Food Production and Competitor Density as the Determinants of Feeding Territory Size

An energy-time budget model is developed which predicts the influence of various environmental factors upon feeding territory size. For nonbreeding animals maintaining noncontiguous territories, territory size should (1) vary inversely with food production, but directly with competitor density, for feeding-time minimizers (defined here as animals that exhibit relatively fixed daily energy requirements); and (2) vary inversely with both food production and competitor density for food-energy maximizers, animals whose potential reproductive success is positively correlated with their net energetic intake. Concomitant predicted changes in time budgeting provide operational criteria for testing the model. Besides the primary effects of food and competitors, other factors may also influence territory size. Any competitors which successfully invade the territory can decrease the availability of food, forcing both time minimizers and energy maximizers to expand their territories. Where territories are contiguous, differential social dominance between neighbors can force certain individuals to contract their territories below the otherwise optimal size. Finally, sexual selection and other factors may result in males defending as large an area as possible. Whether between or during breeding seasons, territory size of such "area maximizers" should not be affected directly by changing food production, but should vary inversely with competitor density. As the breeding season progresses, subsequent reproductive activites may or may not result in territorial contraction, depending upon such factors as the degree of habituation between neighbors and the level of parental care. Although some of the predictions of the model are qualitatively supported by field evidence, data necessary for adequate quantitative tests are presently unavailable. Suggested experimental investigations of the time budgets of territorial animals under controlled environmental conditions will undoubtedly provide major refinements and revisions of the tentative ideas presented here.