Seasonal Correlations of Reserve Energy of the Red-Winged Blackbird

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Abstract
A study of the reserve energy available to the Red-winged Blackbird (Agelaius phoeniceus) was undertaken in Centre County, Pennsylvania, during 1961 and 1962. The body weight of males decreased from a mean of 67.9 g in March to a mean of 62.0 g in July. The mean weight of females was 42.1 g in May and decreased to 38.9 g in July. Regressions for weight decrease of both males and females in 1962 were not statistically different from those in 1961. Regressions for males and females also were not statistically different. The weight of fat in the 3 areas of deposition decreased at a faster rate than did the total body fat. The rapid decrease of fat in deposits appears to be correlated with the rapid increase in testicular size in April. The relationship between temperature and reproduction and their effect on reserve energy of a population is discussed. The survival value of increased fat deposition in the spring may pertain, in part, to the fact that the reserve energy supply is utilized largely for reproduction or during inclement weather rather than for migration per se.