On the move
More information about when and how birds migrate.
Different types of migration
Most migrating birds usually fly at a height of between 200 and 1,500 metres above sea level. When the wind is against them (a headwind), they stick closer to the ground, where ridges, trees and buildings slow the wind down. With the wind blowing behind them (a tailwind), they fly up high to where it will whisk them along faster. Mountains also force birds to fly higher.
Bar-headed geese in central Asia regularly cross the Himalayas – the world’s highest mountains – by reaching heights of 18,000 metres. No mammal could survive long at this altitude. But birds’ lungs can take in more oxygen from the thin mountain air.
Many large birds migrate by day in order to make use of thermals. Birds such as swifts and swallows that feed on flying insects also migrate by day, feeding as they go.
But many songbirds, including warblers, thrushes and starlings, migrate mostly at night. The air is calmer and cooler then, so they lose less energy by flapping and less water through evaporation. They’re also much less likely to bump into a predator after dark.
Most birds try to cross the sea in a single flight. Landing would spell disaster, since they’d quickly become waterlogged and drown. But seabirds, such as gannets and shearwaters, often take a break on the sea to feed and rest. They are strong swimmers and their well-oiled feathers keep them afloat.
Young auks, such as puffins and guillemots, often leave the nest before they can fly properly. They set off on their migration by swimming. For penguins, which are flightless, swimming is the only option.
Many birds migrate in flocks. This provides safety in numbers, since each individual is less likely to be attacked by a predator than if it were travelling alone.
Some, such as geese and cranes, travel in their family groups inside larger flocks. Migrating flocks of small birds tend to be much more broadly scattered. Each one looks after itself, but they keep in touch other using contact calls, and may gather at roosts along the way.
Some large birds, such as geese, fly in V formation. This shape helps the flock to make better progress. The front bird breaks up the wall of air that the flock flies into. This leaves a wake of swirling air behind, which helps give a lift to the next bird along.
In this way, each bird in the V gets some help from the one in front of it. After a while, the leader drops back and another bird takes over. Experienced flyers usually do most of the work.
Scientists have shown that birds in V formation can fly 70 per cent further than one bird flying alone.
Formation flying also helps a flock of birds to stick together. Geese stay in close contact by honking as they fly, and the white markings on their rumps work as 'landing lights', helping each bird to see its neighbour.
If one goose becomes injured and has to land, a few family members will stay with it until it recovers. When it is ready to fly again they all set off and look for a new flock to join.
The weather can help or hinder migration. The sun heats up the land, causing warm air currents called 'thermals' to spiral upwards.
Large birds - like birds of prey, storks and herons - use these thermals to gain height without using up precious energy by flapping. Once they are up high, they glide towards their destination on the wind, just like hang-gliders. Without thermals, many large birds could never get across seas and deserts. Flying speed depends on wind.
In still conditions, small birds fly at an average speed of 30–35 kph and medium-sized ones at 45–55 kph. But with the wind behind them, birds can easily cover 1,000 km (625 miles) in just 24 hours. Wind allows some birds to perform amazing flying feats. Bar-tailed godwits have been recorded migrating from Alaska to New Zealand – a distance of almost 11,000 km (6,800 miles) – in just six days.
Sooner or later the urge to move will overcome any barrier the weather may create.
Packed lunches and service stations
Different birds migrate at different speeds, depending on how they get their energy. Small birds like warblers use their fat reserves like a packed lunch, to see them through as quickly and directly as possible.
They can get from the UK to Africa in less than three weeks. But many larger birds make slower progress. An osprey may take over two months to reach Africa. It can’t store enough energy for non-stop flying, because the extra fat would make it too heavy to fly.
Instead it relies on ‘service stations’, such as lakes and estuaries, where it can rest for a while and recharge its batteries with a juicy fish or two.