TERRITORIALITY (Concluded)

Floaters

If all individuals who attempt to obtain territories are successful, territoriality will simply result in a more uniform (even) distribution of individuals in suitable habitats. However, Julian Huxley suggested that territories may be likened to compressible "elastic disks." At low densities territories would be large and territory size would decrease as densities increase, but there would be a lower limit to territory size, i.e., the size of the territory can be compressed only so far by intruder pressure. Although the feeding territories of nonbreeding nectivorous birds and sanderlings seem to continue to shrink as intruder pressure increases, and those territories may be abandoned completed if intruder pressure is high, the territoriality in many breeding birds (and some nonbreeding birds, e.g., Red Grouse) seems to fit the Huxley's elastic disk model, i.e., there is a lower limit to territory size. Thus at high densities some individuals may be excluded from holding territories by the territorial behavior of others (see handout); such adults are called "floaters."

Numerous studies of breeding bird populations have indicated the presence of nonbreeding individuals, i.e., floaters which do not establish territories. Susan Smith (S.M. Smith. 1978. Am. Nat. 112:571-582) demonstrated that surplus male and female Rufous-crowned sparrows (Zonotrichia capensis), occur in some instances and are capable of breeding if they have a chance.

Recent studies are increasing our appreciation of the behavioral strategies of adults that are physiologically capable of breeding but are unable to compete successfully for territories. Although such adults are called floaters, this is an inappropriate label for nonbreeders in at least some species. As one example, Susan Smith (referenced above) found that nonbreeding adult Rufous crowned Sparrows of both sexes establish within-sex dominance hierarchies in areas where territories occur (see handout). The locally dominant nonbreeder will become the breeder if the territory owner of the same sex dies (or is removed).

In some instances excluded adults may adopt "alternative strategies" and achieve some breeding success. For example, Great Tits are hole-nesters that defend all-purpose territories, and they readily nest in nest-boxes. A study of a high-density population in Belgium (Dhont and Schillemans. 1983. Anim. Behav. 31:902-012) showed that some birds, ìintruders,î that were initially excluded by territorial birds, ìhosts,î would then attempt to nest in unused nest boxes on established territories. Some "intruder" pairs did nest successfully; on average they produced 4 fledglings vs. an average 8 fledglings/nest by territorial owners. Reproductive rates of ìintrudersî were higher the farther they nested from the ìhostsî and the longer they laid after the ìhosts.î Pairs nesting alone had higher success (8.9 fledglings/pair) than those nesting on territories with intruders (7.3 fledglings/pair). Thus territorial birds were unable to maintain exclusive use when the population was at high densities. This example demonstrates that exclusive use of a territory can be eroded not only by intrusions by foraging birds, but also by birds adopting alternative breeding strategies once they have been apparently excluded from breeding because they could not obtain territories.

Superterritories

Jared Verner suggested that individuals should defend territories larger than the size they need if this results in exclusion of more individuals from territories, i.e., they should behave spitefully. His idea drew considerable criticism. Individuals that defend superterritories, by implication, are defending territories that are larger than those which would maximize the benefit:cost ratio. While it is true that they may exclude some individuals from territories as a consequence and thus have higher success relative to those individuals, their success relative to those defending territories of optimal size would be reduced. Therefore, superterritoriality would not be an Evolutionarily Stable Strategy (ESS).

Wolf territoriality and predation on deer

David Mech (1977. Science 198:320-321) has shown that spacing behavior of wolf packs results in bands of unutilized areas up to about 2 km wide between territories. In a declining deer herd in Minnesota, surviving deer inhabited these edge areas (see handout, Figure 9-24). Wolves only hunted there when desperate in order to avoid possibly fatal encounters with neighbors. Wolf territoriality apparently reduces wolf numbers and predation pressures, allowing surviving deer along territory edges to repopulate the area through presaturation dispersal of prime, less vulnerable, individuals into territory cores. Thus wolf territoriality in Minnesota has implications for population control and pack persistence.

Prior experience

As studies of nonbreeding adults begin to accumulate, it is clear that such individuals behave in ways that increase the probability that they will obtain territories in the future. For example, nonterritorial male Red-winged Blackbirds that live in area one year are more likely to obtain a territory the following year than ones that first arrive the following year. Judith Stamps (1987. Behav. Ecol. Sociobiol. 21:273-277) experimentally showed that Anolis lizards that lived near available home sites were more successful in obtaining them than newcomers--the former made fewer mistakes of attempting to inspect home sites already occupied by larger lizards who would attack any intruders.

