Two current approaches to studying interspecific competition in the field provide inconclusive results. These are behavioral observations of individuals in the presence and absence of their putative competitors, and population removal experiments. Behavioral studies rarely measure the effects of competition on individual fitness, and population studies seldom elucidate the underlying causal mechanisms of competition. A synthetic approach is needed which simultaneously measures resource availability, effects of competitors upon their shared limiting resources and on one another, and effects of competition on individual fitness. This study will apply this synthetic approach to foraging competition between Townsend's solitaires and other species which feed upon juniper berries during the nonbreeeding season. It will quantify both interference and exploitive effects of competitors on individual solitaires defending winter feeding territories. It will also measure the fitness of 50-60 solitaires over two years by monitoring overwinter survival and fledgling success. Quantifying the relationship between the intensity of competition experienced by individual solitaires and their fitness will provide an empirical test of the evolutionary significance of interspecific competition to this species.
