Friday, April 15, 2005

Blakeman on Progeny -- a retreat of sorts

Below, Blakeman's new thoughts on the progeny question. At the end I add a small rejoinder of my own, just to keep the conversation going.


Marie,
You know that I went on at length back in December about how all the other red-tails in the Central Park area weren't likely to be Pale Male progeny. I expounded on how the adults will drive off the young to protect the larger adult hunting territory. Secondly, I noted that an incursion of Pale Male kids would promote mating of close kindred hawks, resulting in genetic problems.

But from your many descriptions of pale colored birds, and other cogent observations you and others have made, I've retreated from my former answer of rather standard field biology, that the kids shouldn't be there. It looks like they are, for whatever reason(s) and in whatever densities.
Now the question. Did the new NYC ledge-nesting red-tails select those sites because they were raised in and fledged from the 927 ledge nest? Or, might there be another explanation?

I really don't think the birds are nesting on ledges because they were hatched on ledges. As I mentioned before, little eyasses are rather stupid and involve their minute, developing brains with vocalizing, feedings, flapping wings, and defecating. Frankly, until an eyass can fly, it doesn't know at all what’s supporting the nest – nor does it care.

Why, then, the recent preponderance of ledge nests? I'm guessing that it involves the multitude of people under the trees that would be the conventional sites of red-tail nests. Even in the West, where big trees are available, trees are used over ledges. Red-tails prefer trees when they are available.

I've never elaborated upon the inordinate tolerance of humans, dogs, and who knows what other animal and mechanical disturbances the Central Park red-tails have endured. Out here in rural areas, a sitting red-tail is likely to abandon the nest and eggs when a single human walks within 100 or 200 yards of the nest. In Central Park, people are walking right near perching and nest trees. For overnight perching, the birds have learned to accommodate the multitude of strange bipeds (people) strutting hither and thither below.

But when it was time to build a nest, I think the presence of dogs and people might have been a bit over the edge. Trying to get a few sticks to lodge in the small crotch of a tree 40 ft up is no easy task. It requires full attention to the multiple construction tasks inherent in getting a secure nest erected. I'm guessing that the birds simply chose nest sites remote from any human experience, and those were way up on the sides of tall buildings. Up there, they could poke and probe and thrust and re-arrange sticks without any diversions.

One of the first anomalies of the 927 nest I noticed was not that it was on the side of a “cliff.” I've seen wild Western red-tails do that. But it was the inordinate height of the selected nest site. The western nests I recall observing were seldom much taller than tall trees, usually in the 75 to 100 ft range. Many were lower. I never recall seeing a red-tail nest either so high, or so close to the canyon rim (here, the roof line).

<>But no one can deny that both the 927 nest and the Trump-Parc nest are about as far from humans as physically possible. I think that’s the factor, the desired remoteness from humans and dogs. This might also involve a perceived nest predation threat on the part of these two-legged ground animals. Who, among red-tails, knows if humans couldn't just shinny up a park tree with a nest in it and take a pair of eggs for breakfast? Perhaps the hawks are building high, remote nests on ledges because it’s the only place in Manhattan where a sitting hawk can be confident that a human won't disturb its calm incubation. As urbanized and acclimated to The City as these hawks are, they may still retain a requirement for a bit of private remoteness, some wild solitude and security. The high ledges are the only places that provide these.

But as always, merely thoughts off the top of my head (reflected, however, from watching both wild and captive red-tails for 35 years or so).

Sincerely,

John A. Blakeman

John: You note that the little eyasses are too stupid to take note of their ledge origins, and therefore are not likely to be drawn to ledge nests when they're ready to breed themselves. But would this be a matter of intelligence? Might it not involve instinct.?Just like the principle of philopatry brings phoebes and orioles back to their former nest locations by instinct, not intelligence, might there not be some instinctive drive on the part of these eyasses raised on a ledge to replicate their early experience?