A comparative study of breeding traits in colonial birds

I examined the effect of an evolutionary transition to coloniality on sever al breeding traits in birds using 29 pairs of congeners that included one c olonial and one solitary breeding species. Males in colonial and solitary s pecies, as well as females, had similar body masses. The extent to which th e sexes differed in body mass and plumage characteristics was also similar in colonial and solitary species. Although sexual selection is often assume d to be stronger in colonial species, coloniality appears to be a weak forc e in the sexual diversification of body mass and plumage characteristics in birds. Colonial species produced heavier eggs and invested more in their o ffspring per unit of female body mass than their solitary counterpart. The results support the hypothesis that coloniality is associated with greater foraging efficiency, which allows colonial birds to increase parental effor t. Although offspring predation risk is often assumed to be smaller in colo nial species, a fact that could lead to the production of larger clutches, colonial and solitary species produced clutches of similar size. The result s thus provide little support for an effect of differential predation press ure on changes in clutch size in colonial species.