How Do Birds Know Where to Migrate?

A classic experiment involved young birds raised in isolation and then released at migration time. They oriented in the correct direction despite never having seen the route or another migrating bird.

This points to an inherited programme that specifies a direction and roughly how long to fly. Experienced adults perform better, correcting for being blown off course in a way inexperienced birds cannot, so genuine navigation appears to be learned on top of an inherited compass.

Birds appear to carry several navigation systems and to cross check between them:

• •A sun compass, using the sun's position corrected for time of day
• •A star compass, learned as young birds watch the sky rotate around the pole
• •A magnetic compass, sensing Earth's magnetic field
• •Landmarks such as coastlines, rivers and mountain ranges
• •Smell, which some seabirds appear to use over open ocean

This is one of the most interesting open questions in biology. The leading hypothesis involves cryptochrome proteins in the retina, where a light triggered reaction produces pairs of molecules whose behaviour is affected by magnetic fields.

If correct, birds may partially see the magnetic field as a visual pattern overlaid on the sky. A second possible mechanism involves iron rich particles in the beak acting more like a physical compass needle. The evidence for both is suggestive rather than settled.

Migration distances can be extraordinary. The Arctic tern breeds in the far north and winters in the Antarctic, covering something like seventy thousand kilometres a year and seeing more daylight than any other animal.

The bar tailed godwit holds the record for non stop flight, tracked flying from Alaska to New Zealand in about eleven days without landing, feeding or drinking. Before departure it nearly doubles its body weight in fat and shrinks its digestive organs to save weight.

The main cue is day length rather than temperature, which is more reliable from year to year. Changing daylight triggers hormonal shifts that drive feeding, fat storage and restlessness.

Caged migratory birds show this clearly, becoming active at night during the migration season and facing the direction they would travel. That built-in restlessness has a German name, Zugunruhe, and it is one of the neatest demonstrations that migration is deeply instinctive.

Much of what we know comes from ringing, where a numbered band is fitted to a bird's leg and reported if the bird is recaptured. It is cheap and has produced over a century of data, but it depends on the small chance of a bird being found again.

Modern methods are more direct. Lightweight geolocators record light levels to estimate position, satellite tags report continuously, and weather radar detects the mass movement of birds at night. Isotope analysis of feathers can even indicate roughly where a bird was when it grew them, since local water chemistry leaves a signature.

Some of the clearest evidence for the star compass came from experiments by the American biologist Stephen Emlen with indigo buntings, small songbirds that migrate at night. Emlen placed birds in funnel-shaped cages lined with paper that recorded footprints, so their restless hopping showed which direction they were trying to go. Under the artificial sky of a planetarium, the birds oriented correctly. When parts of the sky were blocked out, he found that they relied mainly on star patterns near the celestial pole, which stays almost still while the rest of the sky turns around it.

The most revealing test involved young birds raised indoors. Some saw a planetarium sky that rotated, as normal, around the pole star. Others saw a sky that rotated around Betelgeuse, a bright star in Orion. When autumn came, each group treated its own centre of rotation as north and headed away from it. Birds that had never seen a rotating sky could not orient properly at all. The result showed that the star map itself is learned, but the rule for building it, find the still point of the turning sky, is inherited.

Migration routes are not fixed forever, and one small European songbird has shown evolution reshaping them within living memory. Blackcaps breeding in southern Germany and Austria traditionally fly south-west in autumn to winter around the Mediterranean, in places such as Spain and North Africa. During the second half of the twentieth century, birdwatchers noticed growing numbers of blackcaps spending the winter in Britain and Ireland instead. Ringing recoveries showed that many of them came from central Europe, which meant they were flying north-west, a direction that would once have been a costly mistake for a small bird facing a northern winter.

Researchers led by the German ornithologist Peter Berthold caught blackcaps wintering in Britain, bred them in captivity and tested their young in orientation cages. The offspring, which had never migrated, chose a westerly to north-westerly heading like their parents, showing that the new direction was inherited. Other experiments that crossed blackcaps from populations heading in different directions produced young that tended to pick an in-between course. Milder winters and garden bird feeders probably made Britain a viable destination, and birds wintering there have a shorter journey home, so they can reach the breeding grounds early and claim good territories.

Many songbirds migrate at night, when the air is often cooler and calmer and fewer predators hunt, and they use the stars among other cues to steer. Bright artificial light can disrupt this. Birds flying over cities may be drawn towards illuminated buildings, especially in cloudy or foggy weather, and then circle in confusion or collide with glass. In the United States, researchers estimate that collisions with buildings kill hundreds of millions of birds every year, and night migrants are among the most frequent victims. A navigation system shaped over millions of years can be misled by conditions that did not exist until recently.

An unusual memorial in New York shows the effect clearly. Each year on 11 September, the Tribute in Light sends two powerful beams into the sky, and during migration season thousands of birds can become trapped circling inside them. Scientists using radar and acoustic monitoring found that when the lights are switched off for a short time, the birds disperse and continue their journeys, so observers now ask for the beams to be turned off whenever too many gather. Many cities also run Lights Out schemes, asking building owners to dim unnecessary lighting on peak migration nights, a simple change that helps reduce collisions.

Birds navigate migration with an inherited sense of direction plus a set of compasses based on the sun, stars and magnetic field, refined with experience, allowing journeys that in some species span the entire planet.