Foraging behaviour of common murres in the Baltic Sea, recorded by simultaneous attachment of GPS and time-depth recorder devices

Global positioning system and time-depth recorders were deployed in combination to investigate the foraging behaviour of common murres Uria aalge breeding at Stora Karlsö Island, Baltic Sea, during the chick-rearing period. In the pre-breeding period the main prey species of murres, sprat Sprattus sprattus, is targeted by commercial fisheries, likely reducing prey availability during the breeding season. Foraging trips typically consisted of a short flight followed by a period sitting on the sea surface (0.39 ± 0.48 h), followed by several (5.3 ± 3.8) diving bouts interspersed by flights and water surface activity. Following the final diving bout, murres returned directly to the colony. Overnight foraging trips lasted longer than daytime trips, and that result corresponded with greater diving activity and reduced dive depths around dawn and dusk, likely times of high prey accessibility. High outward flight groundspeeds (20.0 ± 2.8 m s-1) were aided by tailwinds, and lower inward flight groundspeeds (15.1 ± 2.5 m s-1) were impeded by headwinds. Flights following the wind direction may reflect a strategy to reduce crosswind drift. Foraging intensity was lower than reported by most other studies of murres, suggesting more abundant or aggregated prey