Offshore diplomacy, or how seabirds mitigate intra-specific competition: a case study based on GPS tracking of Cape gannets from neighbouring colonies

How do seabirds deal with intra-specific competition for food? We addressed this question in a study of the foraging behaviour of 91 Cape gannets Morus capensis from 2 South African colonies, situated 110 km apart, using GPS and time-depth recorders. Theoretically birds should have widely overlapping foraging areas and comparable foraging characteristics. Surprisingly, the foraging areas only overlapped by 13 and 14%, and birds from the 2 colonies also showed marked differences in their foraging patterns. Birds from the larger colony foraged more intensively; their foraging trips lasted longer (22.6 vs 8.5 h), involving longer total flight time (7.8 vs 5.9 h), longer foraging path length (293 vs 228 km), and greater maximum distance from the breeding site (104 vs 67 km). They also travelled faster (50 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 (68 vs 66), the average maximum depth attained (3.4 vs 3.6 m), nor the average or total dive duration per foraging trip (4.3 vs 4.3 s and 5.7 vs 4.3 min, respectively). We conclude that gannets from these 2 colonies are spatially segregated and experience different foraging conditions. We speculate that wind patterns and group feeding could generate such foraging asymmetries. Foraging site fidelity and memory effects could consolidate these asymmetries, and generate 'cultural' differences in foraging patterns.