Migrating songbirds check their direction each night before take-off – by taking a bearing on the setting sun.
Many migrating creatures – honeybees, certain fish, many birds, and even monarch butterflies – possess built-in compasses to follow the lines of the Earth’s magnetic field, and today in the journal Science ornithologists report on how they tried to mislead thrushes by exposing them to magnetic fields distorted towards the east.
It seemed to work. Released after dark, the birds flew west instead of north to their summer breeding grounds. They were fitted with radio transmitters, and the German and American ornithologists followed them by car for up to 1,100km (680 miles).
However, once free to decide where they were, the birds noted the direction of twilight and corrected their flight northward. The conclusion: thrushes steer by compasses at night, and update them from the setting sun every 24 hours.
Migration has been a mystery. Some birds return to nesting sites every year, flying up to 25,000km. Monarch but terflies breed in the eastern United States and Canada and every autumn their offspring fly 2,500km to the same roosting areas in Mexico. Salmon and sea turtles, and even a species of mole rat, make epic journeys using very precise navigation.
Scientists proposed mental maps, based on memorised landmarks, or navigation by sun or stars. But oceans have no landmarks, and the sun is no guide at night; any birds using the stars would be confused going north to south, as fresh constellations appear.
For a while, researchers decided nature provided a compass – tiny fragments of magnetite inside the birds’ heads. They tested this by trapping migrants in a room with a false magnetic field. Satisfyingly, on release they flew in the wrong direction; others with no field distortion flew in the correct direction.
That did not answer all the questions. Magnetic north is around 1,000km from the North Pole, and moves from year to year; even more confusing, there are cyclic fluctuations in the Earth’s field. If magnetism were the only guide, any migrant leaving north Alaska and following magnetic south would head west.
So William Cochran of the Illinois natural history survey, Martin Wikelski of Princeton University, and Henrik Mourit sen of the University of Oldenburg in Germany tried to find how Catharus thrushes – Swainson’s and grey cheeked thrushes – knew how to navigate. They conclude that the Swainson’s thrushes, at least, relied on a compass, but checked it at twilight each day.
Tim Radford, science editor
Friday April 16, 2004
The Guardian