Marine Ecology Progress Series 268: 265-279 (2004)
Offshore diplomacy, or how seabirds mitigate intra-specific competition: A case study based on GPS tracking of cape gannets from neighbouring colonies
Gremillet D, Dell'Omo G, Ryan PG, Peters G, Ropert-Coudert Y, Weeks SJ
Most seabirds live offshore lives, only touching ground to breed in dense colonies. Due mainly to social attraction these aggregations can number several millions of individuals. Intense intra-specific competition for food occurs around breeding colonies, and potentially between breeding locations. In theory seabird colonies tend to be spaced so as to minimize inter-colony competition. In practice, limited availability of breeding habitats does not always allow for such optimisation. We studied the foraging behaviour of the cape gannet Morus capensis, a seabird endemic to southern Africa, at two neighbouring colonies of different size (Malgas, 70,000 pairs and Bird Island - Lambert's Bay, 17,000 pairs) using GPS- and time-depth-recorders. In theory both colonies should have widely overlapping foraging areas, with comparable foraging characteristics for most individuals. We equipped 91 breeding cape gannets with data loggers in December 2002. Birds from both locations showed marked differences in their foraging patterns. Total foraging areas were of different size (29,500 km2 versus 22,300 km2), and only overlapped by 12% - 15%. Birds from the larger colony exploited a larger marine area, and foraged more intensively. Their foraging trips lasted longer (22.6 hrs vs 8.5 hrs), involving longer total flight time (7.8 hrs vs 5.9 hrs), longer foraging path length (293 km vs 228 km), and greater maximum distance from the breeding site (104 km / 67 km). Birds from the larger colony also travelled faster (50 km h-1 vs 44 km h-1), and had a larger number of foraging locations during each trip (252 vs 121), with more sinuous foraging paths (1.4 vs 1.1). However, there were no significant differences in the number of dives per foraging trip (67), the average maximum depth attained (3.5 m), nor the average or total dive duration per foraging trip (4.3 sec and 5 min, respectively). We conclude that, contrary to predictions, gannets from the two neighbouring colonies are spatially segregated and experience different foraging conditions. We propose a series of factors which might explain this phenomenon. These include patterns of marine productivity, abiotic parameters such as wind speed and direction, as well as behavioural patterns linked to group dynamics. We speculate that wind patterns and group feeding could generate foraging asymmetries. Foraging site fidelity and memory effects could consolidate these asymmetries, and generate 'cultural' foraging patterns. Our case study, based on GPS tracking, highlights the great potential of this new technology for the study of apex predators and the understanding of marine food chains.
BD04-03
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