The Lipid Reserves of White-Crowned Sparrows on the Breeding Ground in Central Alaska

King, James; Farner, Donald; Morton, Martin

The variation in lipid reserves in migratory birds during the vernal postmigratory-prenuptial phase and the autumnal premigratory phase of the annual cycle has not been systematically investigated previously in any species. Because descriptive data on changes of lipid metabolism on the breeding grounds are an essential basis for the formulation of meaningful hypotheses concerning the regulation of metabolic adaptations for migration, we initiated an investigation of the White-crowned Sparrow (Zonotrichia leucophrys gambelii) on its breeding grounds near Fairbanks, Alaska. Samples of the free population were obtained periodically from mid-May to early September, and observations were made on variation in body weight in a group of captives exposed to normal temperature and photoperiod. Lipid reserves were minimal (3 to 4 per cent of total body weight) in the earliest samples, and were held at a low level until immediately before autumn migration. At this time there was a rapid increase in reserves in both adult and immature birds to a mean level of ca. 8 per cent of total body weight. Maximum lipid levels in individual birds, perhaps best representing the fraction of the population that was prepared metabolically for migration, reached ca. 16 per cent of body weight. Total body weight in captive White-crowned Sparrows began to increase concurrently with fat deposition and the onset of migration in the feral population. The increase in body weight in captives was rapid and relatively precisely timed among individuals, closely resembling the pattern observed in captives in spring at the latitude of the wintering grounds. In the feral population, total body weight did not increase (adults) or increased only slightly (immatures) during premigratory fat deposition. This was the result of a concurrent decrease in fat-free body weight. The variation in fat-free body weight was closely correlated with the postnuptial molt. It is evident that variation in total body weight is not a reliable index of variation of lipid reserves in the White-crowned Sparrow in summer and autumn. The data are discussed in relation to possible regulatory mechanisms which induce the postmigratory depletion of lipid reserves in spring and the premigratory restoration of these reserves in autumn. As a working hypothesis, it is proposed that the decrease of reserves from the vernal maximum reflects the development of a vernal metabolic photorefractory phase. If this phase dissipates during the summer, then premigratory fat deposition may be photoperiodically induced again in the autumn.

The Lipid Reserves of White-Crowned Sparrows on the Breeding Ground in Central Alaska

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