Starling Roost Dispersal and a Hypothetical Mechanism Regulating Rhthmical Animal Movement to and from Dispersal Centers

A hypothetical mechanism is proposed that may regulate the dispersal pattern of animals aggregating at cores and dispersing from them into the surrounding terrain to forage. This hypothesis is that the number of individuals per unit of area in the space foraged by individuals using the same dispersal center declines with distance from the core, relaxing intraspecific competition for available resources, thereby compensating individuals that have ranged farther afield for the increased energy expenditure and time required to reach these more distant ranges. The compensation is the increased availability of resources due to reduced resource exploitation rates. The balance of advantages and disadvantages may be accomplished by comparison, each individual comparing and modifying alternative courses of action available to it. Since all individuals will be using essentially the same criteria for comparison, the result will be equivalent strategies for all individuals in terms of energy gain and loss. These hypotheses are applied to a California winter starling roost. In winter, California starlings aggregate in large numbers at roosts from which they disperse into the surrounding countryside to forage. Some individuals travel up to 50 miles daily to reach their feeding grounds. Other individuals disperse much shorter distances from the same roost. Strategies represented by 10, 20, 30, 40, 50 and 60 mile dispersal from the roost with respect to intraspecific competition, time available for foraging, and relative amounts of energy expended in various activities are compared. Only intraspecific competition promotes more distant dispersal from the roost. Measurements of starling densities at various ranges from the roost following morning dispersal show that the number of birds per unit of area declines steadily with distance from the roost. From these data it is assumed that competition is reduced at more distant ranges. Time and energy factors become less favorable at greater distances from the roost. Competition, both intraspecific and interspecific, promotes more distant dispersal from the roost. If the relative values for the components evaluated are assumed to be correct and the further assumption is made that the starlings are comparing alternatives and selecting optimum strategies, it follows that those strategies used by dispersing starlings will be equivalent to one another. It is thus possible to determine the relative weighting of the several strategy components and thus to measure variables such as time and competition in terms of calories.