The formation of foraging aggregations in a highly social seabird, the thick-billed murre (Uria lomvia), at small and large scales

Analyzing how animals are distributed in space and time is important to understand the behavioural interactions that underlie population dynamics, especially for highly social species. Thick-billed murres (Uria lomvia) breed in some of the largest and densest colonies of any seabird. Although this bird is known to aggregate at sea, little is known about when, where, and why the birds form aggregations. We examined the spatial and temporal patterns of foraging aggregations during the breeding season through various scales via (1) measurement of the synchrony of arrivals of adults feeding their chicks at the colony, and (2) use of both GPS and camera loggers attached on the birds to examine the proximity of birds at sea. Adult arrivals at the colony were synchronised when bringing capelin (Mallotus villosus), a gregarious pelagic fish, but not when bringing sculpin (primarily Triglops spp.), a solitary benthic fish. Camera loggers revealed very close encounters of foraging conspecific (< 4 m), much closer than what was predicted by chance, despite low prey densities. GPS loggers also showed diffuse at-sea aggregations with minimal distances closer than expected by chance. However, those study birds did not typically share foraging trajectories. We suggest that, at smaller scales, murres form tight groups to increase searching efficiency underwater. At larger scales, murre aggregations are most likely a result of foraging individuals converging in the more prolific areas, either by independently encountering prey hotspots, or by cueing on other foraging birds.