Abstract
Group foraging contradicts classic ecological theory because intraspecific competition normally increases with aggregation. Hence, there should be evolutionary benefits to group foraging. The study of group foraging in the field remains challenging however, because of the large number of individuals involved and the remoteness of the interactions to the observer. Biologging represents a cost‐effective solution to these methodological issues. By deploying GPS and temperature–depth loggers on individuals over a period of several consecutive days, we investigated intraspecific foraging interactions in the Socotra cormorant Phalacrocorax nigrogularis, a threatened colonial seabird endemic to the Arabian Peninsula. In particular, we examined how closely birds from the same colony associated with each other spatially when they were at sea at the same time and the distance between foraging dives at different periods of the day. Results show that the position of different birds overlapped substantially, all birds targeting the same general foraging grounds throughout the day, likely following the same school of fish. There were as many as 44,500 birds within the foraging flock at sea at any time (50% of the colony), and flocking density was high, with distance between birds ranging from 8 to 1,380 m. Birds adopted a diving strategy maximizing time spent underwater relative to surface time, resulting in up to 72% of birds underwater in potential contact with prey at all times while foraging. Our data suggest that the benefits of group foraging outweigh the costs of intense aggregation in this seabird. Prey detection and information transmission are facilitated in large groups. Once discovered, shoaling prey are concentrated under the effect of the multitude. Fish school cohesiveness is then disorganized by continuous attacks of diving birds to facilitate prey capture. Decreasing population size could pose a risk to the persistence of threatened seabirds where group size is important for foraging success.
Highlights
Many predator species aggregate at the intraspecific level for the purpose of foraging, a phenomenon known variously as communal foraging, cooperative foraging, social foraging, or more generally as group foraging
Despite its obvious relevance to different fields of ecology, the study of group foraging in predators remains challenging
We studied the vulnerable Socotra cormorant (Phalacrocorax nigrogularis), a species endemic to the Arabian Gulf known for large at-sea aggregations (Jennings, 2010), but for which there is little information on its foraging behavior
Summary
Many predator species aggregate at the intraspecific level for the purpose of foraging, a phenomenon known variously as communal foraging, cooperative foraging, social foraging, or more generally as group foraging. Group-foraging seabird species, such as the Guanay cormorant (Phalacrocorax bougainvillii, Weimerskirch, Bertrand, Silva, Bost, & Peraltilla, 2012) or the Cape cormorant (Phalacrocorax capensis, Cook et al, 2012), tend to specialize on shoaling prey, usually small epipelagic fish, and on krill, as in the case of the short-tailed shearwater (Puffinus tenuirostris, Hunt, Coyle, Hoffman, Decker, & Flint, 1996) or murres (Uria spp., Hunt, Harrison, Hamner, & Obst, 1988) These seabirds may aggregate into foraging flocks of up to several hundred thousand individuals (Gould, Forsell, & Lensink, 1982), suggesting they rely on extremely high prey densities. We used a biologging approach to study group foraging in this species by deploying GPS and temperature–depth recorders on breeding adults, accurately measuring their behavior in three dimensions and with detailed temporal resolution
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