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.

Black-legged kittiwakes Rissa tridactyla on the Isle of May, southeast Scotland, feed predominantly on the lesser sandeel Ammodytes marinus, an abundant, pelagic fish that is currently the subject of the largest fishery in the North Sea. The population of black-legged kittiwakes on the Isle of May is declining, and the fishery has been implicated. In order to assess this concern, there is an urgent need to improve our understanding of the factors that affect black-legged kittiwake foraging behaviour. During 1999, we carried out a detailed study of the foraging strategies of black-legged kittiwakes using purpose-built activity loggers that allowed us to distinguish 4 key behaviours: travelling flight, foraging flight, presence on the sea surface and attendance at the nest. We used the data to model 2 key aspects of time allocation at sea: (1) the relationship between the travelling time and trip duration and (2) the ratio of time spent actively foraging to time of inactivity on the sea surface at the foraging grounds. We found that a broken-stick model with a flat asymptote was the best fit for the relationship between travelling time and trip duration. Using published flight speeds for this species, we calculate that breeding black-legged kittiwakes on the Isle of May had a maximum range of 73 ± 9 km from the colony. We speculate that this upper limit is dictated by the distribution of prey rather than any energetic constraint on flight costs: a large sand bank complex, known to have high concentrations of lesser sandeels, lies entirely within this range. There was no consistent pattern in the ratio of the active to inactive components of the foraging trip, suggesting that this species exhibits highly flexible foraging strategies at sea, probably reflecting the patchy and unpredictable distribution and availability of its prey. Our findings suggest that the birds are feeding on sandeels at the same time and in the same area as the operations of the sandeel fishery.

The North Water polynya is an area of open water that occurs year round between northwest Greenland and Canada. Oceanographic conditions differ between the western ('latent heat' polynya type) and extreme eastern ('sensible heat' polynya type) sectors of the polynya, and the effects of this variation on prey availability and foraging conditions for high trophic-level predators are unknown. Thick-billed murres Uria lomvia breed on both sides of the polynya, and we conducted inter-colony comparisons of their foraging ecology. We measured time allocation and foraging efforts of chick-rearing adults using electronic data-loggers which recorded dive profiles and flight activity. Murres on the western side of the North Water polynya foraged at relatively shallow depths and rarely (4.4% of dives) explored depths beyond 70 m. In contrast, murres on the eastern side searched for prey at > 70 m in 23% of all dives, and spent a greater share of each trip actively diving. The Canadian birds made foraging trips of longer duration than the Greenland murres, but they also spent more time 'resting' at the sea surface. Rest time at sea was apparently the only time buffer available for increasing foraging effort. The Greenland birds had little room for increasing foraging effort, while the Canadian birds had spare capacity for additional work. Maximum potential foraging ranges were equal at the 2 colonies (75% within 50 km), and the murres had access to approximately equally-sized areas of open sea. We estimate that the density of foraging birds at sea within the 50 km of colonies would be 6.5 times higher on the Canadian side due to the large breeding colony at Coburg Island. Intra-specific competition for food should increase with increasing colony size because the predators may reduce food resources within their foraging range. Nevertheless, the Canadian birds worked less than Greenland murres, indicating a relatively good food availability in the western part of the polynya. However, the higher foraging effort by Greenland murres paid off in a higher chick growth rate, so it remains unclear why the Canadian birds did not also make use of their spare capacity to increase foraging effort. Since high-level zooplankton/fish stock interactions are seldom part of oceanographic studies in the Arctic, seabird foraging behaviour and breeding ecology serve as two of the few indicators of possible local variation within the polynya ecosystem.

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