A life in the fast lane:: energetics and foraging strategies of the great cormorant

Body insulation is critically important for diving marine endotherms. However, cormorants have a wettable plumage, which leads to poor insulation. Despite this, these birds are apparently highly successful predators in most aquatic ecosystems. We studied the theoretical influence of water temperature, dive depth, foraging techniques, and prey availability on the energetic costs of diving, prey search time, daily food intake, and survival in foraging, nonbreeding great cormorants (Phalacrocorax carbo). Our model was based on field measurements and on data taken from the literature. Water temperature and dive depth influenced diving costs drastically, with predicted increases of up to 250% and 258% in males and females, respectively. Changes in water temperature and depth conditions may lead to an increase of daily food intake of 500-800 g in males and 440-780 g in females. However the model predicts that cormorant foraging parameters are most strongly influenced by prey availability, so that even limited reduction in prey density makes birds unable to balance energy needs and may thus limit their influence on prey stocks. We discuss the ramifications of these results with regard to foraging strategies, dispersal, population dynamics, and intraspecific competition in this avian predator and point out the importance of this model species for our understanding of foraging energetics in diving endotherms.