Sexual differences in the foraging behaviour of parents have been observed in a number of sexually sizedimorphic birds, particularly seabirds, and the usual inference has been that these sex-specific differences are mediated primarily by differences in body size. To test this explanation, we compared the foraging behaviour of parents in a monomorphic seabird species, the northern gannet Morus bassanus. Using specially designed instruments and radio telemetry we found that individuals of both sexes were consistent in the directions and durations of their foraging trips. However, there were significant differences in the foraging behaviour of males and females. Female gannets were not only more selective than males in the areas where they foraged, but they also made longer, deeper dives and spent more time on the sea surface than males. As the sexes are morphologically similar in this species, then these differences are unlikely to have been mediated by body size. Our work highlights the need to investigate sexual differences in the foraging behaviour of seabirds and other species more closely, in order to test alternative theories that do not rely on differences in body size.

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To investigate how a generalist marine predator modifies its foraging behaviour by prey type, we attached time-depth-temperature recorders to chick-rearing thick-billed murres (n = 204) at Coats Island, Nunavut, Canada from 1999 to 2007. Predators varied their behavior along 3 major 'axes': foraging effort, prey depth and prey lifestyle (benthic/pelagic). Dive behaviours for different prey -fish doctor, squid, sandlance, amphipods, snakeblenny, daubed shanny, sandlance and Arctic shanny -were discriminated from one another in a discriminant analysis of dive variables and these prey were therefore considered 'specialist' prey items. Specifically, amphipods were captured during V-shaped dives near the colony with a slow bird descent rate, squid were captured during deep V-shaped dives in cold water and fish doctor were captured during a long series of U-shaped dives in relatively warm water far from the colony. Arctic shanny and snakeblenny tended to be taken at moderate distances from the colony, with snakeblenny taken at deeper depths. Daubed shanny captures showed a bimodal distribution, with some taken at shallow depths far from the colony and others at deep depths close to the colony. Dive behaviours for Arctic cod, capelin and sculpin overlapped both with each other and the behaviours for specialist prey items and, therefore, were classified as 'generalist' prey items. In general, V-shaped dives preceded deliveries of pelagic prey items and U-shaped dives preceded deliveries of benthic prey items. Our results strongly suggest that generalist marine predators use stereotypic behaviour to forage for prey items, based on previous knowledge about what locations/strategies maximized intake for a given prey type.

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We studied the diving behavior and diet of the sexually size-dimorphic blue-footed booby Sula nebouxii (BFB) breeding on Isla Lobos de Tierra, Peru, to evaluate whether sexual dimorphism is associated with feeding niche divergence between the sexes. We also discuss the foraging behavior of BFBs in relation to what is known about the vertical distribution of their main prey, and compare the BFB's foraging behavior with that of other tropical and temperate Sulidae. After analyzing data on 23 females and 28 males, we found no sexual differences in diet composition, crop mass, trip length, number of trips per day, proportion of time in flight and on the water, and number of dives per hour. We argue that this is because males and females were foraging in the same areas close to their colony. Conversely, females dived significantly deeper and longer, and consumed larger prey than males indicating that segregation may occur underwater. Body size (regardless of sex) was positively correlated to dive depth and prey size, suggesting that feeding niche separation is mediated by size dimorphism. However, the effects of body size and sex on foraging behavior could not be separated. BFBs and larger temperate gannets attained similar depths, but BFBs dived much deeper than did tropical boobies. The Peruvian anchovy Engraulis ringens was the most important prey consumed by BFBs, accounting over 79 to 97% of the total mass of the regurgitations. The temporal distribution of dives and dive profiles seems to be linked to the behavior and vertical distribution of anchovies.