Davies N.B. (1980) The economics of territorial behaviour in birds. ARDEA 68 (1-4): 63-74

I. If it is assumed that evolution has occurred by natural selection, then territorial behaviour can be interpreted in terms of the contribution it makes to an animal's survival and reproduction. Most studies of territoriality have been qualitative and although various selective pressures have been invoked, they have usually not been measured and are often just a reflection of the ingenuity of the observer; such factors as 'predator pressure' or 'patchy environment' can be used to 'explain' any social behaviour whatsoever. 2. Because, by definition, natural selection is an optimizing process, a more rigorous method for identifying selective pressures is to construct an optimization model. The model can be tested either by observing how well it reproduces the behaviour of the animal or by an experimental test of its predictions. If the model works, then it can be provisionally concluded that the selective pressures responsible for the behaviour have been correctly identified. 3. Early quantitative studies, inspired by Brown's concept of economic defendability, measured the costs and benefits of territoriality and showed that it was of net benefit. However, ideally we want to know whether it is more profitable than alternative strategies. This raises the difficult problem of defining and measuring the fitnesses of alternatives, where the payoffs may be frequency dependent and. where measurements should be made over an individual's lifetime. 4. In this paper I examine some quantitative studies of territorial behaviour in nectar-feeding birds (sunbirds, hummingbirds and honeycreepers) and an insectivorous bird (wagtail) where attempts have been made to answer in a quantitative way questions such as: When should an individual defend a territory? How should it allocate its time between defence, feeding resting, etc' and what sized territory should it defend? 5. All these species exploit renewing food supplies and search their territories systematically so as to allow for resource renewal between successive visits. Intruders are evicted because they reduce the owner's return times to patches in the territory. Because intruders often feed in patches that, unknown to them, have been recently depleted by the owner, the territory is less profitable to them than it is to the owner. 6. Defence strategies vary depending on the renewal pattern of the food resource. Experiments with hummingbirds show that individuals vary their territory size and defence in response to manipulations of the food supply. Observations show that honeycreepers have an upper and a lower threshold for economical defence of their territories. The use of an optimization model reveals that sunbirds adopt a territory size and time budget that minimises daily energy costs. 7. Wagtail owners may sometimes allow another individual (a 'satellite') to trespass on their territories. Under these conditions it. can be shown that an owner benefits from the association; its increased feeding rate arising through benefits gained by help with defence outweigh the costs incurred through having to share the food supply with another bird. When the owner would have a. higher feeding rate by being alone, it evicts the satellite from its territory. 8. Finally I discuss some of the problems associated with the quantitative approach to territorial behaviour, including alternative strategies, long term benefits and the measurement of payoffs when the territory is not solely concerned with feeding.