Patterns of sexual size dimorphism in seabirds of the Southern Hemisphere

A review of published data on 99 populations (60 species) of seabirds shows that the degree of sexual size dimorphism varies considerably among species, with males larger than females in some species but smaller in others. Males tend to be larger, relative to females, in populations (1) that have large average body mass, and (2) that feed in areas of the ocean (especially, high latitudes) where primary productivity is high. These correlations are not due to phylogenetic conservatism, because the relationships hold even when the data are analysed cladistically. These patterns are similar in some respects to those seen in other avian lineages, but their adaptive significance remains unclear.