The Adaptive Role of Winter Fattening in the White-Crowned Sparrow with Comments on its Regulation

Abstract

In White-crowned Sparrows wintering in southeastern Washington, there is an inverse correlation between the quantity of body fat and mean air temperature below about 3 C. Between 3 C and 9 C the fat reserves are independent of air temperature. Because of seasonal variation in fat-free body weight in this species, variation in total body weight is not a highly reliable index of variation in lipid reserves. A review of published data indicates that winter fattening is a common phenomenon in small birds which winter in cold habitats or high latitudes, although exceptions can be found in species that evidently in part substitute behavioral adaptations for winter fattening. It may be concluded provisionally that the magnitude of winter fattening tends to be less inwarmer habitats or at lower latitudes.It is absent in a subequatorialwintering population of Yellow Wagtails. Although it is reasonable to assume that winter fattening is an adaptation to the thermoregulatory demands of low ambient temperature, the tacit assumption that cold is the proximate factor that induces it is not fully substantiated by the evidence now available. Rather, it appears at least equally plausible that the low environmental temperature of winter must be regarded as the primary ultimate factor. The proximate factor, or battery of proximate factors, which could include reduced environmental temperature, involved in the physiological regulation of fat storage in winter remain an open question.