Broad-winged Hawk Migration
In light of the rapidly advancing hawkwatching season of fall 2014, permit me to present a few of my ramblings on the subject. These ramblings are dedicated only to the migration of the Broad-winged hawk, therefore the term hawk will mean Broad-winged hawk. One of the greatest mysteries of the Broadwing migration is: Just what triggers the migration? How do the hawks know when to leave the breeding grounds and when to leave the wintering grounds. If we refrain from anthropomorphizing, we come to realize that the hawk, as all birds, has a very small brain. How much information can they store and how much does perception or stimulus drive their actions? I submit that stimulus is the driving source for all of their actions and that such a stimulus for migration must be a very compelling stimulus - a stimulus that results from a great change in the environment surrounding the hawk where it would be content to remain. I also submit that there are two environmental stimuli that provide the triggers to migrate.
The Broad-winged hawk breeds in the northern temperate zones and winters primarily in the northern tropics with some extending to the southern tropics. While on the breeding grounds the hawk must know when to start its migration in order to guarantee it will have an ample food supply on its journey south. The primary food supply for Broad-winged hawks consists of reptiles, amphibians, and insects - all of which thrive in summertime climes in the temperate zones. Therefore the Broad-winged is dependent on long warm days for its food supply. It would stand to reason that the Broad-winged hawk would be stimulated to migrate when the days shorten.
I have plotted the length of daylight hours during a year's time at Boston, Mass. and found that the plot resulted in a sine wave. The slope of a sinusoid represents the greatest rate of change. Therefore, around the middle of August the rate of change of the length of day is at its greatest and the days are becoming noticeably shorter. There is your trigger for the migration south. The Broadwing now knows that it had better be moving southward lest it run out of food. And, it had better move fast. We see just how fast, as nearly all Broad-winged hawks have passed the northern latitudes by the end of September. They are settling on their wintering grounds in the tropics north of the equator by October.
Now, we have our trigger for the southward migration and it seems reasonable. So, what triggers them to return. While they may have ample food supply in the topics, will it suffice for raising a family? Pre-wiring compels them to breed and perhaps compels them to breed in the northern temperate zone, but what will stimulate them to return to the breeding grounds? There is very little or no variation in the length of day in the tropics. What brought them to the tropics will not serve to trigger their return to the temperate zones. Also, the amount of prey species in their tropical wintering grounds will remain constant. We must look for another severe change in environment that will trigger the northward migration.
I submit that such a change will be the transition from rainy to dry seasons. Conventional wisdom dictates that a vast majority of Broad-winged hawks winter between 15 degrees north latitude to 15 degrees south of the equator. The change from the rainy to dry season is not as well defined as one approaches the equator. Some locations are almost always rainy while others do not show a change that coincides with the timing of the return migration. But, at locations 10 to 15 degrees south, the rainy to dry season is well defined. The rainy season in these locations plotted over a year present a curve similar to the Bell Curve with a slope resembling the slope of a sinusoid. The four samples I used showed a change from rainy to dry occurring from January to May. The timing is excellent. We now have a trigger occurring in the wintering grounds that is very similar to the trigger that occurs in the breeding grounds.
These stimuli are most pronounced at the extremes of the hawk's habitat so it is apparent the hawks at these extremes will lead the migration. As they fly over other hawks they will be seen and that sighting provides an added stimulus to start to migrate. Sort of like the dominoes we like to line up and tip over in a chain reaction. Call it a sharp environmental change reinforced by a domino analogy.
David J. Holt
August 11, 2014