PREY DISTRIBUTION AND PATCHINESS: FACTORS IN FORAGING SUCCESS AND EFFICIENCY OF WANDERING ALBATROSSES
Résumé
Seabirds are considered to rely on prey that are patchily distributed and whose abundance differs greatly according to physical processes or water masses. However, very little information is available about the environmental factors and individual factors that affect the foraging success of marine predators. We studied the distribution of prey encounters, foraging success, and efficiency in different water masses and during two stages of the breeding season when energy requirements differed in Wandering Albatrosses (Diomedea exulans) of known experience and sex. The birds were simultaneously fitted with satellite transmitters, stomach temperature sensors, and activity recorders. Only 27% of prey were captured in patches (distance between two prey , 1 km); the rest were caught at an average distance of 64 km. Prey in patches were smaller than isolated prey and were caught mainly at night. Diet analysis indicated that albatrosses preyed mainly upon adult squid: squid taken at night were smaller (bioluminescent species), whereas those taken during the day were larger (nonbioluminescent species). Birds spent more time foraging farther from the colony during incubation than during brooding, when their energy requirement was highest, but foraging effort (landings per hour, or kilometers per hour), foraging success (grams of prey per hour), and efficiency (grams of prey per landing per hour) were similar during the two stages. The rate of prey encounter and foraging efficiency did not differ between water masses or between oceanic and shelf-slope waters. We found no differences between the sexes in terms of foraging success and efficiency. Young, inexperienced individuals had foraging success and efficiency similar to those of older, experienced birds but had a different strategy: they foraged more actively and caught more prey at night than did experienced birds. These results suggest strongly that Wandering Albatrosses rely on prey that are highly dispersed, catch few prey within the same patch, and do not adjust foraging effort according to energy requirements. The unpredictability of the location of prey and the type of prey caught by Wandering Albatrosses indicate that the species has a unique foraging strategy compared to most seabirds, which generally concentrate in more predictable foraging areas.