Bird migration is a two way journey from resting or breeding place to feeding or nesting place and vicversa.  It is the periodic , regular and to and fro movement. Birds are the greatest migrant however some species of birds never migrate.

For a bird, traveling hundreds or thousands of miles between its breeding and non-breeding ranges is a difficult, perilous journey, one that not all birds survive. So why do birds migrate? What reasons send millions of birds into the risky skies every spring and fall?

There is more than one single reason for different birds to migrate, but it all comes down to survival, not just for each individual bird, but also for the families they hope to raise.

Migrating for a Meal

For all birds, one of the principle driving forces behind migration is food scarcity. If all birds were to stay in the same rich, tropical areas year-round, food would become scarce and breeding would be less successful with undernourished parents and hungry hatchlings

Migrating for Family

Over millennia, birds have evolved different migration patterns, timing, and destinations to disperse around the world to breed. This helps birds take advantage of a wide variety of suitable conditions to raise their young, increasing the chances of healthy, viable offspring

Climate: Birds have evolved different types of plumage to survive different climates, and changes in those climates can affect migration. Many birds leave their Arctic breeding grounds, for example, when temperatures begin to dip and they need more temperate habitat because they cannot survive the brutal cold. Similarly, the hottest tropical regions can be a harsh environment for raising delicate chicks, and it is advantageous to lay eggs further north in cooler areas.

Predators: Habitats with abundant food sources year-round also attract a greater number of predators that can threaten nests. Birds that migrate to different habitats can avoid that onslaught of predators, giving their young a better chance of reaching maturity. 

Disease: Any large group of birds crammed in one type of habitat is susceptible to parasites and diseases that can decimate thousands of birds in a short period of time, and diseases can and do occasionally devastate breeding colonies

Good migration allows birds to survive for another generation and allows birders the pleasure of witnessing another year's migration.

Importance of migration of birds to ecosystem

Migratory wild birds play as a vital part of biodiversity in all ecosystems. Migratory birds serve key functions in the interconnected systems that keep nature healthy, including pollination and seed dispersal of crops for human and livestock consumption, pest regulation and as an aesthetic source of pride for cultures across the globe.

Birds roles in ecosystem

Birds occupy many levels of trophic webs, from mid-level consumers to top predators. As with other native organisms, birds help maintain sustainable population levels of their prey and predator species and, after death, provide food for scavengers and decomposers.

Many birds are important in plant reproduction through their services as pollinators or seed dispersers. Birds also provide critical resources for their many host-specific parasites, including lice that eat only feathers, flies adapted for living on birds, and mites that hitchhike on birds from plant to plant and even between countries.

Some birds are considered keystone species as their presence in (or disappearance from) an ecosystem affects other species indirectly. For example, woodpeckers create cavities that are then used by many other species. After the extinction of the dodo, it was discovered that a tree whose fruits had been a primary food item of the dodo was unable to reproduce without its seeds passing through the dodos’ digestive tracts, which process scarified the seed coat and enabled germination.

Birds and humans

Birds have been integral to humans since prehistory. To birds’ detriment, they and their eggs have been an important human food source since humans evolved, and we have hunted many species to extinction. Feathers, usually obtained by killing their original owners, have been used as adornment in hats, headdresses, and capes. Birds are popular as “pets” throughout the world, and the pet trade has driven many species to the edge of extinction.

More benignly, birds appear in ancient art and mythology worldwide. Birdsong enriches the outdoors for many people, and birdwatching is one of the fastest-growing recreational activities. Birds also were the primary inspiration for human flight.

Threats to Migratory birds

Migratory birds are increasingly at risk due to the negative consequences of human activity including environmental pollution, toxins, pesticides, electrified power lines, wind turbines, and other physical changes to landscapes and habitats (terrestrial and aquatic). In addition, encroachment from human settlement, agriculture and poaching – be it for subsistence food, a means of income or for high profits in illegal game hunting – are destroying habitat for all wildlife species, including migratory birds. For these reasons, it is imperative that FAO and partner organizations work together to safeguard migratory birds, their migration routes and their habitats.

Recently extinct birds

A hundred bird species have vanished since 1600, nearly all due to human activities, chiefly habitat loss, overhunting, and introduced predators. Island birds are especially vulnerable.

Having been acknowledged all about need of migration to birds; role of birds to ecosystem and human itself; importance of Migration of birds to ecosystem and possible threats for them during Migration, it is undoubtedly clear that it's human action which is hindering the life of birds and to tackle with all these problems for bringing balance environment, World Migratory Bird Day is of utmost significant when possible great ideas and actions from concerned people and authority comes up to implement it all round the globe.

Source: https://www.endangeredspeciesinternational.org/birds5.html