Flexible foraging behaviour of a sexually dimorphic seabird: large males do not always dive deep

Abstract

Sex differences in foraging behaviour have been explained by size dimorphism and/or avoidance of inter-sexual competition for depletable resources. To distinguish between these 2 hypotheses, we examined how intrinsic factors (sex-related differences) and extrinsic factors (year differences) shape the foraging behaviour of size-dimorphic imperial shags Phalacrocorax atriceps albiventer breeding at New Island, Falkland Islands/Islas Malvinas. We deployed time-depth and compass loggers to male and female imperial shags over 3 consecutive chick-feeding seasons. Males and females partly overlapped in coastal foraging areas, which were used mainly for benthic diving. Males additionally used offshore areas over deep water for shallow pelagic diving, suggesting that spatial segregation is involved in the avoidance of inter-sexual competition for food. Stable isotope data suggested differences in prey composition between the sexes, with consistently higher trophic levels in males, as expected for their larger size. Males were 27% heavier than females and reached greater maximum dive depths (98.9 ± 5.3 m) than females (54.1 ± 2.9 m). However, contrary to predictions based on body size dimorphism, the median dive depths of males were similar to those of females. While females used mainly benthic diving, males were more flexible in their benthic and pelagic foraging behaviour. Females also carried out more dives per day in all years, and deeper and longer dives than males in one year. As dive parameters differed strongly among the years, our results suggest that body size dimorphism and the avoidance of inter-sexual competition for food are involved in the evolution of sex-related differences in foraging in this species.