Friday, June 23, 2006

Chris Lyons returns to the mystery of the orangey chests

The Trump Parc nestlings - July 18, 2005
Photo by Bruce Yolton

We've gone a long way in our pursuit of answers to the mystery of the orange-chested nestlings. First we thought it was a genetic aberration marking Pale Male's offspring only. Then we wondered if it was something about the diet of local hawks -- some pigment found in rats and pigeons.Then it became clear that orange-chested redtail nestlings aren't all that unusual-- you can find them in redtail nests all over the country. Indeed, it began to appear that white-chested nestlings were the unusual ones! That brought us back to the genetic origins of the orange coloring. Only now we saw that it was not exclusive to Pale Male's genes; it was universal.

Chris Lyons, who has been fascinated by the subject of the orangey chests, returns to consider some new aspects of the dietary genesis of erythrism -- another, perhaps less clunky word we could use in the future in place of orangey-chestedness.

Chris writes:

While I had thought we'd dispensed entirely with the theory that the chest coloration is related to diet, I find myself wondering if that's really true. It certainly isn't related to the specific types of prey animal ingested by the young hawks. But it could be that the orangey color is too costly an investment for chicks being raised in areas where prey resources are scarce.

John Blakeman has many times reminded us that in areas he typically observes them, Red-Tails and other birds of prey struggle much harder than our urban raptors to find sufficient food to rear their young. It may be that this is one reason why the chest coloration is seen so often by urban and surburban observers of Red-Tail nests--because the proximity of large numbers of humans means the unusual abundance of suitable prey animals like pigeons, rats, and squirrels; meaning that all the chicks may get more than enough food to manufacture these marker pigments without compromising other aspects of their development. And it also means larger brood sizes, which means more competition for parental attention, which might make the chest coloration more useful.

However, I don't see any alternative to the conclusion that at least with some species of buteo, orangey-chested eyasses have been around for a very long time. We have no reason to think that they have become any more common than they were when the first European settlers arrived in America. Except in the sense that Red-Tailed Hawks themselves are more common than they were then, which is hardly the case for most other raptor species. More people are paying more attention in more places with better photographic equipment, and a more effective means of sharing their photos and observations. That's really the only explanation necessary.


I decided I'd better refresh my memory of the difference between carotenoid and melanin-based pigmentation. It's not simply more difficult for birds to synthesize carotenoid pigments, it's essentially impossible. The chemicals they use to make red, orange, or yellow-tinted feathers MUST be present in the food they eat, or they will not be able to make feathers of that coloration. Melanin-based pigments, (brown, black, gray) can be made from essentially any food a bird may eat, but carotenoid pigments must be ingested more or less whole to be incorporated into its feathers.
There's been a tremendous amount of research on this in ornithological circles, with scientists trying to figure out the significance of avian plumage (so beautiful and intricate in some birds that it cries out for explanation in Darwinian terms), and there's also been some speculation as to the different signals conveyed by carotenoid and melanin-based pigments.

This does not, of course, substantiate the theory that Red-Tailed Hawk eyasses only get the orangey chests if they are fed certain foods. This would make no sense, because virtually all Red-Tails develop--well--red tails. Those tails wouldn't be red if the birds weren't getting substantial amounts of carotenoids in their diet, regardless of whether they are eating rats, gray squirrels, ground squirrels, chipmunks, rabbits, pigeons, bats, snakes, etc. The Red-Tail diet is limitlessly variable, but tail color is not--the tail nearly always turns red, across the entire range of the species.
Young raptors eat essentially the same things their parents do. Therefore all Red-Tail eyasses get some carotenoids in their diet. It's possible the amount and quality of nutrition they receive from their parents may to some extent determine whether or not they get the orangey chests, or perhaps simply the extent and brightness of the coloration. I've certainly observed that some eyasses get much more orangey than others. However, there must also be a variant genetic factor, or else "Whitey" (the pale-chested Fordham chick) would have the same chest color as his siblings. If all Red-Tails were genetically inclined to develop orangey chests, then John Blakeman would have seen orangey-chested eyasses by now. By the same token, if the orangey-chested gene was rare, we wouldn't be seeing so many Red-Tail eyasses in so many different parts of the country developing orangey chests.