Sexing and Ageing the Purple Swamphen Porphyrio porphyrio porphyrio by Plumage and Biometry

Abstract

Summary. 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...