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SOCIAL BEHAVIOR • Birds exhibit a wide variety of social behaviors either to live in groups, live cooperatively, or to compete for resources • Main topics = territoriality, dominance, and flocking TERRITORIAL BEHAVIOR • What is a territory? • A territory is a fixed area defended continuously in either the breeding or non-breeding season • Acts of defense or display to discourage rivals that would otherwise enter the territorial space • Primary = limited to the defending individual and perhaps its mate and progeny TERRITORY • The simplest territories are those with only 1 type of resource involved – Example = Hummingbirds defending specific flowers • In contrast, all-purpose territories of many land birds are used for male display, courtship, the nest site, and feeding • These territories space individuals and thus reduce predation, conserve essential resources, and reduce sexual interference TERRITORY SIZE - FOOD • Territory size varies with body size, energy requirement, and food habits of the species • Territory size has also clearly been shown to vary with habitat quality • Examples = – Pomerine Jaegers, an arctic species, will defend a territory of 19 hectares when lemming populations are high but 45 hectares when the lemming population is low – Hummingbirds and Sunbirds decrease territory size as flower density (nectar) increases TERRITORY – Density Dependence TERRITORY – Density Dependence • Territory size is also dependent on population density and the number of competitors • In 1934, Huxley proposed that bird territories could be likened to 'rubber discs.' • Example = – When Tree Sparrow density is low they only use 15-18% of their territories and concentrate activity in a core zone but defend a large buffer zone • As population density increases, territories (like rubber discs) are compressed (reduced in size) but only to a certain point – When population is high, denser packing eliminates the buffer zones between territories thereby reducing size • Therefore, there is a limit to the number of territories in an area which effectively limits the number of birds present 1 TERRITORY – Density Dependence • Several studies with a variety of species have revealed that males (and females) removed from their territories are usually replaced by conspecifics (and, often, replaced very quickly). • This indicates that there are 'floaters' (birds without territories) in at least some populations and suggests, that territorial behavior may limit population density. TERRITORY – Density Dependence • Winter territories of Hermit Thrushes (Brown et al. 2000) • Used radio-telemetry to measure Hermit Thrush movements • dominant individuals maintained stable territories throughout the winter • subordinate birds, floaters, forced into a non-territorial strategy in search of resources • floaters (14%) moved among occupied territories, but most were faithful to a larger neighborhood, apparently awaiting a territory vacancy. • The behavior of Hermit Thrushes confirmed that competition for spatially mediated resources on the wintering grounds, such as food or cover, contribute to limiting populations of many species of migrant passerines Territorial Behavior & Population Size Territorial Behavior & Population Size How territorial behavior might limit population size: Brown (1969) reasoned that, in any given area, habitats vary in quality, ranging from very high quality to low quality habitats: At low population densities, all birds occupy high quality habitats At higher population densities, some birds are excluded from the high quality habitats and must occupy lower quality (but still acceptable) habitats At still higher population densities, high quality and lower quality habitats are 'saturated' with territory-holders & some birds must then occupy unsuitable habitats. Birds in these unsuitable habitats become 'floaters.' Territorial Behavior & Population Size Example: Song Sparrows, British Columbia Territorial w/ submissive floaters Fretwell (1972) proposed that, in any given area, habitats vary in quality or suitability, ranging from good to poor. Proportion of floaters increases as the number of territorial males increases In addition, the 'suitability' (in terms of the fitness a bird could attain) of habitats decreases with increasing population densities (more birds means fewer resources per bird). So, when densities are high in the good habitats, a bird may do better by occupying the lower quality habitat (as long as densities are still low there) Territorial Behavior & Population Size As density increases other population parameters decline: fecundity juvenile survival Ideal Free Distribution Likely due to food limitation and increased competition 2 Ecological Traps High Quality Habitat Ecological Traps The correlation between population density and habitat quality can break down if organisms make errors in appraising habitat quality This can occur if the cues that illicit settlement become decoupled from the underlying resources Low Quality Habitat Gilroy and Sutherland 2007 Gilroy and Sutherland 2007 Ecological Traps COSTS AND BENEFITS • What is the central requirement for the cost of maintaining a territory to outweigh the benefits? • The resources must be economically defensible • Defensibility is influenced by temporal and spatial variation in the distribution of the resource HUNTER’S SUNBIRD COSTS AND BENEFITS • Resources that change rapidly in time are used opportunistically and difficult to defend • Example = – Aerial insects whose locations and densities shift frequently are not defendable resources – Rarely observe swallow (aerial insectivores) exhibit aggressive behavior between individuals while foraging BENEFITS MUST OUTWEIGH COSTS Defend territory Cost or Benefit Costs • Individuals that may be territorial over one type of resource may not defend other resources • Example = – Sunbirds are highly territorial