Currencies in a diving bird: are imperial shags (Phalacrocorax atriceps) pushed for time or economising on fuel?

SHEPARD, E.L.C.; WILSON, R.P.; QUINTANA, F.; GÓMEZ LAICH, A.

Hakodate, Hokkaido, Japan

Congreso; 36th Annual Meeting of the Pacific Seabird Group; 2009

Time and energy are fundamental currencies in the foraging ecology of diving birds. Birds may maximise their foraging success in relation to either the time or energy spent, and can alter the currency by varying the allocation of time or energy to different phases of the dive (e.g. in travel to and from the foraging depth, prey searching, prey handling). While time budgets have been widely used to consider the allocation of time to the different dive phases, energy costs within dives have not been measured directly, even though there is evidence that they vary according to circumstance. Overall dynamic body acceleration (ODBA), which correlates linearly with energy expenditure in a number of species, was used to estimate foraging costs in male Imperial cormorants (Phalacrocorax atriceps) using animal-attached loggers. The duration of all dive phases increased linearly with depth, whereas overall effort per dive increased curvilinearly. Derived time and energy costs were used to model the dive behaviour that would optimise bottom time (where prey are encountered in this species) as a function of (a) dive-cycle duration (bottom time per unit time, bTPUT) and (b) dive effort (bottom time per unit effort, bTPUE). Birds appeared to maximise bTPUE at depths > 25 m, but diving behaviour was more consistent with a strategy to optimise bTPUT at shallow depths. This was interpreted as a response to depth-related changes in prey availability, as greater depths are associated with more profitable prey (measurable by head-mounted transducers e.g. IMASEN and acceleration), which, when located, may free birds from foraging under time constraints.