture in the ground will delay the superheating of the ground as it becomes a heat sink.

When that first superheated parcel of air begins to rise it is replaced by the surrounding cooler air which in turn becomes superheated and it in turn starts to rise. Soon you have a column of air rising and it will continue to rise, cooling as it rises until it cools to the temperature of the surrounding air at some altitude above sea level. That altitude will depend upon the vertical stability of the air and the humidity of the air in the thermal. Also the detection of a thermal will depend upon the relative humidity at the earth’s surface. If the air is sufficiently humid then a cloud will form capping the thermal when the dew point temperature of the rising air reaches the dew point temperature of the surrounding air.

This cycle of superheating, rising air, and cooling of the air will continue as long as the sun is shining. But, anyone who has spent some time hawkwatching has noted that by one o’clock PM on a hot and humid day the cumulus clouds spawned by the thermals have begun to amass and at some point amass sufficiently to block the sun from striking the ground. And, you guessed it, the thermals will stop forming. This cessation of the thermals can cause a decrease in the number of hawks counted, as they may not find a thermal to soar upon within the purview of the observers at the hawkwatch.

Not to worry, as the thermals may reappear as the clouds caused by the early morning thermals begin to spread and in some cases dissipate allowing the sun to once again illuminate the ground. If this is the case the cycle will repeat itself albeit in different places until nightfall. The foregoing has described a model of the birth, life, and death of a solar induced thermal. But there are some wrinkles to this model that should be addressed for the hawkwatcher stationed in the Blue Ridge Mountains of southwest Virginia.

In September the mountains in southwest Virginia are covered with the rich foliage of deciduous and coniferous forests. Stand at your site and look around and you will see a sea of uniform green some fifty to a hundred feet above the ground all around your site. How is the sun ever going to illuminate the ground unevenly through this canopy of green? To my knowledge the sun cannot possibly provide a rich source of thermals along the mountainous territory of southwest Virginia and yet many hawks are counted in this venue. And, during that same time the thermals are the rule while all other soaring means are the exception to the rule.

In the vicinity of Harvey’s Knob there is a black asphalt strip in the middle of that sea of green. There is an occasional outcrop of rock and a mowed area that resembles a clear cut. The parking lot at Harvey’s Knob is a non-uniformity in color that provides a source for a thermal. How many hawks have you counted directly overhead in the Broad-winged hawk season? Those of you who are reading this from other lookouts make the same survey. Remember all you have to consider is a circle with a two to three mile radius centered on the site. That will indicate possible thermal sources within your purview thus providing a reason for your counts on those days when the winds are light and variable.

And there are other questions regarding the anatomy of a thermal that I will be glad to field. But, remember that I am not a meteorologist. I am merely a hawkwatcher who has concentrated his efforts into understanding why that hawk was in view when I went to some hawkwatching site. This wasn’t meant to be a scientific paper as it wouldn’t pass muster in scientific circles. It is a paraphrase of the science of a thermal as I have understood it from the various sources I have researched. If I have left something out I am sorry, but I do think I have provided something for you to think about during those long hours when no hawks are in sight. I submit my Email address for anyone who wishes to keep his queries private. Doubtindave@gmail.com

Dave Holt 12/20/06

Sources of rising air

2006-12-12T18:38:00.000-05:00

Some Sources of Rising Air Conducive to Hawk Migration

Migrating hawks are extremely dependent upon a feature of meteorology that I will call rising air. That is that certain areas of the earth’s surface are more capable of causing parcels of air to rise than others for a variety of causes including man-made. The dependency of the migrating hawks upon the rising air is due entirely to the fact that of all the bird of prey species the hawk species are the most capable of soaring. That is, they can achieve and maintain altitude with a minimum of wing flapping and in most cases no wing flapping at all. So, it would help the hawkwatcher to know the sources of rising air and how they may be found. The following discussion intends to do just that.

The sources of rising air are first listed in the order of their strength. It must be remembered that the strength of the rising air is dependent upon the concentration of rising air with reference to the area of the earth’s surface that causes the air to rise. As with all energy sources the density of the source is the prime factor. Here are the sources I have observed and if others exist this blog is open to your input.

