Foraging energetics of arctic cormorants and the evolution of diving birds

Efficient body insulation is assumed to have enabled birds and mammals to c olonize polar aquatic ecosystems. We challenge this concept by comparing th e bioenergetics of cormorants (Phalamocorax carbo) living in temperate and arctic conditions. We show that although these birds have limited insulatio n, they maintain high body temperature (42.3 degreesC) when diving in cold water (1-10 degreesC). Their energy demand at these times is extremely high (up to 60 W kg(-1)). Free-living cormorants wintering in Greenland (water temperature -1 degreesC) profoundly alter their foraging activity, thus min imizing time spent in water and the associated high thermoregulatory costs. They then meet their daily food demand within a single intense dive bout ( lasting 9 min on average). Their substantial energy requirements are balanc ed by the highest predatory efficiency so far recorded for aquatic predator s. We postulate that similar behavioural patterns allowed early diving bird s (Cretaceous) to colonize cold coastal areas before they evolved efficient insulation.