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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
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)."