Bird migration
Bird migration is birds’ regular and annual seasonal journey between their breeding and wintering grounds.[1] Birds that migrate usually travel along north and south flyways and often fly vast distances. Animal migration is inherently risky.[1]
"Bird of passage" redirects here. For other uses, see Bird of Passage (disambiguation).
The Arctic tern holds the long-distance migration record for birds, travelling between Arctic breeding grounds and the Antarctic each year.[1] Some species of tubenoses, such as albatrosses, circle the Earth, flying over the southern oceans, while others such as Manx shearwaters migrate 14,000 km (8,700 mi) between their northern breeding grounds and the southern ocean. Shorter migrations are common, while longer ones are not. The shorter migrations include altitudinal migrations on mountains such as the Andes and Himalayas.
The timing of migration seems to be controlled primarily by changes in day length. Migrating birds navigate using celestial cues from the Sun and stars, the Earth's magnetic field, and mental maps.[1]
Nocturnal migratory behavior[edit]
During nocturnal migration ("nocmig"[39]), many birds give nocturnal flight calls, which are short, contact-type calls.[40] These likely serve to maintain the composition of a migrating flock, and can sometimes encode the sex of a migrating individual,[41] and to avoid collision in the air.[40] Nocturnal migration can be monitored using weather radar data,[42] allowing ornithologists to estimate the number of birds migrating on a given night, and the direction of the migration.[43] Future research includes the automatic detection and identification of nocturnally calling migrant birds.[44]
Nocturnal migrants land in the morning and may feed for a few days before resuming their migration. These birds are referred to as passage migrants in the regions where they occur for a short period between the origin and destination.[45]
Nocturnal migrants minimize depredation, avoid overheating, and can feed during the day.[3] One cost of nocturnal migration is the loss of sleep. Migrants may be able to alter their quality of sleep to compensate for the loss.[46]
Adaptations[edit]
Birds need to alter their metabolism to meet the demands of migration. The storage of energy through the accumulation of fat and the control of sleep in nocturnal migrants require special physiological adaptations. In addition, the feathers of a bird suffer from wear-and-tear and must be moulted. The timing of this moult – usually once a year but sometimes twice – varies with some species moulting prior to moving to their winter grounds and others molting prior to returning to their breeding grounds.[96][97] Apart from physiological adaptations, migration sometimes requires behavioral changes such as flying in flocks to reduce the energy used in migration or the risk of predation.[98]
Ecological effects[edit]
The migration of birds also aids the movement of other species, including those of ectoparasites such as ticks and lice,[108] which in turn may carry micro-organisms including those of concern to human health. Due to the global spread of avian influenza, bird migration has been studied as a possible mechanism of disease transmission, but it has been found not to present a special risk; import of pet and domestic birds is a greater threat.[109] Some viruses that are maintained in birds without lethal effects, such as the West Nile virus may however be spread by migrating birds.[110] Birds may also have a role in the dispersal of propagules of plants and plankton.[111][112]
Some predators take advantage of the concentration of birds during migration. Greater noctule bats feed on nocturnal migrating passerines.[25] Some birds of prey specialize on migrating waders.[113]