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On a wing and a prayer: the foraging ecology of breeding Cape cormorants
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International audience. The Cape cormorant Phalacrocorax capensis is unusual among cormorants in using aerial searching to locate patchily distributed pelagic schooling fish. It feeds up to 80km offshore, often roosts at sea during the day and retains more air in its plumage and is more buoyant than most other cormorants. Despite these adaptations to its pelagic lifestyle, little is known of its foraging ecology. We measured the activity budget and diving ecology of breeding Cape cormorants. All foraging took place during the day, with 3.61.3 foraging trips per day, each lasting 8560 min and comprising 6153 dives. Dives lasted 21.213.9 s (maximum 70 s), attaining an average depth of 10.26.7m (maximum 34 m), but variability in dive depth both within and between foraging trips was considerable. The within-bout variation in dive depth was greater when making shallow dives, suggesting that pelagic prey were targeted mainly when diving to o10 m. Diving ecology and total foraging time were similar to other cormorants, but the time spent flying (12251 min day1, 14% of daylight) was greater and more variable than other species. Searching flights lasted up to 1 h, and birds made numerous short flights during foraging bouts, presumably following fast-moving schools of pelagic prey. Compared with the other main seabird predators of pelagic fish in the Benguela region, Cape gannets Morus capensis and African penguins Spheniscus demersus, Cape cormorants made shorter, more frequent foraging trips. Their foraging range while feeding small chicks was 76 km (maximum 40 km), similar to penguins (10–20 km), but less than gannets (50–200 km). Successful breeding by large colonies depends on the reliable occurrence of pelagic fish schools within this foraging range.
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