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

The basic hypothesis is that an animal which is defending a 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. (i) Territorial behavior maximizes net daily energy gain subject to constraints. (ii) Time spent sitting or resting is maximized subject to constraints. (iii) Daily energetic cost is minimized subject to constraints. (iv) The ratio of gross daily energy gain to the daily energy cost is maximized subject to constraints. The constraints that are included are: (i) Net daily energy gain must be nonnegative, and (ii) 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 data collected by Gill and Wolf (1975a), the "optimal" territory size and time budget are calculated under each of the four 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, it is found that 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 somewhat worse agreement between observed and expected values, while the remaining two hypotheses give very poor agreement. The extension of the model to deal with territory establishment and abandonment is discussed, and the present model of territoriality is compared with that of Carpenter and MacMillen (1976b).