Sunday, February 20, 2005

My Daring Hypothesis: A Question For John Blakeman

There is a "plethora" of red-tailed hawks in Central Park and environs these days. I can't resist offering a somewhat audacious thought of my own:

Pale Male has been breeding successfully since 1995. Each of the 3 offspring of that first nest would have been ready to breed themselves by, let's say, 1998, and THOSE birds could have produced Pale Male grandkids by 2001. Then there are the three offspring of the 1996 nest, who could have each had young by 1999, and so on and so on. Meanwhile Pale Male and his mates have been producing eyasses year after year -- a total of 23 who lived to fledge.A mathematician would have to figure out how many possible Pale Male children, grandchildren, etc. there could be by now. Since it is a geometric progression, I'd bet that number is huge!

John Blakeman has previously written of the 23 offspring:"I'm certain that the majority died in their first summer." That was written on Dec.26., almost 2 months ago.[For this and other references to past Blakeman letters, scroll down on the LATEST NEWS page.] I wonder if his thinking has changed about this since then, as he has come to recognize that the NYC redtail explosion represents a possibly different reality than the situation he is most familiar with -- the rural and suburban redtail.

Obviously, I am not a biologist. Nevertheless I have a strong hunch -- partly stimulated by the timing of the local proliferation of redtails, partly by the fact that so many of the local hawks are making attempts to nest on building ledges rather than trees[unsuccessfully, so far, because of absence of spikes anywhere but at 927 Fifth]-- a strong hunch, as I say, that there is a connection between Pale Male and these proliferating NYC redtails. As I wrote to John Blakeman before, how extremely odd that before PM arrived in Central Park there were NO resident redtails in Manhattan. Now they're a dime a dozen.

I'd buy the no-more-boys-in-the suburbs-shooting-redtails hypothesis John proposed in a letter on Feb. 6, 2005, except that an alternate hypothesis makes more sense to me. I'm about to propose it, knowing well that I'm going way out on a limb, and that I really am unqualified to make scientific hypotheses at all! But perhaps it will stimulate an interesting answer, so here goes.

Is it possible that Pale Male is, in fact, some sort of evolutionary breakthrough? While Blakeman has written about the intelligence of hawks as compared to the Corvids or parrot families, [that is, that hawks are not the sharpest crayons in the avian box] perhaps Pale Male is actually a smarter redtail?

He has succeeded at building and maintaining for 11 years a pretty unique nest. He has managed for all that time not to eat a poisoned rat or pigeon, nor to collide with a truck on the highway, nor to fall prey to any of the various slings and arrows of outrageous fortune that others of his kind run into.

Maybe he has passed on this gene to a bunch of others who are managing to survive that perilous first year better than redtails did before, discovering new prey sources, perhaps. [Blakeman has speculated earlier, in a letter not posted, that pigeons might be an important factor here]. Anyhow, I've run this idea up the flagpole. Long may it wave.





Lincoln Karim
Pale Male with sprig of evergreen in bottom photo
[(See Blakeman comment below]Marie,

The intelligent question was asked [by Karen Anne Kolling], “What do they line the nest with?” As the nest approaches full structural size, lining will be an important next step.

Right now, the nest is almost surely just a moderately intertwined pile of sticks. But this would not be able to retain the heat needed to incubate the eggs. Many first-time red-tail nests fail to get properly lined, and cool March winds blow right through the structures. Needless to say, the eggs laid in these insufficient nests fail to hatch. I've seen this quite often, frequently in marginal habitats, almost surely by newly-mated pairs that haven't learned yet how to properly prepare a successful nest. Pale Male’s first efforts may have been of this nature.

The nest lining has to be wind-tight. When watching either of the pair settle down on to the eggs to incubate, notice the deliberate manner in which this is done. What you can't see is that as the bird gently wiggles down onto the eggs, the parent is putting the naked skin of its abdomen right on to the eggs themselves. These bare patches can't be seen when the birds are flying, as they are under the outer body feathers. But before incubation the inner down feathers of the abdomen are lost, creating the “brood patch.” The brood patch seems to be larger on females than males.

So, while the birds are sitting on eggs, the adult’s body warmth is being transmitted directly into the egg. If the lining of the nest isn't tight, too much of that heat escapes and the embryo or unhatched chick (it’s not an eyass until it hatches) won't develop properly.

What, then, is the lining made of? It varies greatly, depending on what the birds have available. It’s usually some light, fluffy, fibrous plant material. The thin flakings of wild grape vine is used, as is the light, shedding bark of smaller dead tree limbs, providing little sheets of paper-like material that can be pressed together in the nest. Out here in rural Ohio, last season’s dead corn leaves are commonly used.

The birds will simply look for whatever is locally available. Most importantly, the material must be able to be tucked and compressed. Clumps of dead grass stems are often used. A fist-full of partially decayed (but dry) leaves works. It would be interesting to document the preferred lining materials of Pale Male and Lola. What does the vegetation of Central Park have to offer?

If they haven't yet, some time soon the pair is likely to start bringing sprigs of evergreens to the nest. No one has ever figured out exactly why this is done. A well-needled tip of a pine branch is brought to the nest and either tucked into it on the side, or sometimes arranged at the edge of the rim. The birds have a strong compulsion to do this, and out here in rural northern Ohio where evergreens of any sort are uncommon, we know that some birds have to fly several miles to find a valued pine or spruce tree. The best explanation is that the evergreen sprigs tend to repel feather lice and other invertebrate vermin – except that eyasses in their first weeks often are pestered by a number of bugs nonetheless. The green-sprigs-at-the-nest story is still an unexplained one.

This is an experienced, successful pair. They aren't doing anything new that they haven't done so very successfully before. Getting the apartment ready for the new brood of eyasses becomes important once again. Watch to see when lining material begins to appear at the nestsite. That’s rather equivalent to the buying of the bassinet and other accoutrements that human moms do when they know that live-giving things are about to happen.

As a falconer who gets to watch my red-tails hunt and kill, where I see close at hand their remarkable power, speed, and predatory determination, to watch their markedly converse behaviors of gentleness and care at the nest is always a striking contrast. At the nest, I still find red-tails almost totally out of character. I'll have some more comments on that when incubation starts. But to watch a red-tail at the nest is a special privilege, seeing it behave in considered ways seen nowhere else.

And New Yorkers can just go to the right spot in Central Park, ask to take a peek through a spotting scope, or just look up there with a pair of binoculars to see this remarkable spectacle. Although I've seen it here in the wild many times, I'm still a bit envious. You can see it both with ease and at length. Very special. Very special.

Sincerely,

John A. Blakeman