Which birds migrate and why?

Nothing ever stays the same in the bird world - the wild birds we see on our walks and in our gardens vary from one month to the next.

Migration is such a fascinating topic and one we still don’t fully understand. The first records of migrating birds date back to Ancient Greece, although little was understood about where the birds went and how they travelled. In his book The Natural History and Antiquities of Selborne in 1789, naturalist Gilbert White reinforced a myth held for thousands of years that swallows hibernate over winter in mud beneath the surface of ponds (often thought to be the case because the first swallows were often seen over bodies of water). Luckily, an amateur naturalist called John Masefield was able to ring a swallow chick on his porch in Staffordshire in May 1911 and received word that the same bird had been accidentally netted on a farm in Roodeyland, South Africa in December 1912. We now know that swallows make the six thousand mile trip to South Africa every year, travelling two hundred miles a day at speeds of around twenty miles per hour. When we see them over ponds and lakes when they first arrive on our shores they are refuelling after their long journey on the flying insects they love to eat. 

Migration carries many risks, including increased chances of predation, so why do birds do it? Food is the primary motivation for migration - birds often travel from areas of depleting resources to areas of high resources to ensure breeding success. It is also  typically easier to raise young in areas where the days are longer and the weather warmer. Four thousand species of bird regularly migrate (around 40% of the world’s species), and their migration paths vary. Some birds are altitudinal migrants, breeding in upland areas in the summer before travelling down to milder temperatures and abundant food at lower altitudes in the winter - skylarks and snow buntings are altitudinal migrants in the UK. Insect eaters like swallows, warblers, swifts and redstarts arrive from their southern winter homes to visit us as summer migrants, and fieldfares, bramblings and redwings are winter migrants, joining us from the north and east to enjoy our comparatively warm winters.

Moult migrants are birds that migrate to quiet areas where they don’t risk being caught by predators while they moult their feathers. Passage migrants stop off on our shores to rest and refuel for a few weeks as they travel north or south to their final destinations during spring or autumn. Partial migrants base their migration patterns on the weather - birds that are resident in the UK all year round might be regular migrants in other parts of the world. Other factors can lead to one off migrations, known as irruption. A successful breeding season followed by the failure of an important food source can lead to thousands of birds needing to migrate to find food.

How do birds know which route to take? How do they manage to stay on course and how do they know when to take flight? Migration patterns are complex and have evolved over thousands of years - they form part of the birds’ genetic make up, which is why birds are able to locate their seasonal homes in their first year of migration despite having never been there before. We don’t totally understand how birds navigate, but it is believed that they use the sun, stars, wind patterns and land forms, as well as sensing the earth’s magnetic fields. Their paths are plotted according to stop off points for food or rest and they often follow the same course each year. Most birds migrate in flocks, which reduces energy use for bigger birds, and a lot of birds time their migration according to the weather and hours of sunlight (although Eleanora’s falcons time their own migration according to the southbound passage of passerines which they use to feed their own young). Male and female birds can migrate at different times of the year, and some birds only migrate during their first year. Nocturnal birds use short nocturnal flight calls to avoid collisions during their night time flights and geese migrate in a V-shaped formation, taking turns to lead and dropping back when they become tired.

Birds can get lost and end up in unexpected places, often prompting twitchers to travel miles to see an unusual visitor. Some birds overshoot their planned destination and end up further north or south than planned, while others are blown off course by storms. Birds are a great benchmark for environmental health and the effects of climate change, and the conservation of migrating birds requires international cooperation because they cross so many borders. Pollution, hunting along migratory lines, wind farms, offshore oil rigs, storms and habitat destruction can all affect migrating birds and impact bird populations worldwide.

The Arctic tern is the migrant with the longest journey, travelling around sixty thousand miles between their Arctic breeding grounds and the Antarctic every year, while the Adelie penguin migrates eleven thousand miles (although does so by swimming rather than flying).