Bird migration is the mechanism behind the seasonal appearance and disappearance of some species of birds, mammals, fish and insects. In India and South Asia, out of over 2000 species and sub-species, about 350 are extralimital migrants. Generally, in birds, migration is seasonal, and in the Indian subcontinent the majority of migratory birds are winter migrants. In India, the physiology and mechanics of migratory bird flight are not very well known. The Bombay Natural History Society (BNHS) has been working since 1926 to rectify this shortcoming.
The first natural historian to write about migration as an observable fact was Aristotle. Though Herodotus described the migration of Cranes from north of the Black Sea to Central Africa 100 years before. Aristotle was an astute observer and as well as recording the times of departure of some species from Greece, and listing Pelicans, Turtle Doves, Swallows, Quail, Swans and Geese correctly as migrants he accurately observed that all migrating birds fatten themselves up before migrating.
In 1251 Matthew Paris writing in Hertfordshire recorded what is the first reference in England of the migration of Crossbills. By the 1600s good evidence had been supplied by the French ornithologist Pierre Belan to refute many claims of hibernation by the simple act of keeping the supposedly hibernating birds in a large aviary supplied with all the facilities it was claimed they needed to hibernate. None ever did.
It is important to remember that until the 19th century optical equipment was extremely rare, bird identification guides non-existent, travel to other countries difficult and expensive and bird ringing of course had not been invented. Moreover, in 1946 the Nuttalls Poorwill (Phalaenoptilus nuttallii) was found to be a bird that actually does hibernate, it does so in the Colorado Desert, California where it lives.
Scientific investigation of bird migration began in 1802 when birds were tagged with metal leg bands. It was not until this century when large numbers of bands with printed numbers and letters became available that this method really began to deliver results. The numbering of the rings is controlled by a national body in most countries and the rings have a contact address on them. These national bodies co-operate with each other in exchanging information on banding records (either live caught or found dead) of birds ringed outside the country in which they are caught.
Hundreds of thousands of birds are banded around the world each year, by amateurs and professionals. This work over the last 20 years has generated a lot of useful information. In India, it was only in the 1960s that effective bird ringing projects became possible.
Birds generally begin migration when they have a favorable tailwind. Once started however, only very bad weather will stop them. Many birds fly high when migrating because of prevailing winds at higher altitudes and also because the cold at these altitudes helps them disperse heat being generated by their flight muscles.
Timing of migration is a mix of internal stimulus which results in a feeding binge to put on fat to survive the journey and then the tendency to aggregate into flocks. Once the pre-migration flock is gathered, the feeding continues while the birds wait for suitable weather conditions. Thus while the birds' internal clock probably releases the hormonal triggers at a fairly accurate date each year, the availability of food and the presiding weather conditions decide when the migration starts and hence when we see the first spring migrants arrive and the last autumn ones leave.
For geographical reasons, i.e. mountains, coasts and rivers, many migrating birds travel certain general flyway or routes. Migratory routes are not fixed and in some species part of the population follows one route and part another. In India, the winter migrants from central Asia and Siberia are thought to use two main flyways; one in the west along the Indus valley and the other in the north-east along the river Brahmaputra. Some migrants fly very long distances. Some arctic terns fly 15,000 km each way. Most flights occur at between 600 and 5000 ft above sea level with an average height of 1525 ft. However, mountains may mean greater heights are needed and heights over 10000 ft are not uncommon.
Little is known about how birds navigate. Experiments show that most migratory birds have a built-in sense of direction and know innately which direction they need to travel. First year Starlings in Europe kept in a covered cage and away from birds which have already migrated once or more, still move to the correct side of the cage when the time comes for them to migrate. Some birds appear to use landmarks and obviously at a height of several thousand feet they can see a considerable distance. A number of elegant experiments involving and/or displacing birds to different geographical regions have shown that many birds use the sun, at least during the day, as a cue to direction when migrating or homing.
Birds of prey, Swallows and Crows migrate by day. Thrushes, Warblers, Cuckoos and Woodpeckers migrate by night. Wildfowl migrate both day and night. Most songbirds migrate at night. There is believed to be some hormonal stimulus to migrate, resulting, at least in the spring, in the development of the gonads. Other stimuli appear to involve temperature, daylight/darkness ratios and an internal clock.
Migrations mostly consist of birds flying south for the winter and north in spring to breed. To a large extent this reflects the distribution of the continents on the planets. There is a lot more temperate and tundra landscape in the north than in the south. Migrations to and from alpine areas are not uncommon, such as the Mountain Quail which breeds at heights up to 3000 m, but winters below 1500 m. Interestingly, the bird is flightless so it walks up and down the mountains in groups single file.