Migration

Main article: Bird migration
Many bird species migrate to take advantage of global differences of seasonal temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, shorebirds, and waterbirds undertake annual long distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the temperate or arctic/antarctic regions and a non-breeding season in the tropical regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.[51][99] Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around 2,500 km (1,600 mi) and shorebirds can fly up to 4,000 km (2,500 mi),[100] although the Bar-tailed Godwit is capable of non-stop flights of up to 10,200 km (6,300 mi).[101] Seabirds also undertake long migrations, the longest annual migration being those of Sooty Shearwaters, which nest in New Zealand and Chile and spend the northern summer feeding in the North Pacific off Japan, Alaska and California, an annual round trip of 64,000 km (39,800 mi).[102] Other seabirds disperse after breeding, travelling widely but having no set migration route. Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons.[103]
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. Irruptive species such as the boreal finches are one such group and can commonly be found at a location in one year and absent the next. This type of migration is normally associated with food availability.[104] Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates.[105] Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory.[106] Altitudinal migration is a form of short distance migration in which birds spend the breeding season at higher altitudes elevations and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when the normal territories also become inhospitable due to lack of food.[107] Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations.[108]

The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s a Manx Shearwater released in Boston returned to its colony in Skomer, Wales, within 13 days, a distance of 5,150 km (3,200 mi).[109] Birds navigate during migration using a variety of methods. For diurnal migrants, the sun is used to navigate by day, and a stellar compass is used at night. Birds that use the sun compensate for the changing position of the sun during the day by the use of an internal clock.[39] Orientation with the stellar compass depends on the position of the constellations surrounding Polaris.[110] These are backed up in some species by their ability to sense the Earth's geomagnetism through specialised photoreceptors.[111]
Communication

See also: Bird vocalization

The startling display of the Sunbittern mimics a large predator.Birds communicate using primarily visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).

Birds sometimes use plumage to assess and assert social dominance,[112] to display breeding condition in sexually selected species, or to make threatening displays, as in the Sunbittern's mimicry of a large predator to ward off hawks and protect young chicks.[113] Variation in plumage also allows for the identification of birds, particularly between species. Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds.[39] The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements;[114] males' breeding success may depend on the quality of such displays.[115]

Call of the House Wren, a common North American songbirdBird calls and songs, which are produced in the syrinx, are the major means by which birds communicate with sound. This communication can be very complex; some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs.[53] Calls are used for a variety of purposes, including mate attraction,[39] evaluation of potential mates,[116] bond formation, the claiming and maintenance of territories,[39] the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season),[117] and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat.[118] Some birds also use mechanical sounds for auditory communication. The Coenocorypha snipes of New Zealand drive air through their feathers,[119] woodpeckers drum territorially,[50] and Palm Cockatoos use tools to drum.[120]
Flocking and other associations

 While some birds are essentially territorial or live in small family groups, other birds may form large flocks. The principal benefits of flocking are safety in numbers and increased foraging efficiency.[39] Defence against predators is particularly important in closed habitats like forests, where ambush predation is common and multiple eyes can provide a valuable early warning system. This has led to the development of many mixed-species feeding flocks, which are usually composed of small numbers of many species; these flocks provide safety in numbers but reduce potential competition for resources.[122] Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases.[123]

Birds sometimes also form associations with non-avian species. Plunge-diving seabirds associate with dolphins and tuna, which push shoaling fish towards the surface.[124] Hornbills have a mutualistic relationship with Dwarf Mongooses, in which they forage together and warn each other of nearby birds of prey and other predators.[125]