Sex-specific foraging behaviour in northern gannets Morus bassanus: incidence and implications

Sexual segregation in foraging and migratory behaviour is widespread among sexually dimorphic marine vertebrates. It has also been described for a number of monomorphic species, yet the underlying mechanisms are poorly understood. We examined variation among years, seasons and age-classes in sex-specific foraging and over-wintering behaviour in the northern gannet Morus bassanus, a species with slight sexual dimorphism. Our results revealed consistent sexual differences in the stable isotope ratios of breeding birds: over 3 different breeding periods, adult females consistently consumed prey with significantly lower delta C-13 and delta N-15 values than adult males. Additionally, GPS tracking data showed that breeding females foraged further offshore than breeding males (a result consistent with the delta C-13 data), and the home ranges of the 2 sexes were distinct. Analyses of stable isotope ratios using a Bayesian mixing model (SIAR) revealed that breeding males consumed a higher proportion of fishery discards than females. Analysis of stable isotope ratios in red blood cells of immature gannets (aged 2 to 4) indicated that sexual segregation was not present in this age-class. Although sample sizes were small and statistical power correspondingly low, analysis of geolocator data and of stable isotope ratios in winter-grown flight feathers revealed no clear evidence of sexual segregation during the non-breeding period. Together these results provide detailed insight into sex-specific behaviour in gannets throughout the annual cycle, and although the mechanisms remain unclear they are unlikely to be explained by slight differences in size.