The Economics of Territory Size and Time Budget in the Golden-Winged Sunbird

Abstract

An animal defending territory will adopt a territory size and a time budget which maximize its individual Darwinian fitness (i.e., which are optimal). Consideration of possible currencies of fitness leads to the following subhypotheses concerning territorial behavior: territorial behavior maximizes net daily energy gain subject to constraints, time spent sitting or resting is maximized subject to constraints, daily energetic cost is minimized subject to constraints and the ratio of gross daily energy gain to the daily energy cost is maximized subject to constraints. The included constraints are: net daily energy gain must be nonnegative, and the total amount of time available per day for the activities of feeding, sitting and territorial defense is fixed. A mathematical model of territory size and time budget is formulated for nectar-feeding birds. From this model and from other data, the optimal territory size and time budget are calculated under each of the 4 hypotheses for golden-winged sunbirds. When the observed average territory size and time budget for the golden-winged sunbirds are compared with the expected (i.e., optimal) territorial behavior, the best agreement is obtained in the case of the hypothesis that daily energy cost is minimized. The hypothesis that sitting time is maximized provides less agreement between observed and expected values, while the remaining 2 hypotheses give very poor agreement. The extension of the model to deal with territory establishment and abandonment is discussed.