COLONIALITY

Coloniality and territoriality are opposites in terms of spatial distribution of individuals, pairs, or groups. Territorial individuals (or groups) are ìoverdispersed", resulting in a more uniform (even) distribution than would be expected by chance; in contrast, colonial individuals (or groups) are far more clumped than would be expected by chance. We can define a colony as ìa clumping of individuals in time and space, a place where a number of individuals or pairs nest in a more or less centralized location from which they recurrently depart in search of food.î

Among birds coloniality is relatively rare in songbirds (exceptions include most swallows), but it is widespread in seabirds. In birds itís likely that coloniality evolved from territoriality, so people have considered the factors that would result in the evolutionary switch from territoriality to coloniality. Presumably territory size shrank, enabling individuals, pairs or groups to clump more closely.

In 1968 Henry Horn presented a model showing that the distance to clumped, moving food resources (e.g., insect swarms, fish schools) would be less if individuals clumped in the geometric center of the food distribution than if they are dispersed (see handout, Figure 8-1). This model either does not apply or would have to be modified for most birds, however, because colony placement typically cannot be in the center of the food distribution, and they cannot forage in all directions from the colony, e.g., seabirds - colonies may be considerably removed from the center of their food supply. However, many people still argue that the chief factor in the development of colonial breeding is the presence of unpredictable or abundant food resources that are far from nesting habitat and thus can't be defended easily. Although most colonial birds do not defend feeding territories, white-fronted bee-eaters are colonial and "clans" defend feeding territories that may be several km away from the colony (we'll later see a short video of this).

Predation is likely a key factor. If individuals are cryptic when on their nests, territoriality is favored because it results in relatively uniform spacing. As densities increase, this strategy becomes less effective. For both Great Tits and Red-winged Blackbirds, predation rates on nest contents are positively correlated with nest densities. Body size is probably an important consideration also. It is more difficult for larger-bodied species to be cryptic. Consequently, territoriality may be ineffective for large-bodied species, particularly when densities are high. To avoid generalist predators, individuals might then clump in areas inaccessible to those predators, e.g., on islands or on steep cliffs.

Once coloniality has evolved, we can consider various benefits and costs. In terms of food, the colony may provide information about locations of food sources, e.g., unsuccessful foragers may be able to detect successful foragers (e.g., full crops, food in bill) and follow them on their next trip. Thus, colonies may act as information centers. This has been documented for ospreys when adults return with schooling fish (alewife, pollock, smelt) (see handout, Figure 13-8) and cliff swallows, but doesnít seem to apply to most seabirds. Studies of most colonial birds have shown that individuals generally DO NOT follow successful foragers when the successful birds next leave to forage. In the Black-headed Gull, however, individuals flying away from the colony give a call to attract others so that they form groups as they fly off. Individuals foraging in groups have higher success finding food patches and may have higher success in capturing prey due to disruption of prey grouping behavior (see discussion article by Gotmark et al.). On the other hand, grouping at food patches may result in increased competition.

Nesting in relatively inaccessible habitats decreases the predation rates by generalist predators but may increase vulnerability to predators that can capture prey effectively in those habitats. Coloniality may result in increased ability to detect predators, for example, prairie dogs in larger colonies (wards) take less time to respond to predators and to give alarm calls than those in smaller colonies (see handout, Figure 8-4) and repel (group defense, mobbing), and others in the group may provide cover ("selfish herdî effects). Most colonial species are monogamous. The potential for extrapair copulations is increased in colonies. This is a benefit for those who participate in extrapair copulations and a cost for those whose mates participate in them. Certainly, the potential for transmission of parasites and diseases is increased by coloniality, e.g., Charles Brown has shown that mite infestations on nestlings are positively correlated with colony size in cliff swallows (see handout, Figure 13-3).

Information centers

There has been considerable interest in whether or not communal roosts and breeding colonies of certain birds are important sources of information on the location of prey; i.e., are they information centers? Evans (1982. Auk 99:24-30) discusses the possible contexts in which information transfer could occur at roosts and colonies. He found that "attractive" calls sometimes were given by Black-billed Gulls leaving the colony and that calling birds recruited additional flock members more often than silent birds. Thus any information transfer or sharing seemed to occur away from the colony rather than among neighbors at the colony. Andersson et al. 1981. Behav. Ecol. Sociobiol. 9:199-202) found that Black-headed Gulls feeding at a rich artificial food source returned alone on subsequent trips.

Cliff Swallow colonies do seem to function as true information centers (Brown, C.R. 1986. Science 234:83-85). At Cliff Swallow colonies, individuals often forage with neighbors. Unsuccessful individuals tend to return to the colony, locate a successful forager (one carrying insects in its bill) and then follow that individual to a food source. Successful foragers, in contrast, did not tend to follow others on their next trip. Individual success varied through time, and all individuals tended to be followers about 40 percent of the time. The handout shows that osprey colonies clearly function as information centers when successful foragers return with schooling fish.

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