Solar induced thermals
Ridge lift
Cold front passages
Sea breezes
Atmospheric low pressure cells

Let us examine each one of these sources of rising air to further round out our understanding of rising air. It begins like this:

Solar Induced Thermals: When the sun heats the earth’s surface unevenly it causes the latent heat of the surface to be uneven. The air in contact with the surface is therefore heated unevenly. Thus, parcels of air are warmer than the surrounding air and those parcels of air will rise. While those parcels rise they are replaced with the surrounding air rushing in to fill the void. That air is then super-heated and also begins to rise. Thus a fairly narrow column of rising air is created as long as the sun strikes that particular patch of ground. The width of the column is dependent upon the amount of terrain that became super-heated. But, for our purposes, it will be a somewhat narrow high concentration of super-heated air. The solar induced thermal will provide the most energetic and constant supply of lift energy of all the sources of rising air.

A: Man made thermal: The cooling towers of a nuclear power plant produce super-heated air, rising from a small area which is just the exact description of a solar induced thermal and yet it is independent of whether the sun shines or not. Even the ever present, cumulus cloud will cap the air rising from a cooling tower. By all means these towers do produce man made thermals that have all the characteristics of a solar induced thermal.

Ridge Lift: Ridge lift describes the condition when a high pressure cell and low pressure
Cell straddles a large area of terrain thus causing horizontal winds to blow. These winds are particularly interesting to the hawkwatcher station in the mountain ranges. As the wind blows across the mountain ridge it is deflected upward on the windward slope. This deflection creates rising air on the windward slope. The energy of this source of rising air, while not as concentrated as a solar induced thermal, can be spread all along the windward slope of the ridge thus making this source of rising air as important to the hawkwatcher as the solar induced thermal. It should be noted that solar induced thermals and ridge lift are not compatible. Horizontal winds will spread the sun’s heat more evenly over the earth’s surface thus defeating the condition necessary for the generation of a solar induced thermal. But, it should be noted that solar induced thermals are the condition of rule and ridge lift winds are the exception to the rule.

A: Mountain Waves (Lee Waves): Mountain waves are a phenomenon of ridge lift. They occur at a time when the ridge lift winds are strong and steadily blowing across the ridges. Other weather factors combine to create the mountain waves to the extent that sailplanes reach altitudes of 30,000 feet over the western mountains and 16,000 feet over the eastern mountains. The conditions for mountain waves are rare but they do occur and anytime a lenticular cloud is seen over the mountains there is a good chance that mountain waves are occuring. If the hawks utilize this condition they will not be seen by the hawkwatcher as the maximum lift air will be displaced to the lee side of the mountain and the hawks will be very high. The mountain wave is probably the best condition for migrating hawks to escape the hawkwatcher's view when the hawk is within the two mile purview of the hawkwatcher.

Cold Front Passages: A front is the boundary between a cold air mass and a warm air mass. Cold air masses usually are in motion descending from the north. As they advance the denser cold air is hugging the ground and it displaces the less dense air of the warm air mass upward thus creating vertical instability in the atmosphere near the earth’s surface. It is such instability in the atmosphere that makes for rising air and should always be of interest to the hawkwatcher. The cold air acts as a wedge and pushes the warm air upward all along the front thus there is a great deal of energetic rising air along the front of a cold air passage. The problems for the hawkwatcher are the conditions ahead of the advancing cold front. They are usually rain or snow or any other condition limiting visibility. The advance of a cold front may provide good hawkwatching but the ability to extract that data is severely limited by the intensity of the frontal passage. Nay, It is what follows the cold front passage that enhances the hawk counts. Especially in the mountains as ridge lift conditions are usually created with the passage of a cold front.

Sea Breezes: At the coast the sun is heating the ground inland and the water out to sea simultaneously. The ground absorbs more heat and therefore becomes super-heated, i.e. it becomes warmer than the air over the sea. Super-heated air is less dense and therefore constitutes a low-pressure area. The air over the sea is cooler and therefore constitutes a high-pressure area. Therefore air moves from the sea to the land. This movement of the air will contain a windward move from sea to land but more importantly it constitutes a diminished frontal movement, where the cooler air is moving underneath the warmer air thus producing rising air. The folks at the coastal watches swear by this effect. It occurs often in that environment.

