Sexing and ageing the purple swamphen Porphyrio porphyrio by plumage and biometry
Published: Volume 63.2, September 2016. Pages 261-277.
Many bird families, including the Rallidae, are characterised by a lack of plumage sexual dimorphism and reduced sexual size dimorphism. In such cases, biometry may still allow the sex of captured birds to be determined. We investigated this possibility in the purple swamphen, a species for which biometric and moult data from southern Europe are scarce. We studied and measured a large sample of wild birds in order: 1) to assess the extent of sexual size dimorphism in adult and immature birds; 2) to determine the period during which plumage characteristics can be reliably used for ageing; and 3) to develop a discriminant function that allows purple swamphens to be sexed using a set of morphometric measurements. Ten biometric traits were measured for 421 wild birds that were also sexed molecularly. We used body and wing photographs from 425 and 232 birds, respectively, in order to classify bird age (adult versus immature, based on the evidence of immature plumage). For most measurements the overall size ranking was as follows: adult males > immature males > adult females > immature females. However, this ranking did not apply to culmen and shield width, because these were bigger in adult females than in immature males. Immature birds moulted gradually during the first winter. The best ageing criterion for immature birds was the presence of median underwing-coverts with whitish tips before the first complete summer moult (second-year birds). This extends the ageing period by another six months relative to prior knowledge. The discriminant analyses using six different biometric traits correctly assigned sex in 88% and 95% of immature and adult birds, respectively. Using only two variables (culmen and body mass) reduced correct sexing to 80%-92%. These equations are a simple and inexpensive way to sex purple swamphens of known or unknown age. Although some caution is necessary, these equations can be useful for sexing birds from other southern European populations.