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Breeding season‐specific sex diagnostics in the monomorphic House Martin Delichon urbicum

Capsule Breeding adult females show a bare brood patch and a smaller cloacal protuberance than males. Aims To examine the degree of sexual dimorphism in various phenotypic traits in House Martins. Methods In the summers of 2005–07 in the northern Netherlands, 160 House Martins were captured. We measured wing length, keel length and body mass. The 115 adults were examined for the presence or absence of a bare brood patch, the colour of the belly plumage (greyish or pure white) and the diameter of the cloacal protuberance. A small blood sample was taken from the brachial vein and sex was assigned on the basis of a standard molecular assay on DNA from blood. Results With regard to wing length and body mass, House Martins are perfectly sexually monomorphic. However, for keel length females are slightly, but significantly, smaller than males. The young of the year are similarly sized but lighter than the adults. Adult females have a greater tendency than males to show greyish rather than pure white belly plumage. In the breeding season, the most reliable diagnostic for females is the unfeathered brood patch over belly and breast, and the small diameter of the cloacal protuberance. Adult females may begin to regrow the brood patch feathers in August, before southward migration. Among the 55 adult males, we identified two with clear bare brood patches, suggesting an emancipation of their role during incubation. Conclusion Although temporary morphological traits related to reproduction (brood patch, cloacal protuberance) make it possible to assign sex with some confidence, even in this sexually monomorphic passerine, reliable molecular sexing assays on the basis of small blood samples remain necessary to accurately describe and analyse interesting mating system variations, including an apparent emancipatory role in some males.