Wednesday, April 27, 2005

Blakeman answers some important questions

Could it be that the two inch gap between the arch and the metal nest structure caused the incubation to fail?

This is not an issue, I'm certain. The majority of wild (rural, non-urban) red-tail nests in the Midwest and East are in trees with supporting limbs that provide no relief from wind. The gap here is not a concern. Red-tails are capable of creating a reasonably "air-tight" bowel in the nest, whether there is a solid, flat support beneath, or (commonly) just a few crossed branches or a 3-way, V-shaped crotch. The failure of the nest was not caused by air shooting under the gap. Air shoots under wild tree nests, with no problems.

What then might have caused the nest failure?

I really think that the problem was caused by the shallowness of the new nest the birds constructed. This allowed the prongs to eventually protrude up through the nest floor. They may have finally pierced the eggs, or caused the parents try to push the metallic objects aside, as they would a displaced stick. The intrusion of the prongs into the nest bowl and rubbing against the eggs would be destructive, either by directly breaking the eggs, or by disrupting normal incubation behaviors, including the digging motions so often seen.

The metal prongs are probably not large enough to be able conduct away enough heat to cool the eggs or nest bowl. But as the nest normally settles during incubation, the eggs sink lower. Note how often observers stated that the incubating parent settled so low into the nest that it essentially disappeared. No one should presume that the bottom of the nest, the surface upon which the eggs rested, was near the height of the observable, protruding sticks on the rim.

When Lola was sitting in earnest, with her naked abdominal brood patches pressed directly against the eggs, her back is about 3.5 to 4 inches above the eggs. So, if her back can't be seen above the nest rim, one can subtract 3 or 4 inches to determine the tops of the eggs. The eggs themselves are almost 2 inches in diameter. Therefore, the vertical width of the hawk's body is, say, 3.5 inches, and eggs are 2 inches, making the bottom of the nest perhaps 5.5 inches beneath the bird's back while sitting. Look at any of the photos of the nest and find the 5-inch depth. This is very, very close to the tops of the prongs. When the eggs begin to thin (for eventual pipping and emergence) in the last week of incubation, a slight jostling against an intruding prong will puncture an egg.

It appeared to me that the majority of the twigs used to construct the nest came from live trees. Even in winter woody plants retain a great deal of moisture. No one tries to build a campfire or fireplace fire with branches from a living tree. They are inherently wet, and must be allowed to dry. As the new sticks in the nest dried out, they markedly shrank in size, especially in diameter. Ask any carpenter about wood shrinkage as it dries. It's significant. The drying of the nest's twigs from January to April could have depressed the nest bowl a half inch or more, bringing the eggs onto the pointed surfaces of the underlying prongs.

In short, the new nest was just that, very new and unsettled. Rural red-tails never have to contend with metal spikes protruding up through the nest's base, so nest settling caused by twig drying and repeated landings of the adults are not a factor. Here, natural settling was probably the initial event that caused the prongs to disrupt incubation or pierce the eggs.

Next year, the birds will be compelled by instinctive, hormone and photo-period driven behaviors to go through virtually the same actions again. More sticks and twigs will be brought to the nest and piled upon the remnants of this year's nest. This, it is hoped, will elevate the bottom of the nest depression sufficiently above the spikes to allow normal nesting. The present sticks will wet and dry in rain cycles, and further settle as the year wears on. The birds will then pile on more sticks nest winter. And because the new sticks won't have any protruding spikes to hold them in place, next year's nest may be initially much less stable. That will be very good, as the early fragility of the new nest layer will prompt the birds to add more and more sticks until it feels firm to them. That feeling of firmness, a probable clue that nest size was sufficient, was artificially contrived by the presence of the naked pigeon spikes this year.

The birds stopped thickening the nest too early this year, when it felt nice and firm. Next year, the new sticks will sit by themselves above the prongs, free to move about in a quite natural manner, a situation the birds know about and can deal with.

Is renesting possible for this season?

No, not by any means. The various required nesting behaviors are both requisitely sequential and photo-period driven. The endocrinology that first drives copulation, then ovulation, and lastly incubation has passed for the year. The birds are likely to perch around the nest site for some time, but this is merely a response to territorial habits. There will be no new eggs or incubation this year. The season has passed.
But the pair, should it survive, will return next year and add another layer of sticks, one that is thick enough to hold the eggs above the spikes, we all hope.

Red-tails frequently have nest failures, so this year's disappointment does not reduce the pair's chance of starting it all over again next season. The birds might even leave the immediate area, to reappear in December or January (although that's unlikely for this pair in this environment). The worst scenario would be the loss of Pale Male. He's now well into his second decade, an aging patriarch. Whether or not a new male, competent in the unique skills required for successful reproduction at this curious site, would appear is a major question, one that I have no answer to.

I hope this brings some understanding of what we have all watched. We are all pioneers here, Pale Male and his consorts in attempting to so completely occupy Central Park, followed by all of us attempting to do what we could to continue the unique experience.