Résumé
Blue-eyed shags are known to be capable of the deepest dives in the cormorant family. Yet, the way these birds balance their energy and time budgets remains enigmatic. Using ventrally attached time–depth recorders on chickrearing Crozet shags (Phalacrocorax melanogenis Blyth, 1860), we simultaneously described their time budget and diving behaviour. Crozet shags are diurnal, benthic foragers (mean foraging range 1.7 km) that spend 37% of the daytime at sea. While at sea, they spend 49% of their time on the water, 44% diving, and 7% flying, and consume mainly benthic nototheniid fish. Larger fish (>20 g) were caught at shallower depths during shorter trips. They made, on average, 4.4 trips/day, each including 2.5 diving periods of 14 dives. On average, Crozet shags dove to 28 m (max. 145 m) for 2 min 2 s (max. 6 min 11 s). Descent and ascent rates became less variable at a depth of 40 m, possibly owing to a change in bird relative buoyancy at that depth. Ten percent of dives exceeded the estimated behavioural aerobic dive limit (4 min) and diving depths showed a bimodal distribution at <5 and ~18 m for dives lasting ~1 min. This dive duration corresponded to dives with higher dive duration/postdive interval ratios. The Crozet shags managed their dive cycle to fully exploit their breathing capacities, thus optimizing foraging performance.