Abstract:What governs the size and location of seabird colonies has long intrigued population ecologists. Previous analysis of the distribution of colonies of four European seabirds revealed a spatial bias ‐ large colonies occurred farther apart than expected by chance alone ‐ suggesting that intraspecific competition for food supplies during breeding may regulate colony size. Here we use computer intensive statistics to show that a similar spatial bias exists in three burrow‐nesting seabirds ‐ Cassin's auklet Ptychora… Show more
The Balearic shearwater Puffinus mauretanicus is the rarest and most threatened Mediterranean seabird. The biology of this shearwater is poorly understood, and its study is important to design conservation strategies. We studied the feeding ecology of the Balearic shearwater at sea in the western Mediterranean (1996Mediterranean ( to 2000, focusing on the importance of fisheries discards for this species. Fieldwork was conducted on board commercial bottom trawlers (demersal fishery with diurnal activity) and purse seiners (pelagic fishery with nocturnal activity), as well as during experimental trawling surveys. The shearwaters made extensive use of discards, mostly those from trawlers. This was especially so during the late breeding season, which could be related to the general impoverishment of Mediterranean surface waters. At this time of year, most birds foraged along the eastern Iberian coast, with the largest concentrations occurring off the Ebro Delta. This distribution seems determined by favourable local hydrographic conditions and by the presence of important trawling fleets. Balearic shearwaters captured discards by diving at some distance behind fishing vessels, thus reducing interactions with other seabirds. A bioenergetic model estimated that 40.8% of the energy obtained by the Balearic shearwater population comes from trawler discards during the breeding season (March to June), although this value was subject to strong variability (± 36.2% SD). In addition to the capture of discards (38% of the feeding instances observed), Balearic shearwaters also obtained food by capturing fish under floating drifting objects (33%), associating with sub-surface predators (10%), capturing small shoaling fish (10%), and feeding upon plankton (10%). The latter behaviour was observed in crepuscular hours, but the shearwaters did not appear to feed at night. In winter, Balearic shearwaters attended fishing vessels to a lesser extent. Upcoming fishing policies could affect Balearic shearwaters in the short term through reduction of discards, although good design of management strategies (such as trawling moratoria) could help to reduce their negative effects.
The Balearic shearwater Puffinus mauretanicus is the rarest and most threatened Mediterranean seabird. The biology of this shearwater is poorly understood, and its study is important to design conservation strategies. We studied the feeding ecology of the Balearic shearwater at sea in the western Mediterranean (1996Mediterranean ( to 2000, focusing on the importance of fisheries discards for this species. Fieldwork was conducted on board commercial bottom trawlers (demersal fishery with diurnal activity) and purse seiners (pelagic fishery with nocturnal activity), as well as during experimental trawling surveys. The shearwaters made extensive use of discards, mostly those from trawlers. This was especially so during the late breeding season, which could be related to the general impoverishment of Mediterranean surface waters. At this time of year, most birds foraged along the eastern Iberian coast, with the largest concentrations occurring off the Ebro Delta. This distribution seems determined by favourable local hydrographic conditions and by the presence of important trawling fleets. Balearic shearwaters captured discards by diving at some distance behind fishing vessels, thus reducing interactions with other seabirds. A bioenergetic model estimated that 40.8% of the energy obtained by the Balearic shearwater population comes from trawler discards during the breeding season (March to June), although this value was subject to strong variability (± 36.2% SD). In addition to the capture of discards (38% of the feeding instances observed), Balearic shearwaters also obtained food by capturing fish under floating drifting objects (33%), associating with sub-surface predators (10%), capturing small shoaling fish (10%), and feeding upon plankton (10%). The latter behaviour was observed in crepuscular hours, but the shearwaters did not appear to feed at night. In winter, Balearic shearwaters attended fishing vessels to a lesser extent. Upcoming fishing policies could affect Balearic shearwaters in the short term through reduction of discards, although good design of management strategies (such as trawling moratoria) could help to reduce their negative effects.
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.
Explaining why animal groups vary in size is a fundamental problem in behavioral ecology. One hypothesis is that life-history differences among individuals lead to sorting of phenotypes into groups of different sizes where each individual does best. This hypothesis predicts that individuals should be relatively consistent in their use of particular group sizes across time. Little is known about whether animals’ choice of group size is repeatable across their lives, especially in long-lived species. We studied consistency in choice of breeding-colony size in colonially nesting cliff swallows (Petrochelidon pyrrhonota) in western Nebraska, United States, over a 32-year period, following 6,296 birds for at least four breeding seasons. Formal repeatability of size choice for the population was about 0.41. About 45% of individuals were relatively consistent in choice of colony size, while about 40% varied widely in the colony size they occupied. Birds using the smaller and larger colonies appeared more consistent in size use than birds occupying more intermediate sized colonies. Consistency in colony size was also influenced by whether a bird used the same physical colony site each year and whether the site had been fumigated to remove ectoparasites. The difference between the final and initial colony sizes for an individual, a measure of the net change in its colony size over its life, did not significantly depart from 0 for the dataset as a whole. However, different year-cohorts did show significant net change in colony size, both positive and negative, that may have reflected fluctuating selection on colony size among years based on climatic conditions. The results support phenotypic sorting as an explanation for group size variation, although cliff swallows also likely use past experience at a given site and the extent of ectoparasitism to select breeding colonies.
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