Social Systems and Status

Social organisation depends on the behavioural relationships between individuals and on their dispersion through the habitat. As a group, birds show a wide range of social systems, in some of which individuals may live alone or in pairs and in others gregariously, spending all their lives in the close company of others. The social system shown by any one species at a particular time depends partly on features of the species itself, such as its mobility and diet, whether breeding or non-breeding, and partly on local conditions, such as food-supply. Social systems are important in a population context because they provide the behavioural framework within which the regulation of density and distribution occurs.

Central to such regulation is the concept of social rank. Anyone who watches birds as they feed together can easily discern that some individuals are dominant over others. The dominants may drive subordinates from food-sources, but are seldom challenged themselves. Social status thus gives priority of access to resources, and can thereby affect the survival and reproductive success of individuals. In competitive situations, it can provide a means by which numbers are continually adjusted to the resources available, whether food, nest-sites, roost-sites, mates or any other requirement that is in short supply or of variable quality.

Aggressive interactions between individuals, whether momentary over food- items or long term over territory defence, provide a form of 'contest competition', in which resources are shared unequally among individuals. In consequence, the effects of resource shortages fall unevenly through the population. As the total numbers of competitors rise, or as resources dwindle, increasing proportions of individuals fail to meet their needs, and leave or die. If the effects of shortage were to fall evenly on all individuals, as in 'scramble competition', this could result in all succumbing. Contest competition through dominance could thus result in more individuals surviving a food-shortage than would occur under scramble competition, and could give much greater stability to populations (Lomnicki 1980). It also has a profound influence on which individuals survive (the most dominant) and which do not (the most subordinate). Among birds, older individuals of a species tend to be dominant over younger ones, and in most species, males (being larger) over females of similar age. Within winter flocks of geese and swans, however, the family is the main competitive unit, with large families dominating small ones, which in turn dominate pairs and single birds (Boyd 1953Raveling 1970Black & Owen 1989). Dominance status is one of two aspects of individual behaviour that can influence the feeding efficiency of individuals, the other being foraging skill.

SOCIAL SYSTEMS

The division of bird social systems into solitary and gregarious is somewhat arbitrary, and in practice gradations exist between exclusive highly defended territories at one extreme, through overlapping and sometimes clumped home ranges, to widely spaced but dense nesting colonies, communal roosts or feeding flocks at the other extreme. Our understanding of the ecological basis of these different systems is still poor. From comparisons between species, dispersion systems have been linked with patterns of food distribution: territoriality with evenly spread and dependable food-supplies, and colonial-flocking systems with patchy and unpredictable supplies (Crook 1965Lack 1968). From comparisons within species, it is clear that other factors are involved too. In particular, individuals can defend territories only when it is practical for them to do so, and when the benefits outweigh the costs – the concept of ‘economic defendability’ (Brown 1969). If food-supplies are too thinly or sporadically distributed, available at one place on one day and somewhere else the next, then over a period of days individuals must range over areas too large to defend. Similarly, if the numbers of intruders on a territory increase, the costs of defence for the owner become prohibitive, or flock- feeding may become more profitable (Tye 1986). While a bird might for a time defend a fruiting tree, for example (and indeed some birds do), so many other birds may be attracted to the feast that defence becomes impossible.

Cost–benefit analyses of territoriality have been done mainly for species that defend feeding territories for short periods of hours, days or weeks, and make continual adjustments to the area defended, as do some nectivorous birds (Gill & Wolf 1975Kodric-Brown & Brown 1978Gass 1979). In such species, calorific benefits and costs are relatively easy to quantify, and to manipulate, as individual birds can be given extra food in the form of sugar solution in special feeders. Territorial activity in Anna Hummingbirds Calypte anna was measured while energy availability on the territory was varied. On days when energy supplies were unlimited, residents defended highly exclusive territories, and primarily by expensive defence behaviour, such as chases. As energy supplies decreased, territories became less exclusive, defence changed towards less expensive (and less effective) methods, and owners spent more of their time feeding elsewhere (Ewald & Carpenter 1978). Hence, food-supply influenced both the intensity of defence and the degree of territorial overlap between neighbours. The same was found by experimental food provision in several other species, including Great Tit Parus major and Carolina Wren Thryothorus ludovicianus (Ydenberg 1984Strain & Mumme 1988).

In some species, rather precise figures on the relationship between energy budgets and behaviour have been obtained. In the nectar-eating Common Amakihi Hemignathus virens on Hawaii, individuals established territories only in areas that could provide at least 2000 μl of nectar per day (van Riper 1984). Experimental provision of extra food enabled birds to breed where they had not bred previously. Up to a point, breeding success was influenced by the levels of nectar available, with better breeding in areas with more nectar. But beyond a level of about 35 000 μl of nectar per day within the territory, successful breeding became impossible because, with higher intrusion rates, the owners spent too much time protecting their nectar sources. In another type of experiment, in which food availability was held constant while intruder pressure was increased (by installing extra feeders), Black-chinned Hummingbirds Archilochus alexandri defended smaller areas (Norton et al. 1982).

At very high resource levels, birds may cease to defend territories for another reason, namely that they have no need of the extra resources gained by defence. For example, in nectar-feeders, one advantage of territorial defence is that it raises the amount of nectar per available flower (by exclusion of nectar-thieves) and thus increases foraging efficiency (Gill & Wolf 1975). But if nectar levels are already high, the extra increment gained by territorial defence saves so little foraging time that the benefits of defence disappear.

In species that defend longer-term territories, over months or years, cost-benefit analyses are complicated by the fact that, while defence costs are incurred continuously, benefits may accrue with time: for example, if by defence now, the bird protects a food-supply or a nest-site for later use. Moreover, the benefits of territoriality cannot always be measured by net energy gain alone; they are likely in the longer term to involve other advantages, such as the ability to breed. In any interaction, owners may place a higher value on the territory (because of their higher investment and local knowledge), leading to some asymmetry in owner-intruder relations. Despite these considerations, however, the fact remains that territoriality is sustainable only if the defence costs do not plunge the bird into a prolonged energy deficit.

As food-sources become increasingly widely separated, or far from nesting areas, close individual attachment to an exclusive feeding area becomes impracticable. Social organisation may then shift to a more aggregated pattern, based on 'central places', such as nesting colonies or roosting sites, from which individuals range over wide areas for food. At such food-sources, birds may defend feeding territories if it is profitable to do so, or forage in flocks. Many rich food-sources, including fruiting trees, fish shoals or large carcasses, are sporadic, but for a short time provide enough food in one place for many birds to feed together. Other benefits gained from a gregarious lifestyle include information on feeding sites or techniques gleaned from other individuals, or increased protection against predation (Krebs et al. 1972Powell 1974Brown & Brown 1996). Such protection has several aspects: (1) the dilution effect in which the larger the group the less likely any one individual is to be captured during an attack, with birds in the centre of a flock being more secure than those on the edges; (2) the protection gained by group-mobbing and other collective defence behaviour; (3) the extra vigilance provided by more eyes. By sharing the vigilance costs, individuals in a flock also gain more time for feeding (Caraco 1979). These various advantages hold whether individuals feed in single-species or mixed-species flocks (Metcalfe 1989). Disadvantages of...