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    Diving pattern and performance in nonbreeding gentoo penguins (Pygoscelis papua) during winter

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    T.D. Williams (United Kingdom), A. Kato (Japan), J.P. Croxall (United Kingdom), Y. Naito (Japan), D.R. Briggs, S. Rodwell and T.R. Barton (United Kingdom)
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    We studied diving patterns and performance (dive depth, duration, frequency and organization during the foraging trip) in relation to diet in nonbreeding Gentoo Penguins (Pygoscelis papua) over 59 days (involving 5,469 dives) in winter. We estimated foraging ranges and prey capture rates, and compared foraging behavior with that of breeding (chick-rearing) birds. Foraging was highly diurnal with 98% of foraging trips completed during the same day. Foraging-trip frequency was 0.8/day, trip duration was 6-8 h, and birds spent 51-62% of the foraging trip diving. Dive depth and duration were bimodal. Shallow dives (<21 m; 42% of total number and 16% of total dive time) averaged 5-7 m and 0.5-1.3 min. Deep dives (>30 m; 55% of total number and 81% of dive time) averaged 74-105 m and 2.7-3.5 min, respectively. Deep-dive duration exceeded the subsequent surface interval, but shallow dives were followed by surface intervals two to three times dive duration. Deep dives showed clear diel patterns, averaging 10-20 m at dawn and dusk and 70-90 m at midday. These results are consistent with the patchy vertical and horizontal distribution and diel movements of Antarctic krill, the main winter prey of Gentoo Penguins (including study birds). We suggest that shallow dives are mainly searching dives, and deep dives mainly for feeding. Foraging activity of non breeding Gentoo Penguins in winter is similar to that of chick-rearing birds. The only major differences are that foraging-trip frequency is 20% less and stomach-content mass on return ashore 30% less in winter. We conclude that foraging activity in Gentoo Penguins is changed by varying the frequency and duration of foraging trips, rather than by changing the pattern and rate of diving.