Territoriality & Coloniality
A 'territory' can be defined as any defended area & most birds are territorial (in the sense that they defend some area, even if just a nest site) for at least during part of their annual cycle. The advantage or benefit of defending a territory is that the 'owner' has access to a resource or more of a resource (or access to a better quality resource) than they would otherwise have. Territories may be classified based on what resource (or resources) is(are) being defended:
|Male Red-winged Blackbirds establish and defend 'Type B' territories with clearly delineated boundaries during the breeding season. All activities occur within territories, but males and females also forage, engage in sexual chases, seek extra-pair copulations, and prospect for other breeding opportunities outside territorial boundaries. Extent of off-territory foraging varies among nesting habitats and locations. Defense is based on conspicuousness, song and visual display, and aggressive responses to persistent trespassers. Mean territory size is approximately 2,000 square meters, but size varies greatly; territories are smaller in marsh than upland habitats.|
Northern Gannet colony
Greater Prairie Chicken displaying at a lek
|Winter territories of Hermit Thrushes (Brown et al. 2000) -- Many species, including perhaps a majority of non-flocking
migrant passerine species, establish resource-based territories that are
defended for at least part of the winter. Territory defense and exclusion
based only on resource availability are likely indicators that habitat
availability and food resources are limiting factors during the winter.
Several detailed investigations of territorial wintering migrants have
revealed that dominant individuals are able to maintain stable territories
throughout the winter, while some subordinate birds are forced into a non-territorial
strategy in search of resources. Floater (non-territorial) behaviors have
only recently been recognized within wintering populations, although the
behavior may be as common as on the breeding grounds. Floaters are also
considered indicators of habitat and/or food resource limitation, providing
important insight into a population's local distribution.
Brown et al. (2000) used radio-telemetry to measure Hermit Thrushes' movements and territoriality, and found that the thrushes saturated suitable patches in the study area. Hermit Thrushes established and maintained territories using the same agonistic behaviors described for breeding birds. A few non-territorial birds (14%) moved among occupied territories, but most were faithful to a larger neighborhood, apparently awaiting a territory vacancy. The behavior of Hermit Thrushes conformed to the emerging view 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.
Photo by David Roemer
in relation to 9 ha mist-net plot (box). Home range contours are
also shown for territorial birds. Lines indicate movements
between successive locations. B. Territory contours
(95% hamonic mean method) of eight radio-tracked Hermit
Thrushes showing variation in territory size and overlap
Costs & benefits of territorial defense
Defending a territory requires time & energy and may, if aggressive defense is needed, may pose some risk of injury (or, in extreme cases, even death). So, time, energy, & risk of injury are the 'costs' of territory defense. The improved access to resources (e.g., food, nest sites, or roost sites) represents the 'benefit(s)' of territory defense. Birds should only defend territories if the benefits of defense outweigh the costs. This idea of 'economic defendability' was first proposed by Jerram Brown (1964).
If a territory is 'economically defendable', the energy available in a territory should not be less than energy expended in territory defense. Gill and Wolf (1975) tested this hypothesis on Golden-winged Sunbirds:
When should a bird defend a territory? What is the range of resource levels over which territory defense is worthwhile? What if resources are:
For example, Gill & Wolf's (1975) study of Golden-winged Sunbirds:
Figure modified a bit from Gill and Wolf (1975)
So, when resource levels (e.g., nectar) are high, the cost of defense likely
exceeds potential benefits.
Territoriality and Reproductive Success
Many birds defend territories only during the breeding period, and these territories often include resources, such as food or nesting sites, necessary for successful reproduction. In polygynous species, differences in the quality of territories may influence the number of mates a male obtains. For example, male Dickcissels (Spiza americana) with better quality territories (greater vegetation density) may attract more mates those those with lower quality territories. In socially monogamous species, territory quality can also influence reproductive success (e.g., Przybylo et al. 2001).
Male Dickcissel singing
How do birds defend territories?
Male Ring-necked Pheasants fighting
Two buzzards interacting followed by a smaller European Hobby 'mobbing' a Common Buzzard -
attempting to evict the buzzard from its territory (where it's a potential threat to
the hobby and its offspring)
Marking territories with feathers and feces? -- Many animals communicate by marking focal elements of their home range with different kinds of materials. Visual signaling has been demonstrated to play a previously unrecognized role in the intraspecific communication of Eagle Owls (Bubo bubo), with a white badge on the throat that is exposed only during vocal displays and a white border of feathers at the edges of mouths of young Eagle Owls just before fledging that becomes less apparent after dispersal. Visual signals may play a role in a variety of circumstances in this crepuscular and nocturnal species. Penteriani and del Mar Delgado (2008) found that a large amount of extremely visible white feces and prey feathers appear during the breeding season on posts and plucking sites in proximity to the nest, potentially representing a way for eagle owls to mark their territories. This novel signaling behavior could indicate the owls' current reproductive status to potential intruders, such as other territorial owls or non-breeding floaters. Feces and prey feather markings may also advertise an owl's reproductive status or function in mate-mate communication. Penteriani and del Mar Delgado (2008) speculate that feces marks and plucking may represent an overlooked but widespread method for communicating current reproduction to conspecifics and such marking behavior may be common in birds.
