Bioenergetics of the Dickcissel, Spiza americana

Abstract

The energy requirements of the dickcissel were measured under outdoor conditions on both its tropical wintering and temperate breeding grounds, and existence energy levels were determined over a period of several days of constant body weight at 3 different photoperiods in controlled temperature cabinets. The lower lethal limit of temperature tolerance was at -1[degree]C (10 hr. photoperiod), -2[degree]C (15 hr. photoperiod), and -3[degree]C (outdoors), and the upper limit was at 44[degree]C for birds at both 12 and 15 hr. photoperiod. No zone of thermal neutrality, but rather a single thermal neutral point existed at 38[degree]C (12 hr. photoperiod) and 36[degree]C (15 hr. photoperiod). Below these temperatures there was an inverse relationship between existence energy and the ambient temperature. This species gains a surplus of energy (productive energy) over the needs of existence by its spring migration from the tropics to its north temperate breeding grounds and by returning to the tropics in autumn. Furthermore, the spring migration is so timed that nesting activities begin at the time this increase in productive energy can first be obtained. Fall migration removes the dickcissel from its nesting latitude just prior to environmental conditions that result in an unfavorable energy balance.