Wednesday, June 28, 2006


Divine Fledglings 28 Jun 2006

More from John Blakeman, prairie expert, falconer, and frequent contributer of Red-tail information to New York City's urban hawkwatchers.

(The paper mentioned below is Sex determination of Red-tailed Hawks (Buteo jamaicensis calurus) using DNA analysis and morphometrics, J Field Ornithology, Vol 77, Issue 1, pp. 74-79. And as most of you know Bruce Yolton is one of our local hawkwatchers. D.B.)


I believe I sent you an email in reference to a photo of the two Divine eyasses next to each other. I noted that the larger one on the left was decidedly a female, the other a male. No doubt in my mind.

Bruce Yolton has just posted a note that sexing red-tails is very hard, and he doesn't recommend it. He references a new article on the subject.

I've read the article, and in fact, it shows that in-hand measurement data of red-tails can accurately reveal the sex (as confirmed by PCR DNA analysis) at 97 and 98% accuracy rates. I don't know the basis of Bruce's caution.

In fact, for those of us who have trapped, handled, and trained literally hundreds of red-tails, sexing is seldom questionable. Males have a male "look," and females a female visage.

All of us falconers have trapped "in betweeners," birds that tend to look one way or the other, but aren't absolutely definitive. If you trap a 1500 gram red-tail, she's a female. A 1100 gram bird is always a male (unless the bird is emaciated from disease, but these birds are then re-fed and they gain back their lost weight). But a 1200 gram bird might be a tiny female, or a giant male. That's where those of us familiar with the species just look at the head to see if it looks like a female or male. Males tend to have narrower heads and smaller bills (minutely).

When there is a decided size difference as in the photo, there is no doubt about the birds' sexes. The size difference was big enough that anyone, even the inexperienced, could have noted it. Males tend to be narrower across the shoulders, as in the photo your description of the female being "chunkier" is perfect. They are just that.

For the experienced, sexing red-tails is not mysterious or erroneous. We are correct at least 97% of the time.

--John Blakeman


Kara said...

Just to let you know, you are correct on the sex 97% of the time if you are using the equations in the paper on the western subspecies. The paper only covers the western subspecies and it was my experience that an 1100 gram bird was usually a female. I have banded a fair number of red-tails and would agree with D. Bruce Yolton that it is difficult to sex a red-tail based purely on observation.

Kara Donohue

Donegal Browne said...


No doubt about it, that sexing Red-tails by pure observation isn't terribly reliable at all. Sometimes we do speculate for fun about the sex of particular birds though I by no means mistake it for fact. Without DNA or physical examination or knowing who-laid-the-egg, no sexing makes the "fact" grade.

Though there do seem to be a very few people like Jemima Parry-Jones of the National Birds of Prey Centre who last I heard was 100% on her sexing by observation once a hawk fledges. Her assistants have been "testing" her for years with "birds in the hand" that can then be sexed by other means to check her accuracy. There is also a rehabber in Iowa, who I understand is remarkable as well.