Atmospheric low-pressure cells: A low-pressure cell defines an area where the air pressure is lower than the surrounding areas. If the air pressure is lower in this area then that must mean that air is leaving that area. The air cannot leave such an area horizontally because the surrounding air is much denser. Therefore the air in a low- pressure cell is rising. But a low-pressure cell can occupy several tens of miles, perhaps, several hundred miles, therefore the rising air from a low-pressure cell is relatively weak.

But, it is rising and it will rise so long as the low-pressure cell exists. In short it is a definite source of rising air for a long enough period of time to be operational for soaring hawks. It is my belief that such a condition existed when several large Broad-winged hawk flights occurred when visibility was low due to rainfall and fog.

As far as the hawkwatcher is concerned these sources of rising air are pertinent to the soaring species. All hawks are capable of soaring but those species occupying the buteo genus are absolutely dependent upon rising air as they spend their entire time in migration utilizing rising air wherein they can soar. In the southern Appalachians where I have gathered much data, the Broad-winged hawk and the Red-tailed hawk, are the species that produce the greatest numbers in migration. I have seen very few of these species flap their wings during their migration flight. They prefer to soar and they actively seek soaring conditions. Soaring conditions include rising air, ergo the importance of understanding the sources of rising air.

David Holt

Dec. 9, 2006

Laurie to the world; Mike to Laurie; Laurie to Mike

2006-12-10T20:23:00.000-05:00

In a message dated 11/30/2006 2:34:53 PM Eastern Standard Time, goodrich@HAWKMTN.ORG writes:

Additionally, if you have ideas on how HMANA can better serve the community of hawk-watchers - please feel free.

Hi Laurie,

I'm not sure if this is what you're looking for, but I would like to see HMANA publish some kind of booklet/book that I think all hawkwatchers would be interested in. It would be an analysis of each species, where they nest, their migration routes, where they winter, and when they migrate in the largest numbers. It basically would say, "this is what we know, this is what we don't know" and challenge hawkwatchers to find out more of "what we don't know". Let's use the Golden Eagle as an example. I see all of the GE's that the Pa. watches and some NY watches report but I don't know for sure where they go after they exit Pa. They certainly don't come down the Blue Ridge where I watch at Harveys Knob. We only average around 12- 15. The two most reliable places to see wintering GE's in Va. are in Highland Co. and Tazewell Co., both on the W. Va. border. Does that mean most of them are wintering in W. Va. counties in appropriate habitat, or do a large number of them continue into Tn. and further south. Since they're not coming down the Blue Ridge I assume they're going down the Alleghenys. Someone must know. Of course it would be great if there were more watches in Va. and W. Va. in the Alleghenys, or if a watch like Hanging Rock Fire Tower on the Va. - W. Va. border had full coverage. Where are these GE's coming from? Some of the field guides cover some of this regarding nesting range but I feel there needs to be more. Using another species, the Redtail, I've suspected that a lot of the RT's that pass thru Pa. are stopping in Va., and north of Harveys Knob. I don't know this, I just suspect it. Maybe someone knows for sure. Another question that could be answered is where are all of the Bald Eagles seen at Hawk Ridge wintering.....and why? What's the draw? I would like to see this publication with detailed maps designed to show what HMANA has learned over the years plus input from the experts. The maps would show migrations routes used by large numbers of each species along with the watches they pass by. The book would also discuss the different hawkwatch sites, what species seen in large numbers and when best to visit those sites to see the most of a given species. I realize a lot of this information is already out there but it would be nice if it were all in one publication. The book might even discuss starting new watch sites, what to look for in starting a new site. You might even say "these are the locations where we'd really like to see watches started, is anyone up for it?"

Obviously I'm not a scientist, just an average hawkwatcher who's been watching for 35 years. Sometimes I feel like the more I watch hawks, think about hawks and read about hawks, the less I know, and the more questions I have.

MP

Hi MP

Thanks for your input! A start of the book you describe is planned for 2007, Status of North American Raptors proceedings, which will have descriptions of watch sites, flyways, and maybe the first stab at a species look at where they concentrate, where they breed and what we don’t know and need to know (we are doing this with SSHA first). I will pass on your recommendation to the Publications committee.