To increase the conspicuousness of fecal signaling, Eagle Owls mark the most prominent rock surfaces.
Population density & territorial behavior
How territorial behavior might limit density:
1) Jerram Brown (1969) suggested that, in any given area, habitats available to breeding birds vary in quality, ranging from very high quality habitats to habitats unsuitable for breeding:
2) Steven Fretwell (1972) proposed that, in any given area, habitats vary in quality or suitability, ranging from good (e.g., Rich habitat in the diagram below) to poor. In addition, the 'suitability' (in terms of the fitness a bird could attain or, in the diagram below, rewards per individual) of habitats decreases with increasing population densities (more birds means fewer resources per bird). So, when densities are high in the good (or rich) habitats, a bird may do better by occupying the lower quality, or poor, habitat (as long as densities are still low there). As a result, birds are not necessarily prevented from breeding (or become floaters) but simply settle where they can attain the highest fitness.
For many species, it appears that Brown's model may best explain the influence of territorial behavior on population size. For example, a population of Song Sparrows on an island off the coast of British Columbia has been studied for several years (Smith et al. 1991).
|Strategies of male floaters -- Male birds lacking a home nest ('floaters') were previously thought to be young, lower quality or subordinate individuals that could not compete with senior males for a territory, a nest, or a mate. But Kempenaers et al. (2001) found that some floaters are fit enough to breed -- they are just not as fit as those males with mates. Kempenaers et al. (2001) studied mating behaviour and paternity in Tree Swallows, and found fewer paired males copulating with paired females than could account for all the 'illegitimate' offspring. Analysis revealed that male floaters were responsible for up to a quarter of extra-pair offspring. Kempenaers et al. (2001) also found that single males reproduce more than stay-at-home (faithful) males, but less than unfaithful, straying males. Which is surprising, "since most arguments about the benefits to females of having extra-couple offspring focus on possible genetic benefits," Birkhead remarks. "It is counter-intuitive to have females being fertilized by sub-optimal males". Kempenaers and colleagues are now trying to understand what benefits promiscuity holds for female swallows, which share family duties with their male partners. "We have shown that eggs are more likely to hatch in nests containing illegitimate young; and sperm from different males is present on the shell of every single egg," Kempenaers explains. This suggests that promiscuity might be a way for females to select the fittest young, and that selection could occur at the stage of sperm-egg interaction. -- Valerie Depraetere, Nature Science Update|
Ecological traps: perceptual errors and undervalued resources (Gilroy and Sutherland 2007) -- Ecological traps occur when organisms choose poor-quality habitats above better alternatives. This phenomenon has received increasing attention from researchers interested in the relationship between population maintenance and habitat alteration. Both theoretical and field studies have suggested that errors made in judging habitat quality influence the fate of populations under environmental change. One example is the high rate of nest predation suffered by birds nesting preferentially at the edges of forest fragments. Human activities re-shape the appearance and biological functioning of habitats, challenging native organisms to adapt to novel surroundings. If habitat alterations cause a formerly suitable area to become inhospitable or if a poor-quality novel habitat mimics features of a better one, organisms might be ‘trapped’ by their evolved habitat preferences, settling in sites that cannot support them. However, populations might also be affected by a reversal of this phenomenon. If a minor change makes a habitat less attractive to settlers, without affecting habitat quality, opportunities for colonization might go unexploited, creating what we call an undervalued resource, e.g., scarecrows that deter avian pests from settling in crop fields. The scarecrow acts as an erroneous indicator of risk, discouraging settlement in an otherwise freely available habitat.
The source or sink status of high-quality habitats depends on the density of individuals populating a site (Figure above). In this simple model, a high-quality habitat (black line) has sufficient resources to support a certain population size. Below this maximum carrying capacity, the habitat will appear to function as a source, as individuals experience increased fitness (and productivity) owing to release from competition. If the population size increases above the carrying capacity, the habitat becomes a pseudo-sink, as competition for resources (or other density-dependent forces) cause mean individual fitness to decline. Poor-quality sites (grey line) will always function as sinks, as settlers will experience low fitness regardless of population size.
