An Experimental Analysis of Mixed‐Species Flocking in Birds of Deciduous Woodland

Abstract

Two general models have been developed concerning the adaptive advantage of participating in mixed—species foraging groups. One model emphasizes the increased feeding efficiency to be obtained in heterospecific groups, while the other model advances the antipredator advantages of participation in such species complexes. From these models, two conflicting predictions were made about the consequences of providing a superabundant of food resource to birds that normally engage in mixed—species flocking. We tested these predictions over the course of two winters birds in two central Ohio woodlots, provisioning one of the two woodlots with a superabundant supply of sunflower seeds and beef suet during the second winter of the study. In addition, we examined the effects of added food on the density of bird species, and on the size and composition of heterospecific flocks. Carolina Chickadees, White—breasted Nuthatches, Brown Creepers, Tufted Titmice, and Downy Woodpeckers all exhibited significant decreases in mixed—species flocking in the area with added food relative to the controls. These results reject protection from predators as a sufficient cause for mixed—species flocking, and support the view that either increased feeding efficiency alone or feeding efficiency and predation protection in combination cause animals to forage in mixed—species groups.