Wednesday, April 13, 2005

Blakeman answers Donna about stick size

Donna,
Let me summarize your good questions submitted to me about nest sticks (or twigs).
1. Why are only little sticks being used in NYC?
Frankly, I'm not sure it's so, although I first suggested this. It was only an hypothesis I related to a perceived (but errant) notion that Central Park had no dead trees or limbs from which large sticks could have been broken off. Marie corrected me on this. NCY red-tails have dead limbs to break off, and they have twigs and limbs of all sizes to use from other trees, too. It’s no longer fair for me to suggest the NYC nests are fundamentally different from other red-tail nests.
Except, to refresh my memories of field studies long ago (1970s), I looked up the fine photos of red-tail nests in G. Ronald Austing’s seminal 1964 “The World of the Red-tailed Hawk.” Ron Austing, another Ohioan in the Cincinnati area, became entranced with red-tails like the rest of us. He was (and is) a remarkable wildlife photographer. And like myself, he engaged in falconry with red-tails. Ron’s book, although written for a general audience, is still a major work on the species. The photography is remarkable, including the numerous telephoto shots of nests taken from carefully erected towers Ron placed next to nest trees.
It’s from these photos that I believe the wild RT nests tend to have slightly larger sticks than Pale Male’s. When I compare the unexcelled nest telephotos of Lincoln Karim with Ron Austing's photo on p. 50, the Ohio twigs do look larger than the NYC material. So I still contend that rural wild nests are made of slightly larger sticks. But whether that accounts for anything on NYC buildings is still an open question.
2. Do red-tails build nests similar to the ones they were fledged from?
Seems as though they do. But from me, that’s a useless statement because I've only seen conventional tree nests (except for two summers of cliff-nest studies in northern Nevada, where I could no watch any progeny build nests.) I think the species builds its nests primarily on instinct and materials available, not on anything learned or seen as an eyass. Eyasses are plainly stupid. They just sit there, vocalize, eat, defecate, and try to learn to fly. They don't intelligently peruse the building materials and techniques used by Mom and Pop. I think it’s all instinct.
3.Just how big are the "larger sticks" the Western Red-tails use?
I don't know. I failed to record their size when I studied those nests. At the time, it seemed to be a useless bit of information. What difference did it make? (Now, I know that it might be important, but I have no info. An unrecognized, lost opportunity.)
4. We've only seen Pale Male and Lola break twigs off trees, in their case live ones. Do your RT's take their bigger dead sticks ONLY off trees or have you ever seen them forage on the ground for them?
Another question for which I haven't the slightest answer. Frankly, in all the times I've spent looking at red-tails in the wild, I've never seen one pick up a stick off the ground, or even snap one off a tree, living or dead. And not many others have either – except you folks in Central Park where everything is concentrated in a small area, with lots of observant eyes. For me, the vast majority of the nests we studied were nests already in place, ones being reused or refurbished. Consequently only a few top-dressing sticks and lining were being added. New nests were built by new pairs, but because we were watching the nests in an area of about 600 sq miles, a new nest could go up in two or three days before it was discovered. To this day, I don't know where Ohio red-tails procure their nest materials. I see them bringing stuff to the nest, but I didn't see them collect it. An Ohio nest typically has a territory about twice the size of all of Central Park, or more.
5. Seems there are a number of variables here. There is only a certain weight of stick that a given Red-tail can carry. Perhaps only a certain diameter of dead stick that a Hawk can break off due to beak gape.

No, red-tails can carry, should they want to, a stick of remarkable size and weight. They can carry a 3/4-lb. squirrel into a tree. Even a half-pound stick would be rather large. It’s neither a weight nor size thing. It’s a question of how effective a stick will be in the nest. Red-tails have a big mouth and even much larger feet.
I've always wondered if red-tails tend to select sticks that are rough, with lots of jagged projections so as to intertwine when placed in a heap. The missing factor in all of this may not be size or weight at all. It might be the roughness or frictional characteristics of the sticks that count the most. A nest made from densely-spiked hawthorn twigs isn't likely to go anywhere unless placed upon a glassy surface. So perhaps we all need to pay attention also to stick roughness. Remember, unlike robins, red-tails do not cement their nests together. They poke sticks together in only a rudimentary fashion. The pile has to stay together pretty much by itself.
6. In regards to weight, different diameters of stick would vary depending on species (density), on moisture content(deadness), and diameter. Therefore even a big stick that was more "dead" would weigh the same as a smaller fresh one and therefore conceivably blow off. Or is there truly something about diameter as opposed to weight that would make them work better in nest building?
Larger diameter sticks, even if lighter, have more surface area rubbing on the supporting substrate (on ledges), so it takes a bigger gust of wind to blown the nest off. But this may be a very small factor. This is a mechanical engineer’s question.
In retrospect, my reference to the successes of western cliff nests may not be valid, compared to the ledges used at Central Park. The buildings of New York are almost entirely of worked, even polished stone, with low frictional qualities. The western nest ledges were rough or coarse, providing a stronger grip.
Once again, good questions – and rather fabricated answers
Pipping is not so far off. How many will it be this year? The waiting is nearly done.
Sincerely,
John A. Blakeman