GPS-Tracking of Great Cormorants Phalacrocorax carbo sinensis Reveals Sex-Specific Differences in Foraging Behaviour

Great Cormorants Phalacrocorax carbo sinensis are considered generalist predators feeding in a variety of marine and freshwater habitats on many different prey species. Recently however, there is increasing evidence that even seemingly generalist species can often be regarded as a diverse group of individual specialists sometimes induced by sex-specific foraging behaviour. To test whether this also holds true for Great Cormorants, the present study characterised the at-sea distribution and examined the time budgets of provisioning Great Cormorants by deploying GPS-loggers on 11 nesting males and females during the breeding season of 2012. Additionally, pellets were collected from underneath the nests of these tagged birds for detailed diet analysis, based on the otoliths and fish bones recorded within them. Diverse foraging patterns were recorded, ranging from exclusively freshwater feeding, to offshore foraging, with the furthest recorded distance from the colony being 28 km at sea. Time budgets during foraging trips revealed relatively long periods of resting ashore (55% of the trip ± 23%), followed by foraging (31 ± 18%), and time spent in flight (15 ± 11%). Flatfishes dominated in the diets of these Great Cormorants, indicating benthic-foraging strategies when foraging in saltwater (feeding on discards could be ruled out). The males in this study avoided freshwater foraging habitat and preferred offshore waters with a sandy bottom or nearshore waters alongside rocky breakwaters and within harbours. Females foraged in all habitat types, but proportionally more in inland freshwater bodies. Females spent more time on foraging than did males, particularly in marine habitats, whilst males rested more. It is proposed that these differences might have been caused by sex-specific foraging strategies, possibly resulting from competition at foraging sites.