Migration in Birds
Bird migration refers to the seasonal journeys to and from a given area undertaken by all or part of a bird population. Not all bird species (or even populations within the same species) are migratory. In contrast to more irregular movements such as emigration, nomadism, and invasion, which are made in response to changes in food availability, habitat, or weather, bird migration is marked by its cyclical pattern.
Migrations typically occur along established routes called “flyways.” The migrating species often return to the area of their birth to breed. The birds are guided by innate behaviors (including hormonal signals) that enable them to know when to depart and that orient them toward a specific location over long distances. However, they also remain flexible to environmental conditions, such as food supply and temperature, which may fluctuate yearly. The primary advantage of migration is energetic. In the Northern Hemisphere, the long days of summer provide greater opportunities for breeding birds to feed their young. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.
Migratory birds have evolved to undertake long-distance flights efficiently, and they undergo physiological changes (such as an accumulation of fat stores) prior to migration that minimize the energetic cost of flight. Bird migration has larger ecological implications that underscore the inter connectedness of life: Migratory cycles are closely attuned to seasonal food productivity cycles, which leads to a mutual gain for both the migrating species and the ecosystems in which they participate. Migratory birds are able to settle in areas where life is not tenable year-round, while the food resources of some regions would not be adequately utilized without the seasonal presence of migrating populations. Migrations may be diurnal (occurring during the day) or nocturnal. Many of the smaller insectivorous birds, including the warblers, hummingbirds, and flycatchers, are nocturnal migrants. By migrating at night, they minimize the risk of predation, and avoid the overheating that could result from the energy expended to fly such long distances.
Those smaller species that migrate during the day tend to be those making movements that are relatively short and weather-driven, like the larks and finches, or that can feed on the wing, like swallows and swifts