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

Abstract

A favorite of bird-bander is White-throated Sparrow (Zonotrichia albicollis), hundreds of which have been trapped and banded in eastern United States and Canada. Capitalizing upon ease and frequency with which species may be obtained, a number of workers have amassed valuable weight, mensural, and other data from banding these birds. In recent years Eugene P. Odum and his coworkers have described weight and fat variations of White-throats wintering in northern Georgia (Athens) and northwestern Florida near Tallahassee (Odum, 1949, 1958; Odum and Connell, 1956; Odum and Perkinson, 1951). Both of these sites are within wintering range of species, not at southernmost edge. Several significant conclusions emerged from their investigations: (1) White-throats arriving in north Georgia, and also those killed at Tallahassee TV tower in autumn, are both light in weight and lean; (2) there is a mid-winter peak in weight and fatness; (3) during prenuptial molt birds weigh less than at any other time during their sojourn in South; and (4) birds reach a reported premigratory peak in weight and obesity in late April and early May. It is purpose of this paper to examine weight and fat data for White-throat population wintering at Gainesville, Florida, a locality representative of southern edge of this species' wintering range. White-throats are fairly common winter residents around Gainesville (Chapman, 1888), but farther south in Florida species is noticeably rarer (Sprunt, 1954). In autumn White-throats generally arrive at Gainesville about first week or ten days of November. The arrival time at Gainesville is about two weeks after birds first arrive at Athens, Georgia, 300 miles to north, and about one week after they arrive in Tallahassee, Florida region, 150 miles west-northwest. Twelve birds caught in November, 1963 at Gainesville in mist nets were subjectively judged to be lean, and their body weights (Table 1) were considerably less (by about 2-3 g) than autumnal postmigratory birds at Athens (Odum and Perkinson, 1951) but only about 1 g less than autumnal arrivals near Tallahassee. In fact, Odum (1958: 106) suggested that the Whitethroats striking tower at Tallahassee had used up all or most all of their fat, which is not unexpected since Gulf coast is near 'end of line' for fall migration .... By comparison, data in Table 1 show, in addition to distributional information cited above, that Gainesville White-throats were at end of line.