Bird colony

A bird colony is a large congregation of individuals of one or more species of bird that nest or roost in proximity at a particular location. Many kinds of birds are known to congregate in groups of varying size; a congregation of nesting birds is called a breeding colony. Colonial nesting birds include seabirds such as auks and albatrosses; wetland species such as herons; and a few passerines such as weaverbirds, certain blackbirds, and some swallows. A group of birds congregating for rest is called a communal roost. Evidence of colonial nesting has been found in non-neornithine birds (Enantiornithes), in sediments from the Late Cretaceous (Maastrichtian) of Romania. Approximately 13% of all bird species nest colonially. Nesting colonies are very common among seabirds on cliffs and islands. Nearly 95% of seabirds are colonial, leading to the usage, seabird colony, sometimes called a rookery. Many species of terns nest in colonies on the ground. Herons, egrets, storks, and other large waterfowl also nest communally in what are called heronries. Colony nesting may be an evolutionary response to a shortage of safe nesting sites and abundance or unpredictable food sources which are far away from the nest sites. Colony-nesting birds often show synchrony in their breeding, meaning that chicks all hatch at once, with the implication that any predator coming along at that time would find more prey items than it could possibly eat. What exactly constitutes a colony is a matter of definition. Tufted puffins, for example, are pelagic birds that nest on the steep slopes and rocky crevices on coastal cliffs, often on islands. Each pair excavates its own burrow. A congregation of puffin burrows on a marine island is considered a colony. Sand martins (called bank swallows in North America) are seldom, if ever, observed to nest in solitude; such a dependence on social nesting would term the bird a colonial nester. A more extreme example of colonial nesting is found in the weaverbird family. The sociable weaver of southern Africa constructs massive, multi-family dwellings of twigs and dry grasses, with many entrances leading to different nesting chambers, accommodating as many as a hundred nesting pairs. These structures resemble haystacks hanging from trees, and have been likened to apartment buildings or beehives. Some seabird colonies host thousands of nesting pairs of various species. Triangle Island, for example, the largest seabird colony in British Columbia, Canada, is home to auks, gulls, cormorants, shorebirds, and other birds, as well as some marine mammals. Many seabirds show remarkable site fidelity, returning to the same burrow, nest or site for many years, and they will defend that site from rivals with great vigour. This increases breeding success, provides a place for returning mates to reunite, and reduces the costs of prospecting for a new site. Young adults breeding for the first time usually return to their natal colony, and often nest very close to where they hatched. Individual nesting sites at seabird colonies can be widely spaced, as in an albatross colony, or densely packed like an auk colony. In most seabird colonies several different species will nest on the same colony, often exhibiting some niche separation. Seabirds can nest in trees (if any are available), on the ground (with or without nests), on cliffs, in burrows under the ground and in rocky crevices. Colony size is a major aspect of the social environment of colonial birds. Some birds are known to nest in colonies when conditions are suitable, but not always. The white-winged dove of southwestern North America was known to nest in large colonies when foraging areas could support such numbers. In 1978, in Tamaulipas, Mexico, researchers counted 22 breeding colonies of white-winged doves with a collective population size of more than eight million birds. But as habitat was transformed through urbanization or agriculture, the doves apparently spread out into smaller, less long-lived colonies. Today, these doves are observed to nest singly and colonially in both urban and rural areas. The term colony has also been applied, perhaps misleadingly, to smaller nesting groups, such as forest-dwelling species that nest socially in a suitable stand of trees. The red-cockaded woodpecker, an endangered species of southeastern North America, is a social species that feeds and roosts in family groups, or clans. Clans nest and roost in clusters of tree cavities and use a cooperative breeding system. Many parrot species are also extremely social. For example, the thick-billed parrot is another bird that nests and roosts communally; individuals of neighboring roosts has been observed to communicate with each other each morning to signal their readiness to form flocks for foraging. However, these complex social structures in birds are a different sort of group behavior than what is normally considered colonial. The habit of nesting in groups is believed to provide better survival against predators in several ways. Many colonies are situated in locations that are naturally free of predators. In other cases, the presence of many birds means there are more individuals available for defense. Also, synchronized breeding leads to such an abundance of offspring as to satiate predators. For seabirds, colonies on islands have an obvious advantage over mainland colonies when it comes to protection from terrestrial predators. Other situations can also be found where bird colonies avoid predation. A study of yellow-rumped caciques in Peru found that the birds, which build enclosed, pouch-like nests in colonies of up to one hundred active nests, situate themselves near wasp nests, which provide some protection from tree-dwelling predators such as monkeys. When other birds came to rob the nests, the caciques would cooperatively defend the colony by mobbing the invader. Mobbing, clearly a group effort, is well-known behavior, not limited to colonial species; the more birds participating in the mobbing, the more effective it is at driving off the predator. Therefore, it has been theorized that the larger number of individuals available for vigilance and defense makes the colony a safer place for the individual birds nesting there. More pairs of eyes and ears are available to raise the alarm and rise to the occasion.