Regulation of the Size of Bird Populations By Means of Territorial Behaviour

Abstract

This paper deals, from the general viewpoint of population dynamics, with some recent evidence from literature supporting the hypothesis on regulation of bird numbers by territorial behaviour. In addition, it considers the conditions, recently drawn up by Watson and Moss, which should be satisfied for showing that territorial behaviour limits breeding populations. In considering the evolution of territorial functions, it is discussed that it is improbable that territoriality has been evolved in relation to the prevention of overpopulation, because density is a group-feature, which cannot be favoured by natural selection. In discussing the conditions formulated by Watson and Moss with relevance to population limitation in any one year, it is argued that the condition stating that a substantial part of the population should not breed, is too extreme. In addition, some recent observations on unsuccessful settlements and the formation of surplus populations are considered, and some striking new results of experiments on removal of territorial birds are discussed. The results show a clear limiting effect of territorial behaviour on the size of breeding populations in several species. As to the role of territorial behaviour in setting a long-term limit to population size, it is stated that the proportion of potential breeding birds, being excluded from breeding by the behaviour of dominant, established birds, is density dependent, i.e., the proportion tends to rise with an increase of the number of potential breeders. As a result the breeding population fluctuates within certain limits set by the mean minimum territory size, which is variable year by year. The methods used to demonstrate a density dependent exclusion in Red Grouse and Great Tit populations are discussed. The results are not conclusive in all respects, and more data are needed on the same and other species. Minimum territory size is not constant. The most interesting variations are those between the means of successive years in one particular area, causing population fluctuations, and those between the means over successive years of various habitats, giving areas with high and low population levels. The ultimate and proximate implications of the determination of territory size are discussed. Most of the intra-specific differences between territory sizes observed in the field seem to be of a modificatory nature. The literature bearing to this problem is briefly reviewed. It reveals that territory size is determined by a complicated interaction of features, which we are now beginning to understand in the Red Grouse only. The model of Fretwell and Lucas, according to which a limiting effect of territorial behaviour on numbers is expressed in the relation between habitat distribution and population productivity, is discussed. It is concluded that studies on productivity can give indications as to the role of territoriality in the limitation of numbers, but that direct observations on behaviour and removal experiments are indispensable for giving the final proof.