The synthesis of banding and migration counts to give us that information will take some time. We are doing the SSHA now and maybe Redtail next, with funding that HMANA and Hawk Mt and HWI have gotten for the Raptor Population Index project. What you describe is needed and would be used…but it will take some time to get the funding to do it all! There are clearly some areas we need to learn more about and southern US or central US is an area in need of more counts.

In the meantime, check out http://www.hawkcount.org/ and http://www.hmana.org/ to find a hawkwatch section. When you go to a site, there are summary tables now for most sites that give mean counts and peak counts, and maps of sites, so some of that information is starting to be assembled by HMANA in that site.

Thanks and stay in touch if other ideas come to mind on how HMANA can best serve the hawkwatchers.

Laurie Goodrich

570-943-3411 x 106

fax 570-943-2284

goodrich@hawkmtn.org

Dave on Flight, Species and Season

2006-12-08T10:43:00.000-05:00

DJ: I found this mail from 5/4/05. I recall that we had a big NW wind day in the heart of BW season this year and saw some of them very, very high and others after they had passed. Without resorting to the Hawkwatcher’s Folly (too high to see), I suspect there were many more that went by. Whether this is true or not, the ones we saw were much higher than RTs seen on similar days confirming, I believe, that there are species specific flight patterns.

DJ answers: From the date, I believe, it was a spring watch and a big day on NW winds at the Harvey’s Knob site is a truly rare occasion. I have only experienced one such day in the spring after many hours (wasted) watching at Harvey’s Knob, in the spring, on NW winds. I wish you had included the wind velocity and if your memory still serves you can insert that factor here.

When hawks reach very high altitudes it is usually due to thermal soaring. But, thermal soaring does not occur when the wind velocity is a Beaufort 4 and above. There is another reason for very high altitude flight and it is called lee waves or mountain waves. These also require a steady wind velocity at the high end of a Beaufort 3 and above. All that this shows is the importance of recording the wind direction and velocity in a more precise manner and offering that precise record for general discussions.

Also, another form of a hawkwatcher’s folly is the comparison of individual species of buteos for the purpose of analyzing the flight patterns of all buteos. For instance: the broad-winged hawk is in a hurry in its migration. It notes that the daylight hours are rapidly decreasing by mid-August and it starts to migrate. The broad-winged hawk is extremely desirous of maintaining its position in the hemisphere where daylight and night are close to equal. That is why it clears all the latitudes of the United States within a matter of weeks. Two and a half weeks to be more precise. Only the western Swainson’s hawk maintains such a pace of migration.

The red-tailed hawk is not interested in going to the tropics nor is it particularly interested in the length of the day where it maintains its residence. It is in no hurry to migrate and although they are seen in passing in September their primary migration season is late October and November. And, during this time the passages of cold fronts increase and NW winds will prevail. The NW winds provide the ridge lift that the red-tailed hawk prefers. Ridge lift flyers do not reach very high altitudes. The red-tailed hawk is also a thermal-soaring species as is the broad-winged hawk. But, the thermals in late October and November do not achieve the heights of those thermals in September. Consequently, the red-tailed hawk is usually seen migrating at lower altitudes than the broad-winged hawk, whether it is thermal soaring or ridge lift soaring. Therefore we cannot compare their migration behaviors even though they are members of the same genus.

The data at Harvey’s Knob will show that in the fall the migration is most heavily influenced by the number of days when NW quadrant winds prevail. Even NW winds whose velocity is too puny to provide ridge lift. Thermal soaring hawks under light NW winds will be driven closer to the west slope of the lookout where they will be counted in higher numbers. Also light winds consistently from one direction can line up the thermals into a cloud street, where thermal soaring migrants can be counted in great numbers. It appears that the leading line for the Broadwing migration in southern Virginia is the Shenandoah Valley not the Blue Ridge.

