Migration is the moving from one place to another, usually in search of more favourable conditions for either feeding or breeding.
Many songbirds migrate at night and feed and rest during the daytime. The air is also cooler and denser at night and so there is:
Other birds fly at very high altitude for a similar effect, for example: airline pilots have observed Whooper Swans at altitudes of 8850 metres (29 000 feet).
Scientists believe that the bird's internal clock and cues taken from seasonal events govern the timing of their migration. At the appropriate time, the birds prepare for migration by building up their fat reserves by eating insects and berries. Some species, particularly warblers, complete their migration in one non-stop flight and can double their body weight, while others stop en route to feed.
Additionally, some species, such as Willow Warbler, may moult their feathers ready for the migration, while others will moult only when they arrive at their destination.
The tremendous feat of travelling thousands of miles is all the more miraculous when some species are known to return to the same location year after year. Scientists think that birds use their sense of smell to follow odours, their remarkable eyesight to follow the Sun, the stars, the Earth's magnetic field, and landmarks, and wind directions to achieve navigation. Interestingly, some species take a different route in their summer migration to the one in the winter.
From the birdwatcher's point of view, there are three types of bird visitors: summer visitors, winter visitors, and passage visitors, and can offer splendid views of large flocks of birds and hundreds of different species.
In Britain, our summer visitors are birds that have migrated in the spring from around the Mediterranean and Africa. They do this to improve the chances of rearing young. In they stayed in Africa the competition for limited food supplies would be high, but in the more northern latitudes there is more food and more daylight hours in which to search for it. However, staying in Eurasia during the winter months when food becomes short would lead to starvation and death, though some of our traditional migrants, like the Blackcap and Chiffchaff, are now over-wintering in Britain.
Willow Warblers and many other warblers fly non-stop and take 4 or 5 days to complete their migration. On the other hand, Swallows can take 2 or 3 months to complete their migration as they stop off every few days to roost and feed.
These species migrate from their breeding grounds in Scandinavia and northern Europe where food becomes hidden under snow and ice.
Examples are the thrush family: Redwing, Fieldfare, Blackbird and Robin. While some of these are true migrants, we also have native migrants, for example: tits and wrens moving from the countryside to urban areas, starlings flying from their city roosts to suburban gardens during the daytime, thrushes flying southwards from northern Britain, etc.
Many passage visitors are sea birds and waders, such as Black Tern, Solitary Sandpiper, but also others such as Serin and Bluethroat.
Irruptions are sudden invasions of birds. One of the better known examples are Waxwings, which sometimes move into Britain when rowan berries have failed in Scandinavia and northern Europe.