Thick-billed murres use different diving behaviors in mixed and stratified waters

Linking diving and foraging behavior of small seabirds with the fine-scale characteristics of water masses has been challenging largely due to sampling constraints. We examined the diving behavior of 12 chick-rearing thick-billed murres (Uria lomvia) at St. George Island, southeastern Bering Sea, in relation to sea-surface temperature (SST) and thermocline depth using ventrally attached depth–temperature–acceleration data loggers. Our results from summer 2004 showed that murres swam in water masses ranging from well-mixed (SST 7–9 °C, estimated distance of 14 km from the breeding colony) to well-stratified (SST 9–12 °C, estimated distance of 30–50 km). Murres dove deeper (modal depth: 60–70 m) in the mixed water mass than in the stratified water, where most dives were to just below the thermocline depth (modal depth: 20–30 m). We suggest that the thermocline is important in shaping dive profiles of thick-billed murres, possibly through its effect on the vertical distribution of both zooplankton and fish prey.