All the preceding is based upon one conclusion: That is that strong winds and solar induced thermals are simultaneously incompatible. Strong winds will distribute the air at the earth’s surface equally therefore defeating the sole factor for producing solar induced thermals. And, even if the strong winds are deflected by a building, hill, ridge top, or other obstruction the thermal generated in the shadow of an obstruction will cease to rise once it reaches the height of the obstruction during strong winds. Beaufort 4 winds blowing across a NE to SW oriented ridge will provide ridge lift only soaring facilities.

The original question was sent on 5/4/05, which is why I assumed that it was concerning a spring watch. As I said before, the Harvey’s Knob site is not a site that intercepts the broad-winged hawk migration in the spring. That is because the Broadwings that have wintered in the tropics of South America take the same route in their return to the breeding grounds. In the spring the birds are coming out of Mexico to spread into the eastern United States and Canada to find their breeding grounds. NW winds will collect them along the western slopes of the western Appalachian Mountains. Harvey’s Knob is too far to the East to offer a migration route for Broadwings other than those that bred in the extreme eastern section of the United States. Therefore, the lower Broadwing counts in the spring as well as the lack of Broadwings in the spring under NW winds.

While I haven’t directly answered your question perhaps I have offered some factors for you to arrive at an answer yourself.

DJ
12/7/06

Sunbathers at Harveys Knob

2006-12-08T10:40:00.000-05:00

Recalling the Rockfish post from earlier this year and not to be outdone, I saw sunbathers on the East side yesterday (12/7/06).

Red Tail Migrant - 11/30/06

2006-12-06T12:17:00.000-05:00

Two Enterprising RTs - 11/30/06

2006-12-06T12:15:00.000-05:00

The Behavior of Hawks in Migration

2006-12-03T11:03:00.000-05:00

The Harvey’s Knob vs. Woodpecker Ridge Nature Center Project

The following is a report on the behavior of hawks while in migration. It is not intended to be a scientific paper but a teaser for those who are truly interested in the behavior of hawks in migration.

The Harvey’s Knob Hawkwatch (HKH) is a hawkwatching site established in the mid-seventies and a site that has been covered continuously for the full fall migration season since the late 1970s. It is situated on the crest of the Blue Ridge Mountains in the State of Virginia. Roughly halfway between the cities of Lynchburg and Roanoke it has been attended by birders from those cities. By the early nineties the hawkwatch at Harvey’s Knob was well established. Those who attended that site thought they knew all that would take place with regard to hawkwatching in that general area of southwest Virginia.

That impression of know-all remained until 1992 when a hawkwatch was conducted at a nearby site where the surrounding territory was diametrically opposite from that at Harvey’s Knob. That site was the Woodpecker Ridge Nature Center (WRNC). The WRNC site was located 7.5 miles SW of the HKH site. It was offset from the crest of the Blue Ridge by one mile off the western slope of the Blue Ridge. It was a site more representative of the Great Valley of Virginia than it was of the mountains. It was a site in the mountains that was not positioned to take advantage of mountain influenced soaring flight.

That hawkwatch at WRNC that began in 1992 continued through 1995. It was conducted only in November for the Red-tailed Hawk migration season. The HKH site was covered as usual during that time providing a simultaneous coverage of hawkwatching sites with differing environments. The results were very interesting and very revealing of the behavior of hawks in migration.

In 1992 when we gathered the data from each site we found that the WRNC site had outscored the HKH site by 3 to 1 in the Red-tailed hawk count! Those of us who were so sure that a ridge oriented NE to SW with an open area facing the west would be ideal for counting the Redtail migration were astounded by the WRNC/HKH count results.

We decided to continue the coordinated watch between the two sites. And, for the next two years the 3 to 1 differential in counts between the two sites remained. The 1992 count was no fluke. It was obvious that the WRNC site was a better place to count migrating Red-tailed hawks in the area of concern. But why was this so, I and others wondered.

The WRNC site at 1600 feet above sea level without a long ridge feeding hawks into it was outscoring the HKH site at 2524 feet above sea level with a long ridge to the NE feeding hawks into it. And it was outscoring it 3 to 1. A ratio that cannot be ignored. I searched the crest of the Blue Ridge with scope and walkie-talkie seeking another site on the crest that would compete with the WRNC sightings, to no avail. After the three year study it became obvious that the WRNC site was a better site for intercepting migrating Red-tailed hawks.
It was obvious that there was another important factor in choosing hawkwatching sites other than a long ridge with an open view to the west. To find that factor it was necessary to delve into the behavior of migration a bit more deeply than before. Since we were mostly looking for the buteos it was necessary to examine the behavior of the buteos.

