Time Budgeting and Group Size: A Test of Theory

Abstract

Foraging flocks of granivorous Yellow—eyed Juncos (Junco phaeonotus) were studied for two winters to test hypothesized relationships between group size and time budgets. Required feeding time is so large that it constrains aggression. At low temperatures (high foraging requirements) a large flock forms in the area of maximal food availability. Group size decreases at warmer temperatures, since the constraints on aggression are reduced. Group size variance responds similarly. As group size increases, individuals scan less often for approaching predators. The time saved is employed to increase feeding rates, unless aggression levels are extremely high. Rates of aggressive interaction increase with group size, and with both ambient temperature and food density. Dominants apparently forage more efficiently than subordinates because of differences in habitat utilization and time allocation. These differences are consistent with the hypothesized correlation between increasing dominance and greater overwinter survivorship.