A Comparison of Energy Reserves during Autumnal and Vernal Migratory Periods in the White-Crowned Sparrow, Zonotrichia leucophrys gambelii

Abstract

Although the physiology of autumnal migration has received substantially less attention than that of vernal migration, it is obvious that it also becomes possible calorically because of the development of a positive energy balance. During the premigratory and intramigratory periods this positive energy balance may be reflected in the accumulation of fat reserves. Such deposition of fat in autumn has been observed in a considerable number of species (see, for example, Wachs 1926; Zedlitz 1926; Groebbels 1930; Merkel 1938, 1958; Wallgren 1954; Rautenberg 1957; Odum 1960; Blyumyental 1960; Dolnik 1960). The most extensive data are from a series of birds killed at a television tower in Georgia near the Gulf of Mexico (Odum 1960; Odum et al. 1961); in these, the fat varied from about 6% of total body weight for Zonotrichia albicollis, which does not migrate farther south, to 33-52% for Dolichonyx ory ivorus, Piranga olivacea, and Piranga rubra, which migrate on into South America. The fat reserve of a migrant at a given time is a function of many variables. Among these must be the rate of energy expenditure during migratory flight, the length and duration of single uninterrupted migratory flights, the time of day at which the bird is taken, the time elapsed after the latest migratory flight, and the rate of replenishment of metabolized fat. For several years we have accumulated data on the body weight and fat content of the whitecrowned sparrow, Zonotrichia leucophrys gambelii, during fall migration. These data suggest that the energy reserves of fall migrants are generally smaller than those of the vernal premigratory and intramigratory periods (McGreal and Farner 1956; King and Farner 1959; Farner et al. 1961), and that these smaller reserves are consistent with the more leisurely progress of fall migration. An additional difference between spring and fall migration lies in the control of changes in metabolism and energy intake which result in fattening. In spring, the positive energy balance in this species is primarily the result of a