Ecological Aspects of Some Mixed‐Species Foraging Flocks of Birds

Abstract

Mixed—species flocks of birds composed of chickadees, titmice, woodpeckers, nuthatches, creepers, kinglets, and wood warblers were studied in several habitats in Louisiana, Maryland, and Maine. Chickadees and titmice usually led these groups. Both the size of the flock and density of birds varied with the habitat. An inverse relation existed between the density of birds in a habitat and the percentage of them that were species participating in flocks. Individuals of species regularly participating in flocks sometimes were found away from them. The tendency for such an individual to be in a flock was inversely related to the density of birds in the habitat. Hostile interactions were infrequent in flocks, being mostly of an intraspecific nature. A dominance hierarchy existed between the different species in flocks. A definite distance was maintained between individuals. As flock size increased the area occupied per individual decreased. The larger flocks were, the faster they usually moved. Each species favored certain parts of the habitat when foraging, though overlap existed between almost all pairs of species. The commonest species (Chickadees, titmice, kinglets) overlapped heavily. Subordinate species foraging in the presence of dominant species changed their areas of foraging, but little or no tendency was seen for subordinates to affect the foraging of dominants. Subordinates might either spread out over alternative foraging areas or concentrate their activities more strongly in a few areas. Most flock members responded markedly to predators; however, few predators were observed during the study. Many of the responses seen were delivered to inappropriate stimuli. Several factors suggest a direct relationship between population density and food supply. Flocking appears to be an effective adaptation to difficult conditions. If a species adjusts its foraging in the presence of another species, possibilities of hostilities should be minimized. Subordinate species usually moved to parts of the habitat to which they were well—adapted. If dominant species are absent, however, opportunism by the subordinate may allow it to take advantage of otherwise unused resources. Dominants may obtain a more predictable portion of the food supply by participating in flocks that if solitary. Constant movement may insure that birds obtain the most readily available food. Large flocks may move rapidly because their individuals are more crowded than those in small flocks. With this restriction either increased speed or more concentrated foraging would be predicted. The low frequency of predators suggests that evasive responses of flock members may have been more advantageous than at present.