Estimating prey capture rates of a planktivorous seabird, the little auk (Alle alle), using diet, diving behaviour, and energy consumption

Interpreting the impact of environmental change on food webs requires a clear understanding of predator-prey interactions. Such knowledge is often lacking in the marine environment where the foraging behaviour and prey requirements of some of the major top-predators remains mysterious. For example, very little is known about the underwater foraging behaviour of the little auk, the most numerous seabird in the North Atlantic. In 2004, we used time-depth-recorders at two breeding colonies in East Greenland to examine the diving behaviour of this small, planktivorous seabird during the chick-rearing period. Due to technical difficulties data were only collected for four individuals, but recordings showed that birds dive up to 240 times a day to maximum depths of 27 m (average 10 m), with maximum dive durations of 90 s (average 52 s). In addition, we collected the chick meals from 35 individuals, which were dominated by Calanus copepods (95%), and also determined the field metabolic rates (FMR) of 14 individuals using the doubly labelled water technique, which averaged 609.9 kJ day(-1). We integrated information on diving duration with chick diet and FMR to estimate the prey requirements and underwater capture rates of little auks using a Monte Carlo simulation. Chick-rearing little auks needed to catch about 59,800 copepods day(-1), which is equivalent to about six copepods caught per second spent underwater. These astonishing results strongly suggest that little auks are, at least partly, filter-feeding, and underline the importance of highly productive, cool marine areas that harbour dense patches of large, energy-rich copepods.