Scientists still mystified by bird migration

BIRD migration is an extraordinary thing, in fact, in some cases, it verges on the unbelievable. Every autumn Aleutian bar-tailed godwits fly non-stop for nine days from Alaska to New Zealand, a distance of more than 11,000km.

Great shearwaters and sooty shearwaters, which are flying off our coasts at present, were hatched on remote islands in the Roaring Forties of the southern hemisphere. Miniature tracking devices attached to some have revealed that in the course of their world travels they fatten up on Canada’s Grand Banks and don’t feed again until they arrive off the east coast of Argentina.

Many long-distance migrants prepare for the journey by undergoing changes that seem as miraculous as Clark Kent’s transformation into Superman. All their non-vital organs, including their digestive systems, shrink in order to reduce weight and make space for the fat that fuels the flight. They reconfigure the amount of haemoglobin in their bloodstream so they can fly at high altitudes where the air is thin but there are beneficial tail winds. All this so that they can enjoy perpetual summer by flying from one hemisphere to another.

Not all bird migration is as dramatic as this and some of it is far from obvious. For the next few months you won’t notice anything different about the blackbirds in your garden but most of them will actually hail from Scandinavia, the Baltic countries and even further east. The same is true of winter starling flocks.

Autumn is a busy season for Irish twitchers trying to add a new species to their life list. Ireland, off the Atlantic seaboard of Europe, is geographically well placed to receive what ornithologists call vagrants — birds that have strayed from their normal migration route.

We tend to think of a vagrant as a bird that’s made a mistake. It might be more accurate to think of them as pioneers or explorers testing out new possibilities and future migration routes. Patterns of migration are continually evolving. Fifty years ago the blackcaps of central Europe all went south in winter. Then some pioneering individuals went west and ended up in Ireland. Nowadays wintering blackcaps are common Irish garden birds.

The question of how migrating birds navigate, often across featureless oceans or deserts, is one that has attracted many theories and practically no hard facts. There probably isn’t one single answer to fit the many different patterns of migration. Some travel at night, leading to theories that they use the position of the stars to establish direction. Others travel by day, and still more by both day and night, and it’s been suggested that they use the earth’s magnetic field.

Scientists are rather bad at admitting defeat, but how migrating birds navigate is a mystery.