The Behavior of Hawks in Migration

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