Bird migration is the regular seasonal movement, often north and south along a flyway between breeding and wintering grounds, undertaken by many species of birds. Migration, which carries high costs in predation and mortality, including from hunting by humans, is driven primarily by availability of food. Migration occurs mainly in the Northern Hemisphere where birds are funnelled on to specific routes by natural barriers such as the Mediterranean Sea.
Historically, migration has been recorded as much as 3,000 years ago by Ancient Greek authors including Homer and Aristotle, and in the Book of Job, for species such as stork, Turtle Doves, and swallows. More recently, Johannes Leche began recording dates of arrivals of spring migrants in Finland in 1749, and scientific studies have used techniques including bird ringing and satellite tracking. Threats to migratory birds have grown with habitat destruction especially of stopover and wintering sites, as well as structures such as power lines and wind farms.
The Arctic Tern holds the long-distance migration record for birds, travelling between Arctic breeding grounds and the Antarctic each year. Some species of tubenoses (Procellariiformes) 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, including altitudinal migrations on mountains such as the Andes and Himalayas.
A flock is a group of birds conducting flocking behavior in flight, or while foraging. The term is akin to the herd amongst mammals. The benefits of aggregating in flocks are varied and flocks will form explicitly for specific purposes. Flocking also has costs, particularly to socially subordinate birds, which are bullied by more dominant birds; birds may also sacrifice feeding efficiency in a flock in order to gain other benefits. The principal benefits are safety in numbers and increased foraging efficiency. Defense against predators is particularly important in closed habitats such as forests where predation is often by ambush and early warning provided by multiple eyes is important, this has led to the development of many mixed-species feeding flocks. These multi-species flocks are usually composed of small numbers of many species, increasing the benefits of numbers but also increasing potential competition for resources.
Group size is a major aspect of the social environment of gregarious animals. However, one has to be careful when using group size measures to characterize animal sociality, because average individuals live in groups larger than mean group size.
In Denmark, there is a biannual phenomenon known as sort sol (Danish for "black sun"), when flocks of European Starlings gather in vast numbers, creating complex shapes against the sky.