Multiple regression analyses of midwinter fattening of the white-throated sparrow

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Abstract
Midwinter lipid depots of the white-throated sparrow (Zonotrichia albicollis) at Richmond, Virginia, are correlated with a suite of environmental and morphological variables. Lipid reserves allow this species to survive even the most extreme winter conditions for several hours. Variables having the greatest individual correlations with lipid reserve are average temperature of the 20 days prior to capture, fat class, body weight, and long-term (32-year) average temperature of the date of capture. A comprehensive multiple regression model based on analyses of all possible independent variables accounts for 87% of the variation in lipid reserves. The most important independent variables in this model are body weight, mean temperature of the 20 days preceding collection, fat class, extreme high temperature of the day of capture, long-term average temperature, relative humidity, chill factor, wet-bulb temperatures of the day before and the day of capture, wing length, and precipitation. The "best" equation using only measurements of environment as independent variables included time of collection in hours after sunrise and hours before sunset, Eastern Standard Time, temperature of the 20 days prior to capture, and mean wind velocity of the day before capture. Models computed solely from temperature measurements included dry-bulb temperatures of the day of capture and the day before capture, low extreme temperatures of the day of capture, wet-bulb temperatures of the day before capture, and the 20-day average dry-bulb temperature of the period prior to collection. Fattening in response to weather conditions appears to be a form of "fine-tuning" of energy reserves superimposed on a more stable, intrinsic cycle of winter fattening.