Time budget and diving behaviour of chick-rearing Crozet shags

Abstract: 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 chick-rearing Crozet shags (Phalacrocorax melanogenis Blyth, 1860), we simultaneously described their time budget and div-ing 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 ben-thic 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 du-ration corresponded to dives with higher dive duration/postdive interval ratios. The Crozet shags managed their dive cy-cle to fully exploit their breathing capacities, thus optimizing foraging performance. Résumé: Dans la famille des phalacrocoracidés, les cormorans à ventre blanc sont connus pour plonger le plus pro-fondément. Pourtant, la manière dont ces oiseaux gèrent leur budget temps et leur budget énergétique reste énigma-tique. En attachant ventralement des enregistreurs de plongée sur des cormorans de Crozet (Phalacrocorax melanogenis Blyth, 1860) élevant des poussins, nous avons déterminé simultanément leur budget temps et leur comportement de plongée. Les cormorans de Crozet sont diurnes et se nourrissent sur le benthos (jusqu’à 1.7 km des côtes en moyenne)