Sex-specific differences in space use of Forster's terns (Sterna forsteri) breeding in the South San Francisco Bay, California

In seabirds, sex-specific parental care during reproduction can lead to sex-specific differences in foraging location, behavior, and use of foraging areas. Sex-specific differences in location and use of foraging areas have been well documented among dimorphic species, however research on sexually monomorphic seabirds has been limited. I studied Forster's terns (Sterna forsteri), a monomorphic and socially monogamous seabird, in the South San Francisco Bay, California to examine sex-specific differences in the distance from the nest, the foraging range, and the home-range and core-area size of terns. Space use by Forster's terns did not differ between the sexes; however, all measures of space use varied significantly by breeding stage, and to some degree, by colony affiliation. Forster's terns were located farthest from the nest during pre-breeding and post-breeding stages, and found closest to the nest during incubation and chick-rearing stages. Home-range and core-area size significantly decreased as the breeding season progressed and was most concentrated in the post-breeding stage. The foraging range of Forster's terns differed significantly between colony sites. This is the first study of a monomorphic seabird that did not document sex-specific differences in space use, however, sex-specific differences may exist on a finer spatial scale than investigated in this study.