SEEN - SE European Bird Migration Network
Morphological variation between sexes in Robins (Erithacus rubecula) migration through the Polish Baltic coast

Katarzyna Rosińska

Bird Migration Research Station, University of Gdańsk, Poland

The aim of this study was to determine morphological differentiation between sexes in Robins migrating through the Polish Baltic coast. Nearly 450 dead birds were collected during spring or autumn migration in 1963–2004. Dead Robins were measured (wing- and tail-length and wing formula) and additionally in 2002–2004, leg colour and amount of greyness on head and flanks (both using a four-score scale) were determined. After the measurements were taken, individuals were sexed by dissection.
Birds were divided into four sex-age classes: immature females, immature males, adult females and adult males. Wing-length, tail length, index of asymmetry (E’) and pointedness (L) were compared among these classes using t-Student test. G-test was used to compare leg colour and amount of greyness on head and flanks between the sexes for immature Robins.
Obtained results indicate that sexes differ in wing- and tail-length and amount of grey colour on head and flanks. Males have longer wings and tails and bigger amount of greyness than females. The overlap of females and males is rather big i.e. in the case of wing-length 69–75 mm in immature and 71–75 mm in adult birds and in the case of tail-length 55–67 mm in immature and 57–64 mm in adult birds.
The Robin is a sexually monochromatic species, therefore biometrical differences between males and females could be helpful in sexing. According to the criteria proposed by J. Pettersson referring to wing-length 60% of Robins could be sexed, but only 29% of birds analysed here could be sexed when applying this criterion. These differences may result from varied composition of populations coming from different breeding areas.

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