A Study of Nonfat Weights in Migrating Swainson's Thrushes (Hylocichla ustulata)

Abstract

Collections totaling 279 Swainson's Thrushes were made during the fall and spring migrations at seven locations along the migration route between Michigan and Panama to test the idea of the constancy in nonfat weights. A gradual and comparatively small decrease in fat-free and nonfat dry weight roughly paralleled the migration sequence, so that mean values were significantly lower in spring arrivals on the U. S. Gulf coast than in Michigan fall birds. Differences between fall birds within the United States or between spring and fall birds in Panama were not significant. The slight losses occurring during southward migration were apparently not recouped during the winter. As wing lengths of the seven populations were homogeneous, the differences in fat-free weights were probably not due to size differences resulting from different racial composition of populations. Ratios of water to nonfat weights were remarkably similar at all localities and indicated a proportionate change in water and nonfat dry components. Only in the posttrans-Gulf migrants was a slight but non-significant dehydration noted.