The buteo genus is especially adapted for soaring flight. Therefore we should concentrate our efforts to intercept the buteos where they can soar. There are two primary topographical/meteorologically conditions that contribute to soaring flight.

Those are solar induced thermals and ridge lift. Solar induced thermals consist of columns of rising air providing rising air for the migrants to soar upon. Ridge lift consists of horizontal winds blowing across ridges that deflect the winds upward thus providing rising air for soaring flight.

To resolve the discrepancy in the Red-tailed hawk flights at the HKH and WRNC sites the two factors contributing to soaring flight must be considered. Obviously the ridge lift factor cannot be considered for the WRNC site but one must consider that the ridge lift factor is the exception rather than the rule. It is the solar induced thermal that is the rule and it is the solar induced thermal that prevails at the WRNC site.

All around the WRNC site there are openings and terrain features that are conducive to the generation of solar induced thermals. Meanwhile the HKH site does not have the direct exposure of the sun, on the west slope, which is necessary for thermal production after the autumnal equinox on 21 Sept. In November thermal production is at a minimum at the HKH site while it is at a maximum at the WRNC site. Thermals are the rule while ridge lift is the exception to the rule. That fact alone explains the 3 to 1 discrepancy in the counts in favor of the WRNC site vs. the HKH site.

I cannot think of another study between two sites that has brought forth the importance of comparing thermal generated soaring with ridge lift generated soaring on the behavior of migrating buteos. The principles put forth in the preceding must be considered at each and every hawkwatching site. Each site must be dedicated to answer one question and one question only: Why is the hawk in view rather than why is the hawk visible.

DJ

November sunset down the parkway from Harveys Knob.

2006-12-03T10:58:00.000-05:00

Wind, Thermals and Hawk Flight

2006-12-01T22:29:00.000-05:00

I'm still looking for a definable relationship between wind and
> thermals and hawk appearance.
>
> What else do I need?
>
> bj

Wind is so variable that it can’t be reduced to a viable factor. I have never believed that wind had an influence on hawk migration when it was at or less than a Beaufort scale of 2. Wind at low velocities is caused by several factors but it is never constant over long distances unless it is influenced by a high pressure system immediately following the passage of a significant cold front.
The wind you may record at the lookout may only be the result of a thermal building in the vicinity. Depending on where the thermal builds relevant to your position the wind you record may just be the air flowing into the thermal build-up to replace the air generated upward by the thermal build-up.
There are other factors affecting the direction of light winds but those factors do not lend themselves to a constant wind from a constant direction. Forget light winds and consider them all as light and variable. The word variable is the key.
I have always considered the fact that the hawk’s brain is very small. I remember my peers at an early age calling me “birdbrain.” The meaning was clear. Birds were stupid and some humans were so stupid that their brains must be as small as those of the birds. In my adult life I have found that modern scientists still believe in the size of the brain as a measure of the intelligence of the individual possessing that brain.
The southbound hawk has no idea of where to go other than south or perhaps southwest if you subscribe to the earth’s magnetic field as the determining factor for the direction of migration. The hawk only knows to soar and glide south to find another thermal to soar upon. Sometimes it doesn’t glide south as it gets confused by a nearby thermal that is east or perhaps west. But, eventually the urge to fly south overcomes all these interruptions and the hawk generally flies in a general direction of south.
The southbound hawk doesn’t give a royal crap about variable winds.
During the time that variable winds are blowing, thermals are booming. It is the thermals the hawk is after and not the winds. The hawk’s minute brain can discern and determine a path for migration that includes thermals but not a path that includes variable winds. All hawks migrating in the absence of the passage of a significant cold front are migrating by soaring up on thermals and gliding to the next thermal in the general direction they wish to go, which is south.
When a significant cold front passes the winds will be constantly from the westerly direction and they will be strong enough to produce ridge lift as they blow across the ridges. Then the puny brained hawks will sense another means of soaring flight in the direction they wish to go, which is south. Then the hawks will congregate along the ridges and the hawkwatchers will ooh and aah as they pass in great numbers. And, if the westerly winds prevail throughout the peak of the migration season the hawkwatchers will have record numbers to ooh and aah over.
Just remember that those same westerly winds blow over flat country where no ridges offer the advantage of ridge lift. Yet the hawkwatchers in those regions still have much to ooh and aah over. The hawks can soar and glide regardless of the lack of ridges. Consider yourself lucky to have a ridge to watch from.
You should only be concerned with a wind that is constant in strength and direction. All those light and variables don’t amount to a hill of ..... . They should be recorded but they should be ignored as an important factor in the migration on the days they occur.
All of the foregoing is from my perspective and I will entertain any notion contradicting my perspective. But I warn you, it had better be good.

