Energetic costs of diving and prey-capture capabilities in cormorants and shags (Phalacrocoracidae) underline their unique adaptation to the aquatic environment

Abstract

Cormorants and shags are foot propelled pursuit divers that forage on benthic and pelagic fish. Previous studies suggested that diving in cormorants is extremely costly, and this is usually attributed to their partially wettable plumage and inefficient mode of propulsion. We investigated the energetic requirements of three Phalacrocorax species during diving. Our results indicate that, when the differences in experimental conditions and calculation methods are accounted for, energy expenditure during shallow horizontal diving in these species is similar and not considerably different from other avian divers. In the absence of direct measurements, thermodynamic modelling has been used to assess the impact of dive depth on the energetic costs of diving. Based on this, the energetic costs of Great Cormorants (Phalacrocorax carbo) diving during the winter in Greenland were estimated to be as high as 64 W kg−1 (∼21 × RMR). A recent study measured the effect of depth on the diving energetics of Double-crested Cormorants (P. auritus) to be much less drastic than suggested by this model. Extrapolating from the latter study to conditions encountered by birds wintering in Greenland shows that the thermodynamic model might greatly overestimate the energetic demand of Great Cormorants during diving. Using an improved model, the daily food intake of Great Cormorants wintering in Greenland was calculated to be ∼1,170 g. Taking into account the recorded time that birds spend diving every day, such food intake would require the highest prey-capture rate suggested so far for an avian pursuit diver (∼41 g min−1 underwater). However, little is known about the prey-capture capabilities of avian divers. We used an underwater video array to study the effect of prey density (live juvenile rainbow trout, Oncorhynchus mykiss) on the prey-capture performance of Double-crested Cormorants. We found that cormorant capture success greatly depended on prey density. Prey capture rate was highest at the greatest fish densities and easily surmounted 41 g min−1 underwater. This would suggest that cormorants in Greenland might be able to achieve these high capture rates, if prey densities are sufficiently high. However, what prey items are encountered by cormorants during the winter in Greenland and at what densities remains to be investigated.