A Biometric Study of Major Body Components of the Slate-Colored Junco, Junco hyemalis

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Abstract
During the winters of 1962-1963 and 1964-1965, 159 slate-colored juncos were collected in central Pennsylvania. Birds were measured, weighed, and frozen. Age, sex, wet weight, total fat, total water, dry fat-free weight, and fat and water indexes were determined. Dry fat-free weight, total water, and water index did not vary significantly during the winter, although water as a per cent of wet weight and fat as a per cent of wet weight did vary significantly and inversely. Individuals collected in the spring and fall migratory periods were significantly smaller and less massive with lower dry fat-free weights than birds collected in winter. Water index was higher during both migratory periods than in winter but was related to basic body weight, not migration per se. Seasonal changes in wet weight were due largely to changes in fat. Highest wet weight (22.87 [plus or minus] 0.42 g) and total fat (3.87 [plus or minus] 0.18 g) were found in Feb. The slate-colored junco is a cold-hardy sp. and should be able to survive without feeding for 2 days in midwinter using available depot fat. Migratory weights were between 20 and 21 g in spring and fall. There was no increase in fat during the spring, but mean reserves were sufficient for flights of about 360 miles at that season. Males were heavier (wet weight difference = 1.22 [plus or minus] 0.29 g) and larger (wing length difference = 3.6 [plus or minus] 0.4 mm) than females. Afternoon birds were heavier (wet weight difference = 1.33 [plus or minus] 0.32 g) and fatter (total fat difference = 0.62 [plus or minus] 0.18 g; fat index difference - 0.092 [plus or minus] 0.028) than morning birds. Dry fat-free weight (mass estimator) and wing length (size estimator) were linearily correlated in females but not in males. This seems to be a valid sex difference in this sp. and precludes any meaningful prediction of weight in male juncos based upon wing-length measurement. Dry fat-free weight was positively related to both total water and total fat. Total water was positively related to dry fat-free weight but negatively related to total fat. It appears that in the junco tissue mass but not necessarily cell number increases during the early stages of fat deposition and decreases during late stages of fat utilization, while fat replaces water throughout fat deposition but less rapidly at higher fat values and vice versa. These processes reach limits at the maximum fat (about 5-7 g) and at the minimum fat (about 0.2 g) for the sp. These interrelationships argue against a strict interpretation of the hypothesis of Odum, Rogers, and Hicks (1964) that nonfat components are stable during fat mobilization and depletion; but the magnitude of lean mass change with fat deposition is probably minor, and there is no evidence that additional adipocytes are recruited. Direct studies of fat organs are needed.