Sexual Dimorphism in the Lekking Great Snipe

Abstract

Many lek-breeding bird species exhibit extreme sexual size dimorphism and plumage dichromatism which has been taken as evidence of a relationship between the mating system (lek) and sexual selection. We find that lekking Great Snipe Gallinago media exhibit no or reversed dimorphism. Data from two different breeding populations and from museum skins show that females are significantly larger than males for body mass and length of tarsus, wing, bill and bill plus head. By using bill length data from museum skins 79.3% of all specimens were classified to the right sex; data for breeding birds separated the sexes even more accurately (99.4%). One character, the amount of white on the tail feathers, differs between the sexes in that males both have more tail feathers with > 50% white and larger areas which are completely white. Discriminant function analyses, based on both morphological measurements and tail coloration of the breeding birds, separated the sexes. Only 1 of 252 birds was classified to the wrong sex by these analyses. We suggest that constraints on sexual selection set by natural selection, phylogenetic effects and sexual selection working on traits other than the ones under study (e.g. acoustic cues) may explain why Great Snipe do not show more striking differences in size and plumage characters.