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.

Many seabirds nesting in areas bordering the North Sea have recently experienced large annual variation in breeding success, including reproductive failures in some cases. In contrast, the breeding success of northern gannets Morus bassanus has remained remarkably stable. The present study examines data from the large gannet colony at the Bass Rock (southeast Scotland) across 3 years, to assess the extent to which such stability may reflect both flexibility and consistency in diets and foraging behaviour. Adults exhibited great flexibility both in the species and sizes of prey consumed and in foraging trip durations, ranges and total distances travelled. They also showed a high degree of consistency in bearings of foraging trips and in behaviour at sea; the sinuosity of foraging tracks and average speed of travel was very similar each year and birds in all years spent about half their time at sea in flight. Adults returned to the nest at higher speeds from more distant foraging locations up to ca. 300 km from the colony, but speeds decreased for the farthest destinations (>ca. 400 km). Moreover, the relationship between trip duration and distances travelled at sea was asymptotic beyond ca. 60 h. These non-linear relationships probably reflected constraints on energy expenditure during flight. As a result, nest attendance was low in years with long average trip durations and chicks were left unattended and vulnerable to attack by conspecifics. These data suggest that while adults have so far been able to maintain high reproductive success in years of low prey availability, they may not be able to do so in future years if providing sufficient food for chicks entails any further increases in trip duration or foraging effort. KEY WORDS: Morus bassanus · Wildlife telemetry · Geolocation · Home range · Optimal foragingResale or republication not permitted without written consent of the publisher

No abstract

No abstract

Summary 1.Movement patterns of predators should allow them to detect and respond to prey patches at different spatial scales, particularly through the adoption of area-restricted search (ARS) behaviour. Here we use fine-scale movement and activity data combined with first-passage time (FPT) analysis to examine the foraging strategy of northern gannets Morus bassanus in the western North Sea, and to test the following hypotheses: (i) birds adopt a hierarchical foraging strategy characterized by nested ARS behaviour; (ii) the locations and characteristics of ARS zones are strongly influenced by physical oceanography; (iii) the initiation of ARS behaviour is triggered by the detection and pursuit of prey; (iv) ARS behaviour is strongly linked to increased foraging effort, particularly within nested ARS areas. 2. Birds on 13 of 15 foraging trips adopted ARS behaviour at a scale of 9·1 ± 1·9 km, and birds on 10 of these 13 trips adopted a second, nested ARS scale of 1·5 ± 0·8 km, supporting hypothesis 1 above. ARS zones were located 117 ± 55 km from the colony and over half were within 5 km of a tidal mixing front ~50 km offshore, supporting hypothesis 2 above. 3. The initiation of ARS behaviour was usually followed after only a short time interval (typicallỹ 5 min) by the commencement of diving. Gannets do not dive until after they have located prey, and so this pattern strongly suggests that ARS behaviour was triggered by prey detection, supporting hypothesis 3 above. However, ~33% of dives in mixed coastal water and 16% of dives in stratified water were not associated with any detectable ARS behaviour. Hence, while ARS behaviour resulted from the detection and pursuit of prey, encounters with prey species did not inevitably induce ARS behaviour. 4. Following the initiation of ARS behaviour, dive rates were almost four times higher within ARS zones than elsewhere and almost three times higher in zones with nested ARS behaviour than in those without, supporting hypothesis 4 above and suggesting that the foraging success of birds was linked to their ability to match the hierarchical distribution of prey.