Why Do Birds Migrate?
Migration is the seasonal journey undertaken by many species of birds. Although bird movements also include those made in response to food availability, weather or habitat these are usually irregular. Migration is defined by its annual seasonality. Birds that are not migratory are classed as resident or sedentary.
The main reason for migration is conservation of energy. Birds will travel north to take advantage of abundant food and longer days during the breeding season. When the weather gets colder and the days shorten they will migrate to the south in the tropics of Africa.
Britain’s best known migrants are swallows and in the autumn you can see thousands of them gather ready to set off on a journey which will see them cover 9,500 km to South Africa before returning once again in the spring.
Swallows will travel by day but many migrating species prefer to travel at night time and it is rare to see them leave. Traveling by night reduces the risk of predation and as the air is cooler and calmer at night it also minimises the risk of dehydration and energy loss.
It is not fully understood how migrating birds are able to navigate their great distances. It is believed that the sun is their most important aid, although using the sun for direction involves the need for making compensation based on the time. During bad weather they will make use of polarised light – UV rays which humans are unable to see – to work out the position of the sun.
At night birds will use the position of the stars and possibly olfactory senses and they will also use the earth’s magnetic fields. And it seems that they also have a genetic predisposition for migration – it is interesting to note that cuckoos who are raised by adoptive species and never knew their parents are still able to arrive in their ancestral wintering habitat with no adult help.
Before they begin their migration birds will lay down fat reserves for the journey, as much as doubling their weight. Many small songbirds will supplement their diet of insects with autumn berries and it is important that you feed the birds at this time of year, particularly if it has been a wet summer which means berries may be scarce.
As well as British birds that migrate south we also play host to many migrating species who have come here from even further north.
Fieldfares, redwings, bramblings and waxwings arrive from Scandinavia, Russia and northeast Europe. Robins are another species that will migrate south, and while many British blackbirds move west into Ireland, blackbirds from Eastern Europe come to the UK.