Effects of food supplementation and predator simulation on nuthatches and parids within mixed-species flocks

摘要

Although many ornithologists have studied mixed-species flocks, few have examined spatial positioning of birds within flocks. Flocks, here defined as three or more birds that move together as a group, vary from loose associations of individuals separated by tens of meters to tight clusters of birds only centimeters apart. Variations in spacing are interesting because they may either enhance or reduce the benefits that birds acquire by flocking. For example, birds that are farther apart may have more difficulty acquiring information about food or predators from each other but may have reduced costs due to fewer aggressive interactions. Thus, spacing within flocks may reflect a balance between attractive and repulsive forces between flock members (Emlen 1952). Distances between flock members are likely to change over time scales of minutes or hours as attractive and repulsive forces vary with changes in flock size and composition (Caraco and Bayham 1982, Pearson 1989), energy reserves of flock members, or environmental factors such as habitat structure, food availability, and predation pressure (Grzybowski 1983, Prescott 1987, Pearson 1991). In this paper I examine spacing among Brown-headed Nuthatches (Sitta pusilla), Carolina Chickadees (Parus carolinensis), and Tufted Titmice (P. bicolor) within heterospecific flocks. The basic goal was to determine whether distances between birds varied either as food availability varied or as apparent threat of predation varied. Three hypotheses were tested by experimentally varying food availability and apparent predator abundance. These are: (1) Nuthatches and chickadees flock less closely when these species have less overlap in their food resources; (2) Nuthatches and parids flock less closely when food is easily obtained by both species; and (3) Nuthatches and parids flock more closely when there is a relatively high likelihood of encountering a predator.