| Summary: | Most seabirds breed colonially, at which time they make central-place foraging trips. Parents must collect food both for themselves and for egg production/chick-rearing. How should they forage? I followed five species across two sites in Sweden in the Baltic Sea using GPS and time-depth recorder (TDR) devices, giving information on both flight and diving activity. I use a movement ecology approach, asking questions about which spatiotemporal aspects determine these species’ movement activities and on how species differ in their foraging activity. Further, I test theories of how birds should optimally vary their flight behaviour in response to winds, and of how individual birds collect information on alternative breeding sites, i.e. prospecting.Chick-rearing Common Murre (Uria aalge) from the island of Stora Karlsö (SK; 17.97°E, 57.29°N), foraged most around sunset and sunrise, when they made more frequent and shallower dives compared to at midday. They made longer distance and duration overnight trips where they visited more distant foraging areas than during daytime trips. In a following study I showed that the GPS device deployed had little measurable effect on the Common Murre’s activity, though murres did lose body mass which remains to be explained.During flights returning from foraging sites to their colony at Stora Karlsö, both Common Murre and Lesser Black-backed Gull (Larus fuscus) optimally adjust their airspeeds to minimise their costs of transport, increasing airspeed under headwinds and cross-winds, but reducing airspeed under tailwinds. While the gulls also adjusted their altitude, increasing altitude under tailwinds, thus benefiting from the faster winds higher up, but flying low, where wind is slower, under headwinds and crosswinds; the murres though always fly low, close to the sea surface.Lesser Black-backed Gull are generalist foragers, at Stora Karlsö they feed both on land and at sea. How do they choose between these? Lesser Black-backed Gull were followed with GPS and observations were made ...
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