Time allocation by a deep-diving bird reflects prey type and energy gain

Central-place foragers must decide how to trade off travel time with foraging time when energy costs and gains differ between available prey types. Oxygen consumption places a fundamental constraint on the available options for deep-diving birds. Dives exceeding the aerobic dive limit (ADL) are considered costly because they extend the interdive surface time. Nevertheless, dives exceeding ADL, combined with short surface pauses, may be an efficient strategy if (1) prey density or (2) the probability of losing contact with ephemeral prey is high. We examined surface pause duration, dive duration and dive depth during dive bouts of Brünnich's guillemots, Uria lomvia, immediately before prey delivery. Surface pauses were more strongly related to dive depth than to duration and were both ‘anticipatory’ (of short dives) and ‘reactive’ (to long dives). Surface pauses decreased weakly, but significantly, with prey mass and were not shorter for ephemeral than for benthic prey once prey mass was accounted for. Dive duration was a decelerating function of dive depth and was unaffected by prey type (benthic versus pelagic). Dive duration (as a function of depth), bottom time (as a function of depth and duration) and an index for prey acquisition rate based on these parameters all increased with prey mass. Thus, surface pause duration was not reduced during dive bouts for ephemeral prey and only slightly reduced during dive bouts for larger prey, suggesting that guillemots alter other components of the dive cycle (bottom time, transit time) to accommodate differences in prey type and energy gain.