Using morphometrics to sex adult and juvenile Soras (Porzana carolina)

Determining the sex and age of individuals can be an essential element of
conservation management, wildlife monitoring, and demographic analysis.
For many members of the family Rallidae, distinguishing between males and
females is challenging, even when the bird is in the hand. The Sora
(<em>Porzana carolina</em>), a secretive rail
that occupies freshwater wetlands throughout the United States and Canada,
represents a species that is challenging to sex in the field. Morphometric
measurements can help sex birds of an array of species, including rails.
However, no comprehensive morphometric model has been fully validated for
sexing Soras. We used DNA analysis to confirm the sex of Soras captured in
the field and logistic regression models to determine which morphological
features were the best predictors of sex. Measurements from 108 Soras (31
hatch year females (HY-F), 29 hatch year males (HY-M), 22 after hatch year
females (AHY-F), and 26 after hatch year males (AHY-M)) were used to
create our logistic regression model. Color definition and connectivity of
the auricular patch to eye or nape was used as an additional
characteristic in adult birds. Our top-ranked model was further validated
using a sample of 72 individuals exhibiting intermediate traits that would
be particularly challenging to distinguish in the field. Our top
performing model incorporated culmen length and tarsometatarsus length as
the features most predictive of sex and had an overall accuracy of 85%. If
higher accuracy is desired, an inconclusive band, which eliminates birds
of low model score, i.e. scores indicative of inconclusive sex (below + or
- 1.2), can be used. The accuracy of remaining birds (75% of sample) will
be increased to 95%. Our model shows that simple measurements of culmen
and tarsometatarsus is useful in discriminating the sex of a large
percentage of live-caught Soras. This morphometric model will facilitate
further demographic studies of this species and may be useful in designing
morphometric studies of other species in the family Rallidae.