| Summary: | Background: Among the most widespread seabirds in the world, shearwaters of the genus Puffinus are also some of the deepest diving members of the Procellariiformes. Maximum diving depths are known for several Puffinus species, but dive depths or diving behaviour have never been recorded for great shearwaters (P. gravis), the largest member of this genus. This study reports the first high sampling rate (2 s) of depth and diving behaviour for Puffinus shearwaters. Methodology/Principal Findings: Time-depth recorders (TDRs) were deployed on two female great shearwaters nesting on Inaccessible Island in the South Atlantic Ocean, recording 10 consecutive days of diving activity. Remote sensing imagery and movement patterns of 8 males tracked by satellite telemetry over the same period were used to identify probable foraging areas used by TDR-equipped females. The deepest and longest dive was to 18.9 m and lasted 40 s, but most (.50%) dives were,2 m deep. Diving was most frequent near dawn and dusk, with,0.5 % of dives occurring at night. The two individuals foraged in contrasting oceanographic conditions, one in cold (8 to 10uC) water of the Sub-Antarctic Front, likely 1000 km south of the breeding colony, and the other in warmer (10 to 16uC) water of the Sub-tropical Frontal Zone, at the same latitude as the colony, possibly on the Patagonian Shelf, 4000 km away. The cold water bird spent fewer days commuting, conducted four times as many dives as the warm water bird, dived deeper on average, and had a greater proportion of bottom time during dives.
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