Flight plans

I heard twittering and saw swallows lined up on the wire outside the window this morning. It's past mid-August and they're getting ready for their long journey south: across the English channel, down the west coast of France, eastwards over the Mediterranean to North Africa and on across the Sahara to West Africa (where, in Nigeria, thousands are slaughtered for food as they roost). Then, following still-mysterious routes, they traverse the equatorial forests to their winter home in Southern Africa. The journey covers 6000 miles and takes four months. Even though the birds have indulged in a feeding frenzy to build up their fat reserves for the journey, a European swallow weighs only around 20 grams (a good deal less than an ounce).

Migration is one of the greatest wonders of the animal world, and is still not fully understood. Although the ancient observers and writers such as Aristotle realised that some birds departed for warmer climes before the onset of winter, for many centuries subsequently there was an intense debate over whether the disappearing birds hibernated instead. Many naturalists believed that swallows overwintered in ponds and claimed that they could be dug out of the mud as evidence.

It's hard to credit the idea that a tiny bird no bigger than a matchbox could take to the air and vanish for months but even when the migration was accepted as a fact it was not until 1912 that the ultimate destination of the European swallow was revealed. Ringing birds' legs first began in 1909. Three years later a swallow ringed by a John Masefield, country solicitor in Staffordshire, was captured 6000 miles away in Natal, South Africa.

The swallows' preparatory limbering-up that I observed this morning is known as Zugunruhe, a German term meaning migratory restlessness - you could call it itchy wings. It's observed in captive birds for the period of time that they would be on the wing if they were free to migrate. Birds that migrate the furthest distances exhibit restlessness for the greater duration. Zugunruhe activity takes place mainly at night, when migration takes place, possibly because birds rely on the stars to navigate, but it's also thought that they use the earth's magnetic field and their own sense of smell to guide them on their journey. Experiments with caged birds show that their attempted flight is always orientated in the direction of their normal migration and it is suggested that the knowledge is implanted in their genetic make-up.

Any day now - perhaps tomorrow - I will perhaps notice an absence and these swooping, diving, twittering, feathery darts will be on their way.

Travelling Birds video about bird migration.