Test of a hypothesis of territory regulation in an insectivorous bird by experimentally increasing prey abundance.

Food availability is frequently hypothesized to be important in the regulation of territorial size. Recent theory suggests animals should respond to increased food availability by decreasing their territory. Most demonstrations of this relationship between food and territory are correlative, and few experimental tests of this hypothesis have been conducted.A field-experimental mainpulation was conducted to test three predictions of the hypothesis that food is a proximate stimulus for regulation of territory. The Rufous-sided Towhee, an insectivorous bird that is a permanent resident of chaparral in Arizona, was used to test the predictions that weekly area, total area, and fluctuations in area would be smaller within experimentally manipulated territories. Food resources were increased within five experimental territories and the response was compared with five control territories.The results did not support any of the three predictions. The hypothesis that food is a proximate stimulus for regulating territorial size was rejected. Two alternative hypotheses, that habitat quality or competition are proximate stimuli for regulating territories, could not be rejected.