Dave on Flight, Species and Season

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