Bird migration refers to the regular seasonal journeys undertaken by many species of birds. Migrations include movements of varied distances made in response to changes in food availability, habitat or weather. These however are usually irregular or in only one direction and are termed variously as nomadism, invasions or irruptions. Migration is marked by its annual seasonality. In contrast, birds that are non-migratory are known as resident birds.

A simple explanation is food and a safe place to breed. Birds which breed in the summer in the extreme north such as the Arctic benefit from an abundance of food as plants and insect life flourish in the long daylight hours; and because few large permanent predators can survive the harsh winter.

Many birds that breed in the Arctic simply lay their eggs on the ground. Being able to fly, they can avoid the harsh winter conditions, and be the first to arrive to enjoy the summer benefits.Migration is affected not only by food supply, but also by wind and oceans currents. These make some routes and locations easier to reach. While many birds migrate from northern breeding areas in the summer, to southern wintering grounds (mainly because there is more land near the northern pole than the southern), there are many other migration patterns. Some birds breed in the far south of South America, Australasia and Africa, and migrate to northern wintering grounds. Some birds migrate horizontally, to enjoy the milder coastal climates in winter. Other birds migrate in terms of altitude; moving higher up a mountain in summer, and wintering on the lowlands.

Birds exploit the winds to their favour so they can go the distance by burning minimal fuel. They may shift altitude to find the best wind "conveyor belt". Winds at high altitude may blow in the opposite direction from wind on the ground, and usually are blowing strongly. Larger birds rely on thermals (hot air) rising from the ground in the mornings to gain altitude by simply soaring. These birds usually migrate during the day. They may also follow strong updrafts along ridges.A few birds fly non-stop, some for several days, covering enormous distances. But most birds break journey at staging posts. A vital aspect of being able to make such long trips is to lay down enough fat reserves. This is why staging posts such as Sungei Buloh are important to migrating birds.Besides laying down fat reserves, migrating birds also need to eat a lot to fuel their regular feather moults. Their feathers must be in tip-top condition for their long trips. Different species moult at different times; for most shorebirds it is just after breeding and before the migration to wintering grounds.Studies suggest birds orientate themselves to the compass points using the position of the sun during the day, and the stars at night. They can also sense magnetic north. In addition they use other clues such as visual layout of the land, smell (of the sea), sound (waves on shores, winds through mountain passes).
The most amazing aspect of bird migration is that the location, route and perhaps even the techniques are hard-wired into their brains. Many migrating birds abandon their young as soon as they fledge, and a short time later, the young make the migration on their own. 

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