The Effect of Acentric Colony Location on the Energetics of Avian Coloniality

Abstract

We evaluate the energetic consequences of avian coloniality when colony sites are not centrally located within a foraging region, and we determine how acentric a colony can be before the energetic advantage of shorter mean flight distance postulated by Horn (1968) is lost for average individuals. We present a general equation for making these calculations for any shape of foraging region and any food distribution pattern, and we specifically calculate values for circular, linear, and elliptic foraging regions. Our calculations show that mean flight distances are less for average colony members than for average members of a dispersed population as long as the colony is displaced no more than 70% of the radius from the center of a circular foraging region or no more than 66.7% of the half-length of a linear foraging region. For elliptic regions the degree of allowable displacement depends on both direction of displacement and shape of the ellipse. It varies from 40%-70% of the semimajor axis length, depending on length of the minor axis, when displacement is along the major axis. It varies from 34%-70% of the semimajor axis length when displacement is along the minor axis. We also present the range of allowable displacements when a colony is not displaced along one of the axes. We discuss the general implication of our results, the fact that not all individuals stand to benefit energetically from coloniality when colonies are acentric, the importance of shape of foraging region, and the fact that the energetic consequences of coloniality, whether positive or negative, must always be considered before the adaptive significance of avian coloniality can be fully understood.