European shag provisioning foraging dives

Foraging dives in birds and mammals involve complex physiological and
behavioural adaptations to cope with the breaks in normal respiration.
Optimal dive strategies should maximise the proportion of time spent under
water actively foraging versus the time spent on the surface. Oxygen
loading and carbon-dioxide dumping carried out on the surface could
involve recovery from the consequences of the last dive and/or preparation
in anticipation of the next dive depth and duration. However, few studies
have properly explored the causal pattern of effects within such dive
cycles, which is crucial prior to any assessment of optimal dive
strategies. Using Time Depth Recorders and Global Positioning System
loggers, we recorded over 42,000 dives by 39 pairs of male and female
European shags (Phalacrocorax aristotelis). Dives either involved a
straight descent and ascent, presumably reflecting an unsuccessful search
for prey, or a descent followed by horizontal movement followed by an
ascent, presumably reflecting active hunting pursuit of pelagic prey.
Males were larger than females, but we were unable to distinguish between
sex effects and the non-linear effects of body mass on dive behaviour.
Path analysis showed that within-individual dive-to-dive variation in
surface times can best be explained as recovery from the previous dive. As
expected in a pelagic hunter with unpredictable dive durations, there was
no evidence of anticipatory preparation of oxygen stores in pre-dive
surface durations. Among-individual variation in dives showed that body
mass directly affected descent durations, but individual variation in all
other dive and surface durations was driven by variation in descent
duration, suggesting a critical role for dive depth in overcoming
body-mass dependent effects of hydrodynamic/wave drag and buoyancy. Our
analyses tests for the first time certain critical assumptions for studies
assessing optimal dive strategies in birds and mammals, thereby revealing
new details and avenues for research concerning adaptive diving behaviour.