Tuesday, June 30, 2009

Will the hawks stay in Philly or migrate?

Now that the eyasses have clearly left the nest and the camera at the Franklin Institute has been turned off, we are all wondering how long the hawk family will stay in this area, and whether they will migrate south for the winter.

John Blakeman, as always, provides us a fascinating response:

"Red-tails, particularly those in both my native Ohio and those being watched in Philadelphia, do migrate. The migratory urge is especially strong in first-year red-tails. In August, they will be essentially separated from their parents, although they may still reside in their natal territory. But sometime in September or early October they are likely to soar up on a thermal and notice the passing stream of other red-tail migrants drifting southward. One day an eyass will be seen on a common perch, but gone the next. It will have begun its first migration to some place further south.

The migratory urge is not nearly as strong in adults. Pale Male spends his entire life in Central Park, and the Franklin parents are likely to persist through the winter. Experienced adults tend to spend the winter when ample food is available, which is surely the case in Philadelphia. But if food becomes harder to capture, even adults will head south.

Here in northern Ohio and across the northern tier of states the deciding factor is the depth and persistence of snow. In northern areas that are snow covered during much of the winter, sustenance prey species of mice and voles cannot be seen under the snow. The hawks then fly south where there are no persistent, deep winter snows.

That won’t be a problem in Philadelphia. The Franklin Institute adults will almost surely hang around the nest area this winter, staying within a half-mile radius or so. They have everything that makes a red-tail happy - lots of food, good perches, and no hassles from humans or anything else. Ample food is the big thing.

But the eyasses will likely disappear in September or October, if not before. Any of them can get killed by crashing into a fence, getting struck by a car or truck, or by acquiring some lethal parasite or disease. About 60% (a guess) of fledged eyasses never make it through the first year. Perhaps a half of them die in their first summer, usually in late July and August, after the parents stop feeding them. They just plainly starve, unable to learn how to adeptly capture enough food. Nature is tough, especially for dumb, inexperienced young predators. Only a few survive to full adulthood. So don’t take any special lament when one of the eyasses is found dead. Sorry to convey this news, but it’s very, very likely to occur (albeit undiscovered).

Those that survive to early autumn will begin to have a strong urge to migrate south, promoted mostly by decreasing day length. This turns on the migratory urge, and when these young birds see other red-tails passing along overhead, they find a thermal and effortlessly wheel themselves up a mile or so. Up there, the urge to lean off to the south, along with the dozens of other red-tails and other hawks which they can so easily see up there is overwhelming.

By November, they will be anywhere from Virginia down to the Gulf Coast. They will search for food down there, and when the days start to get rapidly longer in March or April, they will reverse their migratory trek, heading back to the general area of their nest in Philadelphia.

And here’s where some significant science could be conducted at the Franklin Institute nest. It would be so good to get next year’s eyasses banded, both with conventional numbered aluminum bands, and also with separate, unique color-coded bands. If this is done, it could be determined if the birds come back to the immediate nest area, or just to the general area.

This is a contention in New York. There are a good number of new red-tail nests there, and everyone wants to think that these are all direct descendants of Pale Male. Maybe they are, maybe they aren’t. Only good banding can reveal this. I hope that all future eyasses at the Franklin Institute can be color banded. In a few years, this could answer a great question in red-tail biology.

–John Blakeman