but will sit on the same branch with other Sunbirds when they are foraging on passing insects Benefits Optimum Size Territory Size DOUBLE-COLLARED SUNBIRD 3 Economics of Territory Defense • A territory is economically defendable if the energy expended in defense is less than the energy available • Gill and Wolf (1975) tested this hypothesis on Golden-winged Sunbirds – Estimated energy expended and energy gained by: • Recording time spent by sunbirds sitting, foraging, & defending • Using lab experiments to calculate energetic costs (calories per hour) of each of these activities and established relationships between nectar availability and foraging time Economics of Territory Defense Found that: 1 microliter of nectar, sunbirds needed 8 hours of foraging 2 microliters of nectar, sunbirds needed 4 hours of foraging 3 microliters of nectar, sunbirds needed 2.7 hours of foraging Defense of a territory: ensured that no other sunbirds took nectar from available flowers nectar availability in each flower increased from 2 to 3 microliters In sum, territorial sunbirds: saved 1.3 hours of foraging time per day, or about 780 calories per day (= benefit) expended about 730 calories per day in defending territories (= cost). made a small energy 'profit' (benefits > costs) so territories were 'economically defendable.' COSTS AND BENEFITS - NO DEFENSE • Sites that attract hordes of potential competitors are usually undefended • Example = no gull would or could defend a dump site • Sanderlings only defend moderate density prey areas – do not defend dense concentrations of isopods because there are too many conspecifics – Nor do they defend low prey densities because it is not worth the cost TERRITORY—INTERSPECIFIC DEFENSE • Territorial defense is usually against conspecifics but is also commonly interspecific • Examples: – Hummingbirds will defend territories against other nectar-feeding species – Mocking birds will defend berry-rich feeding areas from all species – Woodpeckers will eject other species from cavities MOBILE RESOURCE DEFENSE • Territorial birds can defend a mobile resource, most commonly a mate = mate guarding – Constant defense of a female and her immediate space can help ensure paternity • Sanderlings will defend Willets from other Sanderlings when the Willets are feeding on large crabs – Bits of crab will fall and be eaten by the defending Sanderling DOMINANCE BEHAVIOR • Dominance and reinforcement of its status is very common among birds and is the driving behavior behind territoriality • As social ranks are established, losers seldom challenge the dominant individual and when they do they almost always loose • Dominants then have better access to space, food, mates, etc. 4 DOMINANCE BEHAVIOR DOMINANCE BEHAVIOR • Birds assert themselves more effectively when on familiar ground • Three hypotheses for why owners win: • 1) They are better fighters • Therefore, territory owners win encounters more often than intruders • 2) They have more information about resources and fight harder – Predicts original owner will win – Predicts gradual reversal of dominance • 3) Arbitrary asymmetry of ownership – possession is 9/10 of law • Territory owners know the area and have a known investment to protect – more likely to fight harder – Predicts present owner wins • Krebs (1982) tested these hypothesis with great tits in England – Removed owners and kept them in cages – Newcomers took over almost immediately – Data showed Hypothesis 2 was most plausible with newcomers more likely to win once they had time to learn territory • Example = Acorn Woodpeckers vigorously defend their granaries from squirrels, jays, and other woodpeckers DOMINANCE ADVANTAGES • Dominance confers selective advantages • Examples = Dominant juncos and field sparrows lived longer than subordinates FLOCKING BEHAVIOR • Birds commonly form flocks and there are only a few species that lead solitary life styles outside the breeding season Subordinate wood pigeons obtained less food per hour than dominants – Woodpeckers and both diurnal and nocturnal raptors are mostly solitary Low-rank individuals were also the first to leave the area • For most other species they come together in intra- and interspecific flocks at some point during the annual cycle During winter, dominant whitecrowned sparrows fed closer to the protective cover than did subordinates, which thus had a higher predation risk NEW ZEALAND WOOD PIGEON FLOCKING BEHAVIOR • Benefits of flocking include: – Reduced predation risk – Increased time foraging – Cooperative foraging strategies – Behavioral thermoregulation – Information exchange • Form when benefits outweigh the cost of being surrounded by competitors • Most common in non-breeding season • Food whose location is unpredictable in space and time favors intraspecific flocking FEEDING IN FLOCKS - COSTS AND BENEFITS • Insectivores often form mixed species flocks – Contain only few individuals of each species, reducing competition – Each species retains own niche within flock – Contain nuclear species plus attendant species – Increases foraging efficiency by increasing predator detection 5 Anti-predatory Benefits • Safety in numbers – minor benefit • Confuse predator – minor benefit • Shielding – benefit for some (dominants in center), not others (subordinates periphery) Foraging Benefits • Increased feeding time due to reduced predator scanning time – major benefit • The “beater” effect – Prey flushed by one flock member can be eaten by another • Detection of predators – major benefit – Example = ground hornbills in Africa walk a line across fields cooperatively flushing insects Foraging Benefits • Cooperative hunting – major benefit in a very few species (Harris’ Hawk, White Pelican) • Locating food – Primary reason for intraspecific flocking in species feeding on patchy, ephemeral foods – Benefit in mixed flocks because species forage in different ways within the same patch End Social Behavior 6