DJ

More .......

2006-11-27T09:29:00.000-05:00

Bijam@aol.com wrote:

> Seen this?
>
> http://www.ideasbypaul.ca/research_5d.pdf
>
> Thermals.....

All of that sounds good, but does it take into consideration that thermal activity cannot be accurately forecast and that the hawkwatcher is limited to a two mile radius purview? Naturally, there are more hawks found where thermals are strongest but does that mean that the hawks found represent the maximum that could be found? What are the counts three or four miles away and what is the thermal strength there?
And how much money does each hawkwatching site have to donate in order to acquire the precious instruments that would help forecast this thermal activity?

We hawkwatchers are sort of fixed into our site parameters and we have to know which meteorological conditions coupled with the topography of the site will produce the maximum number of hawks. And, we do not need to know this ahead of time but after the flight has passed. For, we volunteer, amateur, hawkwatchers are not really studying the true numbers of migrating hawks but rather the study of the behavior of hawks in migration.

The BLIP maps would be good for the study of the behavior of hawks in migration but they would not be good for predicting a good site for the study of the behavior of hawks in migration for various reasons. Nor
would they be good for predicting when to go to study the behavior of hawks in migration.

DJ

Dr. Holt Responds

2006-11-25T10:06:00.000-05:00

In my continuing and seemingly futile effort to find a variable that brings hawks to our site, I continually analyze counts and weather data and send speculation to our resident raptoclimactomigratologist who indulges my general thickheadedness for no apparent reason. The last post reflects this speculation and I followed this up with some more data from that day which I thought reflected a 'lift event'. Here is his response to that data:

Bijam@aol.com wrote:

> See anything in these graphs?

I don't see anything in these graphs for the simple reason I don't know what to look for. I do not believe that the daily or hourly variations in the weather have any influence on the hawk migration. I will have to see a mountain of evidence before I change this admittedly biased opinion.

Try to think like a hawk which is not thinking at all. It is feeling and reacting without any conceptualizing. For instance: A hawk rises in the valley to proceed south and it feels strong NW winds. It knows from experience, or from the genes, that now is the time to move to the highest point in view, nothing more and nothing less. Once it reaches the highest point it will find the ridge lift that will expedite its southward journey. If the highest point becomes a long ridge stretched out over a large expanse of land from NE to SW it will provide a collection point for all those hawks rising that same morning. A hawkwatcher stationed on that ridge will enjoy a good day. The hawk felt and reacted and the hawkwatcher was there because he knew the hawk would only feel and react.

Otherwise, the hawkwatcher would have been at WRNC [Woodpecker Ridge Nature Center - a valley floor observation point]. Or take the southern Michigan site into consideration. We both know that there is no high point in this territory, so the hawk does not move anywhere but to the southwest even when the winds are very strong northwest. This is because the hawk has it built into him to migrate to the southwest regardless of the conditions and it helps that the hawk presents a flying object that exhibits near zero drag coefficient to the wind. This is why they can migrate under strong winds in flat terrain and why some species can hover motionless in the face of strong winds while searching for prey. And, the reason for the southwest determination is in the genes.

