Blakeman shoots down theory and proposes new one

Cathedral Dad on Angel Gabriel
Photo by Cal Vornberger
www.calvorn.com

Marie,

Chris Lyon's conjectures on the recent appearance and helpfully-attractive function of the orange-breasted red-tail eyasses are interesting. This is good biological thinking, the start of all good scientific explanations. I'm glad he put forth his thoughts, prompting others to offer theirs.

And of course, here are mine.

First, I have to agree with Chris in thinking that this trait is more common (at least in the East) than I first thought. I've never seen one of these out here in Ohio, but a good falconer friend says that he has seen one this year on a local nest.

The trait is now obviously genetic, not diet-produced. And where present, it may be a dominant trait.

Chris's suggestion that this brighter breast plumage might attract adults to fly over and feed the new fledglings is reasonable. But my experience is that parents have very little difficulty finding newly-fledged eyasses to feed. When young red-tails leave the nest, they are almost always hungry. Therefore, they are prompted to fly after food-providing adults whenever and wherever they are seen.

At the start, before eyass flying skills are well developed, the young birds do have a tendency to sit on branches and vocalize for food. Because they can't fly (or land) very well, these inexperienced birds often get parked in among leafy branches where, indeed, they can be hard to find. A brighter breast would help disclose their location, for sure.

But so would the typical white breast, against the leafy green of the arboreal vegetation. Parents really have no difficulty in finding their young because when hungry, the new yearlings make a lot of noise. It's really the other way around. The question is not how do parents find their wondering offspring. It's how do the young hawks find their parents. The eyasses make a racket to attract their food-carrying parents, and they also flap their wings or fly after them.

By late June, the yearlings are fully on the wing and they tend to chase after the parents wherever they go. When parents retrieve food for the yearlings, they must be careful not to be physically attacked by their offspring. Consequently, parents usually try to remain distant from the young hawks, dropping food away from them.. The yearlings who follow their parents to hunting perches distract the hunting efforts and successes of the parents, so adults often try to fly off without their kids in tow or pursuit. For the first half of the summer, the hawk fledglings simply complicate the parental duties of the adults.

Sooner or later (usually later), the yearlings will begin to hunt on their own. But that's usually after Mom and Pop begin to discontinue daily food provisions. The adults appear to finally get so pestered by the begging calls and thieving pursuit flights of their young that they just fly off and stop feeding them altogether. Often, they turn the tide and fly at the young, driving them to the periphery of the territory. Reluctantly, the yearlings begin to understand that they will have to find and capture their own food. The cutting of raptor apron strings is not an elegant process. It is often downright uncivil, even moderately violent.

So in summary, I don't believe there is any real selective advantage with the orange breasts in allowing more successful feeding of new fledglings. Yearlings could be virtually invisible to their parents, but would still be found as the young fly eagerly to the discovered parents. It's not how do the adults find their young. It's how do the young find their food-providing parents.

This raises another plumage-related question never asked here, I believe. What function does the red-tail's red tail serve? It would be reasonable to presume that it allows identification of adults. First year birds don't have a red tail. All adults do. Therefore, it keeps the immatures out of the mating (pair bonding, not copulation) process.

But there are two complications with this explanation. First, there have been a number of cases where immature red-tails have pair bonded with adults at nests. I'm not certain if these have always produced eyasses, if immatures are capable of inseminating females or producing eggs. Nonetheless, it's clear that the red tail is not an absolute requirement for pair bonding to occur.

Secondly, I studied a pair of Ohio red-tails that produced a single, normal eyass. The father was a quite typical male. But the mother was an almost purely white "leukistic" or albino, without a red-tail. She had a few minor streaks of red in her tail, but in color she didn't look like a red-tailed hawk to anyone, hawk or human. Obviously, this pair formed and produced an eyass without any behavioral prompting of the red tail.

There are a number of buteo hawks (red-tails are in the genus Buteo) with red tails in the world. There is a species in South America that is very, very similar to the North American bird that we know and love so well. In Africa, there are at least two red-tailed buteo species. Why, then, the red tails? Who knows.

But they sure are beautiful, aren't they? Just how exciting would Pale Male be if his tail were a mere brown? Do red-tail females have an elevated sense of beauty and therefore tend to mate (and yes, copulate) with only the most elegant, best-colored males? Is the red tail just a raptorial form of high fashion and aesthetics? Are the aesthetic skills of female red-tails the ultimate source of our delight with the plumage of these fine birds?

--John Blakeman