Sexual Dimorphism and Sex Determination of Juvenile White Wagtail Motacilla alba alba Linnaeus, 1758 (Ayes: Motacillidae) Based on Linear Measurements

摘要

The nominate subspecies of the white wagtail is a common taxon occurring in almost all of Europe. It shows no sexual dimorphism in juvenile and winter plumages and some dimorphism regarding size. The aim of the present study was to propose a method for sexing birds of this species using linear measurements. Through a discriminant function analysis, we identified a set of morphometric traits in juvenile birds (78 males and 120 females) captured in northern Poland during autumn migration. Birds were sexed molecularly. On average, males were larger than females in all measurements and the most sexually dimorphic trait was wing length with twice as high value of size dimorphism index than other measurements. The best discriminant function including wing length and the total head length allowed for correct sexing of 88% individuals (86% of males and 89% of females). The equation with wing length only was also given, as this parameter is the most common measurement taken in Passerines. This formula classified correctly 82% of the birds (81% of males and 84% of females). The two proposed equations should be used for sexing only juvenile individuals from the nominate subspecies, as biometrical differences have been reported among the white wagtail subspecies and age classes. This method could be useful not only in future research but also to sex birds already measured in past studies.