Effect of Season on the Energetics, Body Composition, and Cage Activity of the Field Sparrow

A linear correlation of existence metabolism (Mkcal = kcal·bird-1·day-1) with temperature (T = °C) was obtained for winter acclimatized Field Sparrows (Spizella pusilla) on a 10-h photoperiod (males: Mkcal = 15.59-0.285T; females: Mkcal = 14.69-0.261T). A curvilinear relationship was obtained for summer acclimatized birds on a 15-h photoperiod (males: Mkcal = 16.40 + 0.0001T3 - 0.0002T2 - 0.3662T; females: Mkcal = 15.87 + 0.0036T2 - 0.3911T). Significant differences between summer- and winter-acclimatized birds occurred only at temperatures below 0°C. The lower limit of temperature tolerance was -13° to -14°C under a constant 10-h photoperiod and under fluctuating outdoor conditions and was somewhat lower under a constant 15-h photoperiod. The upper limit of temperature tolerance was about 41°C in the summer. With caged birds under fluctuating outdoor conditions, there was an increase in metabolized energy, existence metabolism, body protein, weight, and lipid content in the autumn and a decrease in the spring. During cold waves in winter, there was commonly an immediate drop in weight before increased metabolized energy brought recovery. In half of the birds, recovery was incomplete and mortality resulted. Coefficients of food utilization varied from 0.71 under a 10-h photoperiod to 0.79 under a 15-h photoperiod and from 0.72 to 0.86 under outdoor conditions. With death from cold or heat stress there was a loss of water, lipids, and proteins. The loss of water was greater under heat stress. Nocturnal activity (Zugunruhe) developed in a different pattern in spring than in autumn and was not accompanied by premigratory fattening in the spring. In several respects, the metabolic and behavioral responses of the Field Sparrow are intermediate between those of long distance migrants and permanent residents, indicating that they may be evolving from a migratory to a nonmigratory status.