Predation Risk and Fasting Capacity: Do Wintering Birds Maintain Optimal Body Mass?

Abstract

This paper tests the optimal body mass (OBM) model developed by Lima (1986) for small birds overwintering in a temperate climate. The OBM model views winter body mass as reflecting a trade—off between predation risk and the risk of starvation during periods of inclement weather. Specifically, stochastic computer simulations of winter seasons showed that costs (e.g., risk of being preyed upon) and benefits (e.g., fasting capacity) of fat associated with an intermediate mass combine to maximize the probability of overwinter survivorship (Lima 1986). The OBM model predicts that when benefits but not costs of extensive fattening are eliminated by predictable resources, the optimal solution will be for bird species to avoid costs by remaining leaner than species utilizing unpredictable resources. In the latter environment, benefits of fattening are important to surviving suddenly imposed fasting spells. Thus if mass and therefore winter survivorship are optimized among environments, size of the fat reserve should vary inversely with resource predictability. As a test of this prediction, this study compares visible subcutaneous fat among three avian foraging guilds that occupy a gradient of winter resource predictability. Strictly tree—foraging species utilizing tree—borne resources were considered to occupy a predictable environment relative to strictly ground—foraging species. In the latter foraging guild, resources may be unexpectedly and temporarily eliminated by moderate to heavy snowfall. Species with a partial dependence on each of these resource environments were assumed to occupy a resource environment of intermediate predictability. In four separate data sets involving small birds from southern Michigan, central Tennessee, northern Michigan, and southern Indiana, visible fat class varied inversely with resource predictability, as predicted. This trend was also observed in a comparison of species assumed alike in physiological factors that may independently influence size of fat reserve (body size, hypothermia, thermal conductance, basal metabolic rate, nocturnal roost microenvironment). In further concordance with theory, a comparison of dawn fat class between the Dark—eyed Junco and Tree Sparrow (from an unpredictable resource environment) and the Downy Woodpecker and Tufted Titmouse (from a predictable resource environment), which factored out all but a slight effect of basal metabolic rate, indicated significantly greater reserves in the two ground—foraging species. I conclude that aside from possible influences of physiological factors, resource predictability likely affects winter body mass in small birds. This influence takes the form of guild—specific trade—offs between predation risk and fasting capacity (see also Stuebe and Ketterson 1982, Nolan and Ketterson 1983). These results support the OBM model (and similar models), and along with previous studies suggest the widespread importance of predation risk as a factor structuring biological systems.