When the migration is flying on thermals the hawk has no plan to fly on thermals, as it cannot plan. It finds a thermal when it rises by observing other flying creatures or by accident. When it finds a thermal and begins to soar it will find other hawks that found that thermal in precisely the same manner. Then they break out of the thermal and they spread out over a large piece of terrain in what we hawkwatchers call a stream. The width of that stream helps them to locate the next thermal and the process repeats itself. Sense and react, and sense and react only.

No migrating hawk ever watches the Weather Channel nor does it have a topographical map of the terrain over which it is to travel. It knows only to fly to the south and to utilize any condition that facilitates that flight that it senses during that flight. Its southerly flight may be influenced by its sensing the earth's magnetic field which causes its flight to be more southwesterly than southerly. The genes say south and the conditions say southwesterly. It is all sense and reaction.

It is interesting to speculate upon the sky conditions when the highest counts of migrants have passed the lookout. Cloud forms and winds are important. But, is the altitude of the flight important? I am speaking of the flight that everyone assumes is too high to see that goes past the lookout undetected. It is quite possible for the human eye to detect a Broad-winged hawk at 3/4 of a mile distance with the naked eye. I think I can safely estimate that 90% of the hawks seen at any lookout are within the purview of the human eye. Is there any reason to believe that there is a massive flight of migrating hawks flying beyond the purview of the human eye?

I think not. Three quarters of a mile is equal to 3960 feet. The naked eye migrant at Harvey's Knob is flying 6484 feet above sea level. Most solar induced thermals have cap clouds in the 6000 to 8000 feet above
sea level range. In some places even lower altitudes are encountered. And, most of the migrants are seen within that range. None are seen above that range for the simple reason that the cap clouds obscure visibility. It is easy to speculate on migrating hawks flying into towering cumulus clouds in order to gain altitude as well as to evade the hawkwatcher’s scrutiny. But, will the migrant expose himself to the turbulence and wind sheer of a towering cumulus in order to gain an altitude it does not need or to evade the hawkwatcher’s scrutiny it knows nothing of.

If the hawks are flying too high to be detected by the hawkwatcher and if they always do this on days when the hawkwatcher has nothing to report, then what is the purpose of reporting what the hawkwatcher has seen. If the hawks fly too high to be detected then the hawkwatching reports cannot possibly measure the population density of the hawks. The only purpose of reported hawkwatches are to provide an alibi for each hawkwatch rather than a piece of evidence that may someday have an effect on the art of hawkwatching. Resolve this contradiction and you have made an important step toward elevating the art of hawkwatching to the science of hawkwatching.

In the meantime, I prefer to remain close to the ground. I prefer to remain in that realm where the most hawks migrate, that realm where most of us hawkwatchers can see. I prefer to concentrate my mental efforts into what I can see or otherwise experience. That realm consists of solar induced thermals and ridge lift. Other systems of rising air are inferior because they are spread out over too wide an area of land. An upper air disturbance creates a low pressure cell which causes air to rise over a very large area of land. Any opportunistic migrant will take advantage of this rising air but it will not mean anything to the not so oppurtunistic migrant.

I have seen Broad-winged hawk migrating on weak rising air because the Broadwings are in a hurry. But, the Red-tailed hawk is not in such a hurry and I have never seen Redtails migrate on puny rising air. We know that Redtails migrate in large numbers when the conditions are conducive to their migration and we must realize that ten to twenty Redtail migrants mean nothing insofar as the science of migration is concerned. Probably, half of those Redtails are locals that will be in the area all winter. There is no-way to distinguish between local Redtails and migrant Redtails in late November.

I hope I have touched on all your questions in this diatribe as I do not have anything more to say, and if I did I don't have the energy to do so.

11/19/06 Migrants of another color

2006-11-22T11:32:00.000-05:00

Tad was on the watch today and it was an unusual day. Hawks were slow in the morning, some accipiters and redtails, and then suddenly a group of 4 ravens went over high, heading northward. Then some Red Shouldereds came up from the valley and then Redtails were high and moving. Then we had a string of these:

Another Loon stream came later, but spread out like the above. Then, along came a small group of these:

Quite a surprise to see Canadas on the mountain.

To complete the unusual day we had a V of these:

Swans, no less.

Unusual day!

I heard on the news later that there was an upper level disturbance that caused some "lift".

What's that about?