ANOTHER THEORY ON REDTAIL FEMALE SIZE SHOT DOWN [gently]
How about this thought? With most species, when a
predator appears, the alert is given, and the birds
flock together to mob the predator. This doesn't
happen with Raptors. Therefore, particularly where
territory is larger than in the case of our city
hawks, the female is the only defense against the
predator until the male can return from hunting and
help defend the nest.
This may be the best explanation yet. Certainly seems plausible.
But as always, I have to play devil's advocate and see why this possibly wouldn't work, or might be an unlikely explanation.
When I stop to consider the predatory threats toward red-tail eggs and eyasses, only a few are prominent. Raccoons are a major concern. Many a nest has been cleaned out by a raiding raccoon the night after a biologist has climbed into a tree to band the young. Modern raccoons have learned to follow human scent through the forest, and almost inevitably it will lead to some sort of food (humans throw out a lot of raccoon-edible things).
Consequently, when I and my partners banded eyasses, or ever climbed into a nest, we always sprinkled mothball crystals at the base of the tree to disguise the scent trail we left. We never had a single raccoon problem. Others did, tainting their research results.
A male red-tail, in the eyes of a marauding raccoon, is essentially the same size as a female, especially at night when raccoons are on the prowl.
The other nest predator is the great-horned owl, and these brutes are so large and powerful that neither a male nor female red-tail would be able to dissuade such a bird from attacking either a parent or the eyasses. But of course, because great-horned owls are quite dependant on the availability of old red-tail nests upon which to raise their owlets, great-horns have been selected against outright red-tail predation. So big owls aren't a real concern, either.
That's just about it. I don't think that the slight (in this case) size difference between male and female adult red-tails offers much benefit in nest defense. The best defense is to get a nest way up there where it's difficult to get to, and red-tails do that well.
Very good thinking. Very plausible -- until the theory is measured against likely nest predators, and I think for our species they are only great-horned owls and raccoons, where the female's larger size is insignificant against either of these much larger predators.
But perhaps in the field (not the mind) this is the answer. Whatever it is, there has been natural selection for female incubation in all raptors. I've only pondered the red-tailed hawk. Perhaps size-dependent nest defense is a more significant factor in other species.
Again, very good thinking.