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

Abstract

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.

Limited movement in blue rockfish Sebastes mystinus: internal structure of home range

“…Individuals only left these centers for brief radial movements or rapid and direct moves to another pinnacle. Similar patterns of radial movement from a central position have been observed among refuging birds, such as starlings (Hamilton et al 1966), and among hammerhead sharks (Klimley & Nelson 1984). In such central-core refuge systems, intraspecific competition for resources is assumed to decline with increasing distance from the center (Hamilton & Watt 1970).…”

Intercolony variations in movement patterns and foraging behaviors among herring gulls (Larus argentatus) breeding in the eastern Wadden Sea

“…Inter‐ and/or intraspecific competition (Corman et al., 2016; Hamilton, Gilbert, Heppner, & Planck, 1967) may thus be an important factor driving decisions on where to forage. Corman et al.…”

Population Ecology of the Gray Bat (Myotis Grisescens): Factors Influencing Growth and Survival of Newly Volant Young

“…In their discussion of starling foraging behavior Hamilton et al (1967) state, "Relatively small numbers of individuals using the same dispersal center will have a smaller impact upon available resources close to the center, reducing the relative value of more distant ranges. One was extremely distant, and the closer area was still the second most distant for the five colonies.…”