Sunday, March 09, 2008

John Blakeman on Red-tail Hawk Nest-Site Selection

Pale Male and Lola Switch on the Nest

Yesterday I published an excerpt from a news article in which the opinion was expressed that urban Red-tails preferred nests on buildings. John Blakeman took exception to this and here is what he has to say.


Someone in one of your postings stated that Red-tails prefer to construct their nests on buildings and ledges, compared to trees, which move around too much.

Not so. Even in the West, where there are many cliffs, Red-tails generally prefer to place their nests in trees (if any are large enough there). This is particularly true with the Red-tails in the East and Midwest, where cliff-nesting Red-tails are virtually unknown in the wild.

Our birds instinctively prefer to put their nests in trees. Their genetic behaviors have this strong preference. But when tree nests don't seem to be preferable to a Red-tail, as when they are harassed by crows in the trees in Central Park, they will, as Pale Male and the others did, head up above the trees and nest on building ledges, free from the avian and human racket down below.

Yes, I designed a Red-tail "Nest Nook" device that could be bolted to a building wall or placed on a building ledge. But I'm not certain that this would actually attract a new Red-tail. Great-horned Owls, Ospreys, and Peregrines can be rather easily enticed to nest on created new nest platforms. But Red-tails seem to want to put their nests where they so arbitrarily decide. Nest site selection for this species is another of its great mysteries.

For example, the photo by of the new (still meager) nest at a New York school window ledge is interesting. Just why there? Certainly there weren't any appropriate trees in the neighborhood, free from human and avian disturbances. But why this particular ledge, compared to the many others that also could have been chosen?

There does seem to be some unknown magic about some nest sites. I've watched in passing (at 65 mph) a single tree in the middle of a small woodlot here that has had a Red-tail nest on and off for over 25 years. Different birds, at different times. But repeatedly, new birds have chosen this tree over the several hundred other similar trees in the woodlot. Why? I don't know. It's hard trying to think like a Red-tail.

--John Blakeman

I don't know if we can generalize about Red-tailed Hawks, particularly those rather creative adaptable urban ones. Watching the sites urban Red-tails choose has given me the feeling that there is a set of criteria, perhaps some wired in and some learned from experience, known to the birds for nest site selection. We'll all most likely agree that there are few if any perfect nest sites and it seems to me that a bonded pair uses their judgement as to which are the most important criteria for them and choose a site accordingly. After a year some pairs build elsewhere, as Momma and Papa seem to be doing, whereas others hang in for years as Pale Male has done.

How much does successful hatching have to do with the change? Hard to tell, as Pale Male and Lola have now stuck into the fourth season of a three season failure run. Momma and Papa were successful last season and moved sites anyway.

What is the criteria? They know and we don't. Is the tree nest overhanging the road of the Riverside pair the best tree to hold a nest in the area? Quite possibly in their opinion and they may be quite right, but it can't possibly be the best place from which to fledge eyasses. Is safe flight of eyasses, one of the nest site criteria? If so it seems lower on the list for the Riverside Park pair than other criteria.

Another problem with urban tree nests at least in Central Park, and at up at Morningside Park as well, is the vulnerability of these nests to Crows. Crows may well have been a good part of the reason that Pale Male and his mate decided to make the revolutionary adaptation to nest on a NYC building. Attempting to protect a tree nest from Crows is what sent Pale Male and First Love to rehabbers in the first place. Pale Male's next serious nesting attempt was from 927 Fifth Avenue.

As forests become more and more truncated in the U.S. many species of breeding birds have become far more vulnerable to Crow predation. Our woods now have too many edges, as does Central Park with it's mostly widely spaced trees. More edges allow the sharp eyed Crows the visibility to spy out nest locations. A pair of Red-tails can stave off a murder of Crows from a nest with a wall to their backs. I've seen Pale Male and Lola do, and I've also seen Lola and Tristan do it. It is much harder for them to do with a 360 degree field of battle over a tree nest. Therefore in my opinion does an urban Red-tail pair prefer tree nests or building nests given their druthers?

It all depends, I think, on the blended hierarchy of the nest site criteria for any given pair of Red-tailed Hawks. The variables are legion, and John's right, it's hard to think like a Red-tail. Only they know their particular criteria. Sometimes that criteria puts them on buildings and sometimes in trees and that's where their criteria choices has led them to be.

Donegal Browne

P.S. We must also take into account that at least anecdotally form Peregrines to Red-tails where they hatched themselves, on a building, a bridge, a tree, or a cliff colors their own choices for a nesting site.

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