Abstract
1. Cormorants are typically considered as wettable diving birds with high thermoregulatory costs and are presumed to exert substantial predatory pressure on fish stocks.
2. The stomach temperatures of seven Great Cormorants and three European Shags were recorded during a total of 108 foraging trips undertaken near the Chausey Islands breeding colony (France).
3. Both species kept a constant body temperature during the dive series which lasted up to 158 min and were conducted in 12°C water. Consequently, assuming that heat loss to the water is equal to heat production in diving Great Cormorants, the minimal insulating plumage air volume was calculated to be 0·371 × 10–3 m3 (corresponding to a 1·62-mm air layer) in males and 0·347 × 10–3 m3 (corresponding to a 1·90-mm air layer) in females.
4. Furthermore, it is shown that plumage air volume and dive depth are the major factors influencing heat flux to the water and that the energetics of diving Great Cormorants may also vary substantially according to fat layer thickness, water temperature and body temperature. Swim speed plays only a minor role.
5. Considering these results, it is postulated that Great Cormorants may have optimized plumage air volume so as to minimize both mechanical costs (upthrust) and thermoregulatory costs of swimming in cold, shallow water.
6. Finally, body temperature patterns recorded in different cormorant species while diving are compared.