I've been watching the Duke Farms Eagle Cam recently, which shows a pair of adult eagles alternately taking care of three recently hatched eaglets. Today, I wondered whether I could tell the adult eagles apart. https://www.dukefarms.org/eagle-cam/
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The answer is from this 1984 paper in The Journal of Wildlife Management, by the aforementioned Bortolotti. He examined 135 bald eagles in museums to come up with the formula in question. https://doi.org/10.2307/3808454
It's a good study in the problems we have to confront for any quantitative analysis. Many possible measures were considered, including length of exposed culmen, bill depth, hallux claw length, bill width, tarsus width, unflattened wing chord, and more. A PCA was used identify relevant factors.
Bortolotti devised a simple predictive model to discriminate between sexes using 2 criteria. It achieved 98.1% accuracy in his sample, and it showed validity via extension to 4 live (captive) eagles which weren't used to derive the formula – demonstrating the importance of out-of-sample testing.
He also mentioned that he could improve model accuracy still further by adding another factor, "but a third dimension introduced unnecessary complexity into the analysis". A good example of penalizing for additional complexity.
Of course, none of this is useful to me, a non-biologist watching an eagle cam wondering whether I'm looking at the mom eagle or the dad eagle. After all, I can't measure their hallux claw length or bill depth. But that won't stop me from enjoying the heartwarming family scenes.
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Sex = (bill depth * 0.392) + (hallux length * 0.340) - 27.694
Sex >0 = Female; <0 = Male
Naturally, I wondered where these values came from.
https://journeynorth.org/tm/eagle/MaleVFemaleMH.html