THE BIOLOGICAL SIGNIFICANCE OF THE TERRITORIES OF BIRDS.

Summary.1 This paper forms an introduction to a symposium on the territorial behaviour of birds. 2 Following Noble (1939), territory is defined as “any defended area”. A simple classification of breeding territories is given, and the diversity of territorial behaviour emphasized. 3 Territorial behaviour can be analysed into at least two components—restriction of some or all types of behaviour to a particular area, and defence of that area. These are often accompanied by self‐advertisement. 4 The aggressive behaviour which arises in different situations depends primarily on the same mechanisms, though the fighting may be specialized to serve different functions (e.g. to defend different objects). This justifies the broad definition of territory given above. 5 The fighting involved in territorial defence is associated with tendencies both to attack and to flee from the rival. 6 The nature of the evidence concerning the biological functions of territorial behaviour is considered. In previous discussions of this subject the term “function” has sometimes been used to refer to any advantageous consequence of the behaviour, and sometimes restricted to consequences through which selection in favour of the behaviour can act. 7 Various possible functions of territory are considered.(a)  Circumstantial evidence suggests that the familiarity with the area which results from site attachment may assist feeding, escape from predators, etc., and may also increase fighting potentiality. (b)  There is strong evidence that territorial behaviour, in addition to producing over‐dispersion, can regulate density in favoured habitats. There is no direct evidence that territory limits the total breeding population in all habitats. (c)  In many species, it facilitates the formation and maintenance of the pair‐bond. (d)  It may also reduce interference in various reproductive activities by other members of the species. (e)  Defence of the nest‐site is an important consequence of territorial behaviour in many species, and the aggressive behaviour is often clearly specialized to this end. (f)  In a few species territory is primarily concerned with food, but in most the food value of the territory is not significant. Even in the species which feed on their territories, the territorial behaviour is not specialized for the defence of food objects:in these cases it is possible, but unproven, that the territorial behaviour does help to ensure an adequate supply of food for the young. (g)  In some species the over‐dispersion produced by territorial behaviour may reduce predation, though direct evidence that this is the case is not available. (h)  Maintenance of a territory may reduce the despotism of other males, but this does not explain the function of territorial aggressiveness. (i)  Territorial behaviour may reduce disease, but this is unlikely to be a significant consequence except in some colonial species. (j)  It is unlikely that the prevention of inbreeding and the promotion of range extension are significant consequences of territorial behaviour.8 The functions of territorial behaviour are extremely diverse, and the quality of the evidence available for assessing them is little different from that available to Howard. Since territorial behaviour has consequences both harmful and advantageous to the individual's chances of ultimate reproductive success, and since the inter relations between the selective forces governing behaviour, structure and physiology are extremely complex, simple answers about the function of territory cannot be expected.