A Review of the Vernal Fat Deposition Picture in Overland Migrant Birds

Abstract

The white-throated sparrow leaves northcentral Flordia at least by early April, at which time individuals are both light in weight and lean. On the average the premigratory bird contains only about 1.2 g of fat, enough fuel to sustain a continuous flight of about 90 miles. Evidently these lean premigrants initiate their northward movements with short flights, making frequent stopovers en route. As they migrate northward and as the season progresses evidently they become more and more obese. Certainly the evidence in this species indicates that only the late premigrants or intramigrants are markedly fat. Vernal increases in weight and fat depositon for other overland migrant passerines are discussed. Two factors, time and place, are important in assessing fat deposition. The data on fat deposition, both qualitative and quantitative, amassed so far indicate that the early premigrants generally do not show a sharp increase in weight involving fat deposition whereas intramigrants generally do. Especially is vernal fat deposition characteristic of late migrants at locations north of the southermost edge of the wintering grounds.