Metabolic and Behavioral Responses to Fasting in the White-crowned Sparrow (Zonotrichia leucophrys gambelii)

During winter, ground-feeding bird species may occasionally be forced to fast when weather conditions (e.g., snowstorms, icing) make food temporarily inaccessible. To describe the response of White-crowned Sparrows to fasting, we (1) investigated variations of body weight and temperature in fasting and fed birds confined in a cold room for up to 64 h, and (2) periodically measured oxygen consumption and locomotor activity of fasting and fed birds through a 39-h night/day/night cycle while simulating natural conditions of temperature and daylength. Our data indicate that the unusual White-crowned Sparrow might survive 3 nights and 2 days without food at air temperatures near freezing, but typical individuals could not be expected to survive longer than 1 day and 2 nights. Males lose weight at a greater rate than females, but are able to endure fasting longer because they are initially heavier. This suggests that males may be more resistant to winter storms and food deprivation, and that this attribute may have contributed to the evolution of geographic variation in the sex ratio in this species. Physiological and metabolic responses to fasting included a nighttime decline in body temperature (ca. 3 C) that was greater than that observed in fed birds, and a 21% decline in oxygen consumption on the second night of the fasting cycle when compared to the first night. The drop in body temperature would be sufficient, in terms of the van't Hoff effect, to account for the decline of metabolic rate, but visual inspection of the fasting birds at night suggested that adjustments in ptiloerection and posture may also play a role in effecting the decline. During the day, locomotor activity was 1.5-10 times greater in fasted than fed birds. Oxygen consumption, however, did not differ, indicating (as one possibility) that fasting birds may be able to utilize heat generated by exercise to meet some of the cost of thermoregulation. The short-term adjustment to starvation under natural conditions may thus include a moderately reduced energy requirement and a partial diversion of energy expenditure from thermoregulation to foraging.