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A flock is a gathering of individual birds to forage or travel collectively.
Avian Avian may refer to: *Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufacture ...
flocks are typically associated with migration. Flocking also offers foraging benefits and protection from predators, although flocking can have costs for individual members. Flocks are often defined as groups consisting of individuals from the same species. However, mixed flocks consisting of two or more species are also common. Avian species that tend to flock together are typically similar in
taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
and share morphological characteristics such as size and shape. Mixed flocks offer increased protection against
predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
, which is particularly important in closed habitats such as
forest A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' ...
s where early warning calls play a vital importance in the early recognition of danger. The result is the formation of many mixed-species feeding flocks.


Mixed flocks

While mixed flocks are typically thought to be composed of two different species, it is specifically the two different behaviours of the species that compose a mixed flock. Within a mixed flock there can be two different behavioural characteristics: sally and gleaner. Sallies are individuals that act as guards of the flock and consume prey in the air during flight. On the other hand, gleaners are those that consume prey living within vegetation. Studies have shown that as resources in the aerial environment increase, the flock will possess more sallies than gleaners. This has been shown to occur during forest fires in which insects have been flushed from vegetation, however this can also be done by the gleaners. When gleaners obtain meals from vegetation it causes the other prey within the vegetation to be flushed out into the aerial environment. It is through this specific behaviour of feeding among vegetation that the gleaners indirectly increase the foraging rate of the sallies. Those birds that are more rare and therefore less abundant in an environment are more likely to perform in this mixed flock behaviour. Despite the fact that this bird is more likely to be a subordinate, its ability to obtain food increases substantially. As well this bird is now less likely to be attacked by a predator because predators have a lower success rate when attacking large flocks.


Safety from predation

The ability to avoid predation is one of the most important skills necessary in order to increase one's fitness. It can be seen that by
ground squirrel Ground squirrels are members of the squirrel family of rodents ( Sciuridae), which generally live on or in the ground, rather than trees. The term is most often used for the medium-sized ground squirrels, as the larger ones are more commonly known ...
s living in colonies, the ability to recognize a predator is rapid. The squirrel is then able to use vocalizations to warn
conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organis ...
s of the possible threat. This simple example demonstrates that flocks are not only seen in bird species or a herd of sheep, but it is also apparent in other animals such as
rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
s. This
alarm call In animal communication, an alarm signal is an antipredator adaptation in the form of signals emitted by social animals in response to danger. Many primates and birds have elaborate alarm calls for warning conspecifics of approaching predator ...
of the ground squirrel requires the ability of the animal to first recognize that there is danger present and then to react. This type of behaviour is also seen in some birds. It is important to note that by making an alarm call to signal members of the flock one is providing the predator with an acoustical cue to the location of a possible prey. The benefit here is if the members of the flock are genetically related to one another. If this is true, even if the bird that signalled the flock were to die its fitness would not decrease according to
Hamilton's Rule Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution ...
. However another study involving thick-knees challenged whether or not an animal had to recognize the presence of a predator for protection against it. Thick-knees are birds that are seen in large flocks during particular seasons in various regions of the world. During the nonbreeding season, Peruvian thick-knees in Chile are reported to have an average of 22.5 birds — a mixture of adults and youngsters — in their flocks. Young birds were observed learning anti-predator behaviour strategies from adults during this time. Researchers believe that the flocking behaviour may help to decrease a predator's success rate when attacking the flock, rather than increasing the ability of the flock to spot an approaching predator. By birds co-existing with one another in a flock, less time and energy is spent searching for predators. This mutual protection of one another within the flock is one of the benefits to living within a group. However, as flock numbers increase the more aggressive individuals within the flock become towards one another. This is one of the costs to living within a flock. It is often seen that flocks are dynamic and thus fluctuate in size depending on the needs of individuals in order the maximize benefits without incurring a large amount of costs. By living in a large flock, birds are also able to attack the predator with a stronger force compared to if the bird was on its own. In the black-capped chickadees it can be seen that flocks of birds can produce a mobbing call when it visualizes a possible predator. In response the black-capped chickadees within the flock surround the predator and attack it in a mob-like fashion in order to force the predator to leave. This is known as
mobbing Mobbing, as a sociological term, means bullying of an individual by a group, in any context, such as a family, peer group, school, workplace, neighborhood, community, or online. When it occurs as physical and emotional abuse in the workplace, suc ...
. This mobbing behaviour is quickly learned by the juveniles within a flock meaning that these individuals will be better equipped as adults to ward off predators and respond rapidly when a predator is in sight.


Foraging in flocks

Bird species living in a flock are able to capture prey, likely injured, from an unsuccessful bird within its flock. This behavior is known as the beater effect and is one of the benefits of birds foraging in a flock with other birds. It can be seen that birds in a flock may perform the information-sharing model. In this situation the entire flock would search for food and the first to find a reliable food source will alert the flock and the entire group may benefit by this finding. While this is an obvious benefit of the information-sharing model, the cost is that the social hierarchy of the flock may result in subordinate birds being denied food by those that are dominant. Another cost is the possibility that some individuals may refuse to contribute in the search of food and instead simply wait for another member to find a food resource. These individuals are known as producers and scroungers, respectively. An intricate hunting system can be seen in the
Harris's hawk The Harris's hawk (''Parabuteo unicinctus''), formerly known as the bay-winged hawk, dusky hawk, and sometimes a wolf hawk, and known in Latin America as peuco, is a medium-large bird of prey that breeds from the southwestern United States south ...
in which groups of 2–6 hunt a single prey together. The group splits into smaller groups in which it then encloses on a prey, such as a rabbit, before it attacks it. By hunting as a group the Harris's Hawk is able to hunt larger animals and decrease the amount of energy spent hunting while each hawk in the group is able to eat from the catch.


Black sun

In the Denmark, there is a biannual phenomenon known as '' sort sol'' (
Danish Danish may refer to: * Something of, from, or related to the country of Denmark People * A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark * Culture of Denmark * Danish people or Danes, people with a Danish a ...
for "black sun"). This is when flocks of European starlings gather in vast numbers, creating complex shapes against the sky during the spring. It is during this time spent in Denmark that the European starlings spend time gathering food and resting as part of their migration journey. By being in groups this large the European starlings are able to decrease their risk of predation by hawks.


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