Tuesday, June 27, 2006

Chris Lyons: It's not just a Pale Male thing

Fifth Avenue Nestlings - May 28, 2003
Photo by Lincoln Karim

Chris Lyons continues his explorations of the Orange-chest mystery:

I broadened my search, and guess what--not only is it not just a Pale Male thing--not only is it not just a New York City Red-Tail thing--not only is it not just a northeastern Red-Tail thing--it's not even just a Red-Tail thing.
Check out the 2004 slideshow on this hawkcam site--pay special attention to the images from just before fledging--and compare with the parents--
A much better quality photo of a Ferruginous Hawk eyass in that pre-fledging stage of development--you can hit 'previous' and 'next' to see more photos of this species.
Very little is known about the juvenile plumage of Rough-Legged Hawks--the article at Birds of North America Online says much more study is needed. They nest far far north of here, usually on cliff faces, and so nest photos are hard to find online--but take a look--

Swainsons Hawks--I'm not sure--again, hard to find photos of birds at the right age--you'll see some photos of a juvenile who definitely seems to be getting some orangey color in various places--but adults frequently have quite a bit as well, though it's placed differently.

Photographs of White-Tailed Hawks at any age are hard to find, because they are extremely rare in the U.S. Maybe I'm just imagining that I see the beginnings of orangey chest color on these cute babies--

Broad-winged Hawk adults ALL have reddish-orangey color all over their underparts, unless they're dark-morphs. However, the juvenile plumage is much paler--except that we rarely see them right around the time they fledge. We usually see the juv broadies in fall migration--when they generally don't have the definite orangey color this one seems to show--

These are all buteos, close relatives of Red-Tails--the two most similar species in terms of general morphology and habits would probably be the Ferruginous and the Rough-Legged, which rival it in size, though not in versatility.

I'd hate to draw any generalizations about the young of these more specialized buteo species on the basis of so little evidence. But it's probably even more dangerous to generalize about a generalist.

This could be seen as evidence for my theory that the chest coloration is somehow related to the fledging period, when the eyasses are vying for the attention of their parents. Does it stimulate the parents come to them more often? Feed them more regularly? Or, as you suggested, does it mark them as recent fledges, and thus protect them from potential territorial aggression from the parent hawks, who will be strongly encouraging their youngsters to seek new horizons in the near future? There is NO evidence that the adults will fail to feed one of their young sufficiently, or drive it away too soon, if it happens to lack this coloration. It may simply be a minor elaboration on a whole battery of signals that have been evolved to make sure the parent/young relationship concludes advantageously to the survival of the species. However, there may be a price to be paid for this slight advantage over pale-chested brethren. Carotenoid feather pigments require an investment in bodily resources that could be used in other ways by the growing chicks. Like so many other things in life, it could be a trade-off.

Basically, the orangey chests seem to occur in buteo species where the young are otherwise pale-chested at the time of fledging. If they were dark-chested, the orangey color wouldn't stand out. But on an otherwise pale chest, the orangey color is very noticeable, and clearly conveys some benefit--otherwise it's hard to see how it could be found in such a variety of species. But I'm still not finding it in birds other than buteos, and it's probably not common to all buteos--I can't find any evidence of Common Buzzards getting it, for example--that species is very dark-chested.

The darker shades of brown and black we see on birds are melanin-derived, I believe--and much easier to manufacture. That book about House Finches I mentioned earlier ("A Red Bird in a Brown Bag") talks about this in detail--the carotenoid pigments are what give many birds red/orange colors, and they are harder to make, and very dependent on proper diet, as well as genetics. A male House Finch displaying a particularly brilliant reddish coloration on the head is informing potential mates that he is skilled at finding food (thus making him a good provider), and he may also be informing them of other valuable genetic traits. However, this doesn't stop males with less brilliant colors from breeding, and in some instances they may be more successful than the brighter males--evolution is a complicated process, and it's rare for a single advantageous trait to prevail over all others, particularly in a highly adaptable species such as the House Finch (or the Red-Tailed Hawk).

Some birds (like Ruby-Throated Hummingbirds) have special prismatic structures on their feathers that refract only certain wavelengths of light, thus creating the illusion of brilliant pigmentation where no pigments actually exist. In all cases, bright colors represent a significant investment of resources for any bird, whereas duller brown and black feathers are easier to grow, and are also more durable, which is why many birds such as Wood Storks have dark tips to otherwise pale wings.

Although my specific theory may prove to be incorrect, I can't see any alternative to the notion that the orangey chests have some evolutionary benefit. Otherwise they'd simply be a waste of energy. They clearly have no value at all in attracting a mate, because in most cases, they are gone well before a young hawk survives its first year of life. It must have something to do with their effect on the parents in the period around fledging.