Pre-fledging speculation about the Trump Parc nestlings: Blakeman on sexual dimorphism

Photo by Lincoln Karim
July 5, 2005

Boy or Girl? Blakeman asks

7/7/05
Marie,

It's time now to start speculating on the sexes of the two Trump Parc eyasses. Of course, for a while, it will be only speculation, until the birds are ready to leave the nest, to fledge. But an interesting developmental observation has been made by biologists and falconers who closely watch eyasses on nests.

As many here know, there is a marked sexual dimorphism between the sexes in virtually all raptors. When fledged, females are larger than males. When the birds are about to fledge, it should be easy to "sex" the individuals (to determine the sex, in biological parlance). If there is a noted size difference, the larger bird will be a female and the smaller a male. If both birds are the same size, they will have to be sexed in comparison to the size of the parents.

But even that is not always so successful. Red-tails in their first-year plumage are actually dimensionally larger than adults. They look bigger because they are. Their flight muscles aren't fully developed when they leave the nest, so they require slightly longer wing and tail feathers to easily hold them in the sky. Consequently, first-year red-tails appear larger than adults. By weight, they are not as large, but their tails can be an inch or more longer than adults, also with longer wing feathers. A female fledgling can appear to be a giant, larger than either parent. But under her feathers, she's a relative weakling.

Our eyasses in the nest, however, can present a curious contrast. Although females in the end are markedly larger than tiercels (males), it's been observed that tiercels in the middle third of nest occupation can be actually larger than a sister hen eyass. Consequently, if a size dimorphism can be observed in the next week or two, it's not likely to hold till fledging. At this stage, tiercel eyasses tend to spurt ahead in size, causing the unaware to label them as hens. But the females soon catch up and overtake the size of the eventually smaller tiercels.

Therefore, watchers of the Trump Parc eyasses should try to keep some track of the differential growth patterns of the birds, should they be of different sexes. Soon, with emerging flight feathers, astute observers should be able to discover minute feather patterns that identify each bird. This will be helpful for dentification. Mention was made, I believe, of a patch of white feathers on the back of the heads of the eyasses. This is a well known eyass feather pattern. Later, dark feathers will be seen emerging here. This curious feather pattern seems to be deeply ingrained in the genetics of red-tails and other related hawks and eagles. I once trapped and studied an adult female who was "leucistic" or albinistic, a nearly all white partial albino red-tail. Most of her body feathers were pure white, but she retained dark pigmentation in this back-of-the-head patch.

Lastly, I wouldn't be surprised if fledging actually deviates from the published, understood common time ranges for such. Remember, this is New York City, Central Park in particular, and everything here related to red-tails is different from the events, forces, and processes acting on typical rural and wild red-tails. So don't be alarmed if the birds fledge earlier or later. We are dealing with brand new red-tailed hawk biology that so far, is academically undescribed. All of this is leading edge, unknown raptor biology. Others, appropriately, have been excited by the discovery of remnant ivory-billed woodpeckers, a species formerly thought extinct. That story is about an old species being rediscovered. But in the heart of Manhattan we have an old species doing utterly new and unexpected biological things -- a story that for me prompts exactly the same ornithological excitement that the rediscovery of the ivory-billed woodpecker did for others.

Sincerely,

John A. Blakeman