Life‐History Theory in the Nonbreeding Period: Trade‐Offs in Avian Fat Reserves?

Abstract

We tested the hypothesis of optimal winter fat storage in birds. The hypothesis predicts that when the survivorship benefits (fasting capacity) but not the survivorship costs (higher cost of flight, lowered agility, increased exposure to predators) of fat are eliminated by predictable resources (e.g., tree— and shrub—borne food), species will be leaner than species exploiting unpredictable resources (e.g., ground—borne food subject to sudden covering by snow). The predicted pattern was found in an earlier study of avian communities in central North America, while snowfall is frequent. Here we tested the predictions of the hypothesis that (1) when ground—and tree—feeding guilds are compared between geographic regions of harsh and mild winter climate, only ground—feeders will have relatively high fat reserves, resulting in a significant guild ° climate interaction term in a two—way analysis of variance, and (2) ground— and tree—foraging guilds will both show low fat reserved in a mild winter environment. To test these predictions, visible subcutaneous fat class was measured in both guilds in Wisconsin and Michigan (harsh winter environments) and southwestern British Columbia, northwestern Washington, and Tennessee (mild winter environments). Both predictions were supported. We suggest that small birds wintering in North America approach local optima in energy storage strategy and winter survivorship. The energy storage strategy thus appears to be a major life—history trait in the nonbreeding period. The trade—off between the costs and benefits of winter fat has two potential implications for understanding population limitation. First, trade—offs that reduce reserves can lead to lower survival in severe winter weather than would occur at maximum fat levels. Second, our study suggest that individual birds are sensitive to, and can control, both predation and starvation risks. Recent theory of population limitation indicates that as food abundance declines, birds feed to avoid starvation, but at the expense of increase mortality from predation. Together these studies suggest that populations are limited by interacting (predation, food supply) instead of single (food supply) factors. Study of trade—offs used by individuals to maximize survival can provide unique perspectives on population limitation.