Thursday, June 02, 2005

New eggs in the nest?? Blakeman answers Mai's questions


Below, John Blakeman answers the questions Mai Stewart sent in and others have been asking.

Marie,

Mai Stewart’s incisive and thoughtful questions are one of the main reasons posting here is such a delight. We are dealing with observant, thinking, and questioning people – exactly the kind that make science such a wonderful endeavor. None of us are going to solve any great biological question here, but the questions Mai raised are good nonetheless – perhaps because I think I can answer them!

Mai asks, “If they're doing what they need to do in order to make babies (no matter why they're doing it), might this not result in new eggs?” If that were so, eggs would be possible. But the creation of viable eggs is a major physiological undertaking that creates distinct physiological stresses, ones the bird can accommodate, but they are stresses nonetheless. For example, a great deal of calcium must be deposited in the egg, both for the shell and for the skeleton that will develop within. “Dissolving” all this calcium out of both the mother hawk’s bones and from the food she eats is not an energetically simple task. (I know, Professor Martin, the calcium isn't just “dissolved,” but we are trying to make this all comprehendible for people who haven't had your ornithology class.) The mother hawk undergoes a bit of osteoporosis in creating eggs. Her calcium deficit is minor and is quickly made up while she incubates. But all of her reproductive hormones have to be perfectly timed, and it’s not just one or two forms of estrogen. (Sorry Dr. Martin, I forget the several other ones.)

In short, copulation does not necessarily lead to eggs. Actually, eggs destined for viability precede copulation. Copulation (in season) allows for the fertilization of an existing, undeveloped egg. It does not cause oogenesis, the production of a haploid female gamete. Copulation merely adds the male’s half of the chromosomes that allow the egg to develop into a new, genetically different adult. The egg comes first. (Raising the question, of course, are new eggs forming in Lola? I don't think so. See below.)

Red-tails often produce infertile eggs. That’s what prompted me to attempt to breed the species in captivity. A red-tail I held for study in a large rural cage, out of sight from humans, dropped down on to the ground in March, grabbed some grass stalks and leaves, and formed a very shallow nest on the ground. I thought it merely curious, and allowed her to “play house” or in her case, “play nest.” A few days later, when I went to feed her, I noticed her sitting on the ground all bunched up. I was certain she was sick. But slowly she raised up on to her feet. She had been sitting on an egg. I was rather certain that this was Ohio’s first, albeit rather contrived, ground-nesting red-tail. Two days later she laid a second infertile egg. As there was no male, nothing reproductively transpired. But that caused me to build a nest support structure the next winter, in which she could form a more proper, above-ground nest. Its eggs, too, were infertile, as she had no male. In following seasons, she accepted a male, but because she had been mishandled by other human beings before I got her, she never allowed the male to copulate with her. (She was imprinted.) She never produced a fertile egg herself, but for many years produced infertile ones.

She was allowed to raise week-old eyasses taken (legally, with others remaining) from wild nests. From these experiences, I've watched close up everything that breeding red-tails do except copulation itself. At the time, in the early 70s, breeding of raptors in captivity was still rare and uncertain. Efforts such as mine, meager as they were, contributed to proper understandings of raptor reproduction and captive breeding, which resulted in the breeding of most raptors in captivity. The peregrine was essentially saved from American extinction by the release of captively-bred birds. Virtually all modern peregrines now breeding in the East and Midwest are progeny of captive breeding efforts. My role in all of that (with red-tails) was inconsequential, but those experiences (and many others in the field) allow me to offer accurate explanations of the red-tails in Central Park.
Sorry for running on.

In summary, copulation by itself is quite insufficient to cause the production of eggs. Just as with my un-mated red-tail that built her nest each year and laid eggs, the prompts for doing these things are very closely tied to expanding daylight periods, which prompt the pituitary gland to start dripping a complex recipe of sex hormones that cause the uterus to begin ovulation. By itself, the fling thing isn't enough
(And watch. If Lola sits on her nest and starts laying eggs in June, I'll have to . . .? This pair has made me the fool several times before. I'm knocking on wood as I write this.)

And Mai, I didn't well explain the problems with late-season hunting. You are quite correct in asserting that Central Park has a year-round abundance of red-tail goodies. The problem is this. Even though there are zillions of pigeons, a few diurnal (day-active) rats, and always some squirrels, by August there are not many newly-hatched or young of any of these species still wondering around oblivious to predators. The majority of the vertebrates cavorting in the various natural precincts of Central Park in August have learned to become hard to capture. The easy pickings have been picked. There will be lots of animals to eat in August, but they won't be as easy to catch as they would have been in June, when many were young and naive to the ways of avian predators.

Secondly, the days are shortening in August and by late September, there are only about 12 hours of daylight in which to hunt. A newly-fledged eyass in June has about 16 hours each day to learn her lethal craft -- lessons that can't be learned with just a few captures, either. The young hawk out on its own needs to learn multiple ways to capture food, and this comes only by repeated practice. These essential hunting lessons have to be well-learned by the time nights start getting long and cold. A young red-tail that’s not an adept hunter by October is a goner. Capturing pigeons, rats, and squirrels is not easy. The craft must be learned to perfection. A shortened hunting season will make it difficult for the eyasses to be able to capture enough food during the autumnal migration, where they will have to spend much of their time flying, not casually sitting around and waiting for vulnerable prey to present itself. In migration, the hawks better have a high rate of hunting success. Otherwise, they become members of the 60-80% that normally die in their first year. It’s tough enough for the eyasses that have an entire summer to hone their killing skills. For a July-fledged hawk, there is little chance. I wouldn't put any of my money on a July-fledged red-tail trifecta.

Lastly, in reference to our pair’s wonderful adaptations to Central Park, Mai writes, “So, would it really be so strange if they defied even this aspect of "normal" hawk life -- and had a late family??" From a strict biological perspective, it would be very strange. But I'm knocking on wood once again. These birds have defied biology several times.

Sincerely,
John A. Blakeman