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First published online December 14, 2007
Journal of Experimental Biology 211, 58-65 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.009811

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A new method to quantify prey acquisition in diving seabirds using wing stroke frequency

Katsufumi Sato^1,*, Francis Daunt², Yutaka Watanuki³, Akinori Takahashi⁴ and Sarah Wanless²

¹ International Coastal Research Center, Ocean Research Institute, The University of Tokyo, 2-106-1 Akahama, Otsuchi, Iwate 028-1102, Japan
² Centre for Ecology and Hydrology, Banchory, Aberdeenshire AB31 4BW, UK
³ Graduate School of Fisheries Sciences, Hokkaido University, Minato-cho 3-1-1, Hakodate 041-8611, Japan
⁴ National Institute of Polar Research, 1-9-10 Kaga, Itabashi, Tokyo 173-8515, Japan

^* Author for correspondence (e-mail: katsu{at}ori.u-tokyo.ac.jp)

Accepted 19 October 2007

To understand the foraging strategies of free-ranging diving animals, time series information on both foraging effort and foraging success is essential. Theory suggests that wing stroke frequency for aerial flight should be higher in heavier birds. Based on this premise, we developed a new methodology using animal-borne accelerometers to estimate fine-scale temporal changes in body mass of a pursuit-diving, piscivorous seabird, the European shag, Phalacrocorax aristotelis. We hypothesized that variations in body mass determined from changes in wing stroke frequency before and after a series of dives would be related to the amount of prey captured. The estimated net gain in body mass during a foraging trip was highly variable, ranging from –30 to 260 g, values that were extremely similar to food loads obtained from shags on the Isle of May in previous years using water-offloading and nest balances. Load sizes estimated using the wing stroke method were strongly and positively related to both cumulative flight time and return flight time. At the trip level, load size was unrelated to cumulative dive bout duration and the total amount of time spent underwater. However, highly significant relationships were apparent at the individual bout level, with birds showing bigger mass gains following longer dive bouts. Results from this study are therefore extremely encouraging and suggest that changes in body mass determined from changes in wing stroke frequency may provide a reliable method of obtaining short- to medium-term information on foraging effort and success of diving seabirds.

Key words: acceleration, data logger, flight, foraging, seabird, Phalacrocorax aristotelis

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