Saturday, July 16, 2005

Blakeman: Fledging dates are way off!

Yesterday I posted some statistics about Red-tailed Hawks' fledging dates, and applied them to the Trump Parc nestlings. According to those statistics redtails take their first flight from the nest [that is, they fledge] 42 to 46 days after hatching. That set of numbers is what is sometimes called the window of opportunity. All scientific accounts of bird species gives you such a window for each species. It gives a range--the earliest possible number of days from hatching and the latest, for fledging. American Robins, for instance, fledge 14-16 days after hatching; Bluejays, 17-21 days; Red-bellie3d woodpeckersd 24-27 days; Ospreys 48 - 59 days.

I never questioned the window 42-46 days for Red-tailed hawks because during all the years I observed the nest at 927 Fifth Ave I could never tell for sure [nor could anyone else] when exactly incubation began or when the chicks hatched.
We always made educated guesses, based on the hawks' behavior. When a baby fledged later than 46 days after we had decided a baby had hatched, well, we figured, our educated guess was wrong.

Even when I sent out the "Window of Opportunity" posting yesterday, something seemed wrong about it. For instance, the babies of past years at the Fifth Ave. nest always began to jump up and down vigorously and flap mightily in the days before fledging. The Trump Parc babies have just barely begun to stand on their two legs in an uypright position. Until a few days ago they were still sort of crawling around on the nest. Their flapping is brief and weak.

Still, my strong belief in scientists' greater knowledge [excuse: my father was one], made me abandon common sense and post the item predicting that the Trump Parc babies could fledge any minute.

Below are two letters I just received from John Blakeman. I don't always agree with him, [as you'll see in tomorrow's posting]. But I agree entirely with what he writes in the two letters that follow:

First letter:


I don't mean to discount the published fledging periods you posted, but for this pair, in this year, at this location, they are way off.

There is no way either of these birds is going to successfully loft into the Manhattan air anytime real soon. The last posted date is next Tuesday, the 19th. I've watched both captive and wild red-tail eyasses mature on nests, and these birds are at least 10 days from fledging, probably closer to two weeks or more.

The flight feathers of the wings and tail are still "in the blood," partially grown with active vascularization within. They are very heavy right now, compared to the very meager thoracic flight muscles. That's why the birds will flap just a few times at this stage, then stop or just plop down exhausted. The wing feathers are heavy, filled with blood. When dry and mature, they are -- light as a feather. But not yet.

Neither the birds' muscles nor feathers are within a few days of the maturity required for actual flight. The coverts, or smaller feathers covering the wings and body are just beginning to emerge. Neither bird presents any workable aerodynamic profile.

So let's not get our hopes up prematurely. Like an infant taking his first toddle, fledging for observing humans is always an exciting moment. But it's fraught with all sorts of threats and challenges, especially in the city.

As eager as everyone is for fledging, I prefer to see the birds hang around on the nest for as long as possible. If either bird has done her reading and wishes to diligently follow the published dates, I hope she lands safely. The more likely outcome will be for the birds to follow instinct, not older, invalid-for-NYC data.

Let's keep our fingers crossed on all of this. Red-tails are large, strong birds that frequently rise to any challenge. Let's see if these two birds can. So far, all is well.


John A. Blakeman

Second letter:


I'm not familiar with the work giving the fledging dates you posted. In fact, I never really quantified the days in the nest for the birds I studied, primarily because I couldn't determine exact days of hatching. Either I wasn't in the field watching the nest on the hatching dates, or, in the manner of the 927 nest, it was difficult to see what was actually happening way up there. My captively-reared eyass didn't fledge in a conventional manner at all, being raised in a cage.

At any rate, I'm wondering if the posted dates are for authentic fledging, the taking of a first flight, or do they refer to the eyasses walking out of the nest onto the supporting limbs of the tree. Falconers are familiar with this process, and for centuries eyass goshawks have been taken for falconry just when the young birds start to climb out onto limbs. The birds are then affectionately referred to as "branchers," having stepped out onto adjacent branches, but not fully feathered or able to fly.

The Trump Parc eyasses are just getting to the "brancher" age. If they were in a conventional tree nest, they would very likely begin to edge themselves delicately out on to larger limbs or branches, thereby "leaving" the nest about now. They would hop right back into the nest at night, but they become ever more venturesome in their pedestrian excursions as their legs get stronger.

Perhaps this is what the author meant. If so, the dates are correct -- except of course in our urban cliff-side nest lacking any branches. But I'm sure the birds have been seen to be wandering about on the ledge. Bipedal mobility precedes winged mobility. I think we have a pair of "branchers" about now.

John A. Blakeman

PS from Marie: The reference book I cited was The Birds of North America, an authoritative text first published species by species and recently completed for all species of North American Birds. It is published by the Academy of Natural Sciences, the American Ornithologists' Union and the Cornell Laboratory of Ornithology.