Are the big and beautiful less bold? Differences in avian fearfulness between the sexes in relation to body size and colour

Conspicuousness towards predators may influence escape behaviour (or ‘fearfulness’) among animals, with more conspicuous species initiating escape behaviour earlier. Among birds, for example, body size and colour may influence differences in escape behaviour between species, and possibly between the sexes of dimorphic species. We examined 19 bird species with varying degrees of body size and colour dimorphism (including individually marked and sexed monomorphic species), to examine whether these two potential measures of conspicuousness influence sex differences in flight-initiation distance (FID). Starting Distance (the distance at which an observer commenced approaching a bird, which is an artefact of investigator behaviour; SD) was not correlated with dimorphism, so we used phylogenetically controlled models which explored the correlation between dimorphism and FID. Modelling indicated that only sex differences in SD correlated with sex differences in FID in these birds, and that dimorphism in either plumage or body size does not apparently correlate with sex differences in FID. These results suggest that, among the 19 bird species investigated, apparent differences in the conspicuousness to predators between the sexes do not influence escape behaviour. This suggests that either conspicuousness to predators does not influence escape distances in these species, or that sex differences in conspicuousness were too subtle to result in variation in FIDs.