Over 40% of birds migrate. Even the blackbird in your garden in winter could be a visitor from eastern Europe. Birds mainly migrate to get to the best places for food and for breeding.
Summer visitors fly to the UK from further south; mostly from the Mediterranean and Africa. During the summer there is more food here and more daylight hours in which to search for it. In the autumn our days get shorter and they return south to their wintering grounds.
Many sea birds and waders also visit our shore as passage migrants, stopping to rest and fuel up before continuing on their long journeys either to their breeding or wintering grounds. Some stop here on both journeys, some on only one of them. They can be different populations of relatively common species which also breed and winter here, such as dunlins from Greenland and Iceland on their way to West Africa. Or they can be the rarer whimbrels on the move between their breeding grounds in the Shetlands and Orkneys to their wintering grounds in Africa. These passage migrants are particularly vulnerable to disturbances as they often have to double their weight before moving on. Birds who are exhausted and underfed from constant disurbance might not make the journey.
Winter visitors leave northern Europe and the Arctic in Autumn to escape freezing temperatures and shortening hours of daylight. The days here in the UK are longer, the weather is milder and food is easier to find. In spring, they fly back to their breeding grounds further north. As summer arrives in the Arctic the snow melts and there is a huge flush of insects. Birds travel there from all over the world to exploit the abundance of food and the long hours of daylight. The Essex coast is special for overwintering birds like the dark-bellied brent goose, pictured on the right.