BODY-TEMPERATURE AND INSULATION IN DIVING GREAT CORMORANTS AND EUROPEAN SHAGS

1. Cormorants are typically considered as wettable diving birds with h igh thermoregulatory costs and are presumed to exert substantial preda tory pressure on fish stocks. 2. The stomach temperatures of seven Gre at Cormorants and three European Shags were recorded during a total of 108 foraging trips undertaken near the Chausey Islands breeding colon y (France). 3. Both species kept a constant body temperature during th e dive series which lasted up to 158 min and were conducted in 12 degr ees C water. Consequently, assuming that heat loss to the water is equ al to heat production in diving Great Cormorants, the minimal insulati ng plumage air volume was calculated to be 0.371 x 10(-3) m(3) (corres ponding to a 1.62-mm air layer) in males and 0.347 x 10(-3) m(3) (corr esponding to a 1.90-rmn air layer) in females. 4. Furthermore, it is s hown that plumage air volume and dive depth are the major factors infl uencing heat flux to the water and that the energetics of diving Great Cormorants may also vary substantially according to fat layer thickne ss, water temperature and body temperature. Swim speed plays only a mi nor role.5. Considering these results, it is postulated that Great Cor morants may have optimized plumage air volume so as to minimize both m echanical costs (upthrust) and thermoregulatory costs of swimming in c old, shallow water. 6. Finally, body temperature patterns recorded in different cormorant species while diving are compared.