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Related to this question where the difference in size might possibly be due to one of the 3 peregrine falcon chicks being male, how do they tell whether a peregrine chick is male or female?

Eventually, it's possible due to size difference but at this stage how do they tell?

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    Google finds various papers on a combination of morphology measurements being useful, but some of the measures seem to require handling. (Measures such as beak length, various wing and leg bones, and others, taken in combination and compared with a model based on a large data set.) – Jon Custer Jun 6 at 2:32
  • For the falcon in the linked question, I believe when the state DNR came by to do their check-ups, they collected blood and a feather or two for later lab analyses. This on the basis of local TV news reports. In general, some birds can be sexed by inspecting the cloaca, but I do not know if falcons are among those, nor if that was done in this case. – cobaltduck Jun 6 at 11:45
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According to A morphological model for sexing nestling peregrine falcons ( Falco peregrinus macropus ) verified through genetic analysis published by ResearchGate, they input various features of the size, color patterns, and shape of the nestling into a model, which then spits out the answer to 96% accuracy or better, as verified by genetic testing. I have not been able to find out yet which specific features go into the model.

Adult peregrine falcons (Falco peregrinus macropus) have monotypic plumage and display strong reversed sexual dimorphism, with females significantly larger than males. Reversed sexual dimorphism is measurable among nestlings in the latter stages of their development and can therefore be used to differentiate between sexes. In the early stages of development, however, nestlings cannot be sexed with any degree of certainty because morphological differentiation between the sexes is not well developed. During this study we developed a model for sexing younger nestlings based on genetic analysis and morphometric data collected as part of a long-term banding study of this species. A discriminant function model based on morphological characteristics was developed for determining the sex of nestlings (n = 150) in the field and was shown to be 96.0% accurate. This predictive model was further tested against an independent morphometric dataset taken from a second group of nestlings (n = 131). The model correctly allocated sex to 96.2% of this second group of nestlings. Sex can reliably be determined (98.6% accurate) for nestlings that have a wing length of at least 9 cm using this model. Application of this model, therefore, allows the banding of younger nestlings and, as such, significantly increases the period of time over which banding can occur. Another important implication of this model is that by banding nestlings earlier, they are less likely to jump from the nest, therefore reducing the risk of injury to both the brood and the bander

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This answer expands on the answer above given by ab2, and is in response to her comment that she could not find the factors that are measured for the morphological model. The five factors are indeed given in the article she linked to.

"The five features measured were; body mass, wing chord length, tip-cere length, tarsus length and head-bill length."

And then later...

"These variables were selected because they are frequently measured by field researchers (e.g. Olendorf 1972; Arroyo et al. 2000; Balbontin et al. 2001) and are likely to differ between sexes (Baker-Gabb 1984; Olsen 1995)."

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    Thank you dthuff for finding what I did not find. You read that article more carefully than I did, and provided a useful answer to the question. – ab2 ReinstateMonicaNow Jun 10 at 23:46

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