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Foraging behavior and physiological adaptations for diving were studied in Thick—billed Murrer, Uria lomiva, in the field and laboratory. Electronic, light—emitting diode, and capillary recording devices were used to measure foraging behavior. Individual dives were a flattened U shape in profile, and occurred in bouts lasting °15 min. Dive patterns were nocturnal; most dives occurred between 2000 and 0400. Murres probably concentrate their foraging effort at times when prey is most available as it migrates closer to the surface in the evening as part of the deep scattering layer. Although dives averaged 18 m in depth and 55 s in duration, most time—at—depth was spent between 21 and 40 m. Thus, murrers made a large number of shallow short—duration dives. Maximum dive dpeth was 210 m, while maximum dive duration was 224 s. Descent and ascent rates averaged 0.94 and 0.85 m/s, respectively. Hematocrit, hemoglobin, blood volume, and pectoralis myoglobin levels were measured in the laboratory as 52.8%, 18.0 g/100 mL, 12.3% body mass, and 1.9 g/100 g, respectively. Total usable oxygen store was calculated as 44.8 mL/kg, giving an estimated aerobic dive limit (ADL) of 47 s. Murres exceeded the calculated ADL in 48% of their dives. Long—duration diving is probably a more efficient foraging strategy for murres given their relatively small size and limited oxygen storage capabilities. The observed dive depths raised questions of potential problems with decompression sickness (bends) and lung collapse.