The correlation between population density and habitat quality is most likely to 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 [Figure above; perceived (blue bars) versus actual (yellow bars) resource quality]. Under accurate habitat assessment (a), population size will increase until mean fitness falls below a level attainable in alternative unoccupied habitats (in this case fitness ŵa). Hence, at population size a, settlers will begin to use alternative sites. If habitat quality is overestimated (b), population size will increase beyond point a, reducing mean fitness (ŵb) and creating a pseudo-sink (point b). If habitat quality is underestimated (c), population size might remain small (point c), and individual fitness might increase owing to competitive release (ŵc). However, significant underestimates of habitat quality will result in under use of the habitat and, thus, the total population size might be lower than otherwise attainable.
Refs: 6 - Gates and Gysel (1978), 9 - Flaspohler et al. (2001), 45 - Donald et al. (2002), 46 - Odderskær et al. (1997),
47 - Kershner and Bollinger (1996), 7 - Boal and Mannan (1999), and 48 - Shochat et al. (2005).
The impact of a trap can be minimized if habitat quality is improved so population growth becomes positive (e.g. through predator control in traps caused by high predation rates). An alternative approach is to discourage settlement in the trap, by removing the erroneous cues that cause organisms to prefer a poor-quality habitat. A third approach is to focus on alternative habitats. If an undervalued resource is present, the introduction of an appropriate settlement cue might offset the negative effects of the trap by encouraging a higher proportion of settlers to choose productive habitats. Gilroy and Sutherland (2007) applied this framework to several published cases to illustrate the appraisal process (see Table above). Of the five examples considered, removal of settlement cues in the trap habitat was likely to be effective in just one case, whereas the introduction of cues to undervalued resources might have been effective in four. Improving habitat quality in a trap was deemed to be feasible in all five examples, generally involving the control of predator or brood parasite numbers to improve the success rate of nesting birds. Application of this framework reveals several key requirements for effective ecological trap studies, including the specific identification of settlement cues, consideration of available alternative habitats, and quantification of the population response to manipulations of both habitat quality and cue distribution.
|Colonial nesting in Yellow-headed Blackbirds -- Yellow -headed Blackbirds (Xanthocephalus xanthocephalus) in Manitoba breed in dense colonies in cattail marshes. Their reproductive success is affected mainly by predation. The most important predator on blackbird nests is the Marsh Wren (Cistothorus palustris), which breaks blackbird eggs and kills small nestlings. Picman et al. (2002) examined whether colonial nesting in Yellow-headed Blackbirds may represent an adaptation to reduce Marsh Wren predation. Marsh Wren predation may be reduced by (1) mutual nest defense by adult blackbirds, (2) predator satiation or dilution, or (3) selfish-herd effects. They tested these hypotheses using experimental nests and found that their safety increased with decreasing distance to the nearest blackbird nest and with increasing density of simultaneously active blackbird nests located nearby. Safety also was higher for nests placed inside a blackbird colony rather than outside. These findings support the nest-defense hypothesis. Picman et al. (2002) also found that Marsh Wrens are capable of destroying a whole blackbird colony in a few days, and that colony size is not correlated with nest safety. These results suggest that the satiation or dilution benefits are negligible. Finally, they found that central nests were safer than peripheral nests in a blackbird colony, but not in an artificial colony, providing weak support for the selfish-herd hypothesis. Picman et al. (2002) concluded that nest predation is reduced mainly by mutual nest defense of adult birds and may represent an important selective force favoring colonial nesting in this species.||
Male Yellow-headed Blackbird
Photo by Jim Stasz
House Wrens also destroy the eggs and nests of other species, including Eastern Bluebirds.
|Genes Drive Cliff Swallows in Group Choice - In the classic debate of nature versus nurture, Brown & Brown (2000) have scored one for heredity - at least when it comes to Cliff Swallows. Their study suggests that genes guide Cliff Swallows when selecting the size of colony in which to live. "We clearly found that individuals have a genetic basis as to where they choose to live," said Charles Brown, who has studied Cliff Swallows along the Platte River in Nebraska for 19 years. The Browns switched almost 2,000 young birds from nests in big colonies to nests in small colonies (& vice versa), & discovered that when these birds returned about 9 months later to select their own nest sites, they chose the same colony size as that in which they were born. "Our study suggests that there is a genetic difference between birds that choose to live in large groups versus birds that choose to live in small colonies," Brown said. "They return to where they were born irrespective of where they were raised. They are picking the colonies that their parents picked; so it is not environment, it is genes that appear to be dictating their choice."|
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