Where and when to flyHigh flyersMost 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. Night flightsMany 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.By waterMost birds try to cross the sea in a single flight. Landing would spell disaster, since they’d quickly become waterlogged and drown. But sea birds, 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. |
