Temperature Regulation in the Sooty Tern Sterna Fuscata

SUMMARY Temperatures of eggs, young and adults of the Sooty Tern were measured under a variety of environmental conditions on Midway Island. The mean daytime body temperature of incubating adults (40·5° C.) was significantly higher than the mean (39·4° C.) for adults on nests at night. The mean incubation patch temperature (39·‐6° C.) was significantly higher than the means for incubated eggs without visible embryos (34·8° C), incubated pipped eggs (36·6° C), or brooded, newly‐hatched chicks (38·3° C). Severe heating by solar radiation is the principal environmental problem faced by nesting Sooty Terns. Shading by adults of eggs and young chicks prevents overheating; older chicks seek natural shade. When placed in the open sun, chicks of all ages can lose heat by vigorous panting. However, body temperature may rise almost to 45° C. and usually stabilizes in the lower range of prevailing black bulb temperatures. The relationship of physiological and behavioural mechanisms of thermoregulation to environmental conditions during the terns' nesting season on Midway Island is discussed.