How the homing instinct works

Scientists are still trying to determine how birds, butterflies and bats migrate the planet on their annual long-haul flights, writes ANTHONY KING

THE MAP AND GPS capabilities of our smartphones allow us to pinpoint our location, and tell us how to get from A to B. But long-distance migrating animals solved this problem long ago, with their own internal GPS and maps.

Early displacement experiments revealed impressive abilities in animals; in the 1950s, a seabird moved from Wales to Boston returned to its Welsh burrow within two weeks. It somehow knew which way was home. Scientists are still trying to find their way to answering how birds navigate huge distances and correct for misadventures – such as being blown hundreds of kilometres off course or “displaced” by scientists from one continent to another.

“We think they do something akin to using a map and compass,” says Richard Holland, an animal-navigation expert at Queen’s University Belfast. “First they locate their position – the map step – and then they use a directional bearing to fly in the direction they need.”

A classic 1950s experiment moved more than 10,000 starlings from Holland to Switzerland; adults reoriented their co-ordinates and flew to their normal winter destinations in England and northern France. First-timers travelled in the expected direction and distance and ended up in southern France and Spain; they had an innate clock and compass, but apparently no map to tell them they’d been moved. How the adult map works remains elusive.

“They can use the magnetic field, the sun and the stars as a compass. There are still arguments over which is the most important, but they are all well established as being involved in the compass system,” says Holland. For example, birds can become confused if star patterns are varied experimentally in a planetarium. Holland has manipulated the magnetic sense of birds by treating them with a strong magnetic pulse and reported that this did not disorientate the birds. But scientists have long suspected that birds can recalibrate their position using multiple cues. And additions to their instrumentation are turning up.

“We’ve provided the first evidence that birds might use smell to locate their position, which is quite surprising given the scale over which the navigation system needs to work. It had been thought that odours could not be consistent enough cues to be used in a navigation system over this distance,” says Holland.

There is behavioural evidence for detection of a magnetic field, but how birds do this is disputed. US researchers recently reported on a group of cells in the brain stem of pigeons that recorded both direction and the strength of the magnetic field. They concluded the information came from the bird’s inner ear.

“This is an important discovery,” says Holland, although he says the paper ignored an older study where nerves connecting to the inner ear were severed. “While the pigeon’s brain may respond to magnetic information and that information may be coming from the inner ear, it does not seem to be important for pigeons in finding their way home.”

There has been evidence that birds carry a light-dependent chemical compass in their right eyes. While magnetic fields potentially allow for latitudinal calculations, how birds fix longitude is unknown. “We really have no good experimental evidence as to how these birds are solving that problem,” says Holland.

For years, scientists had carried out migration experiments with caged birds. “Somebody discovered that if you put a bird into a funnel, it tries to escape and will hop consistently in the direction it would migrate. This has been a useful tool for measuring direction of migration and even looking at the compass cues involved in migration,” explains Holland.

But hopping and migrating thousands of miles is not the same. “Electronic devices that allow us to track the path of the bird over its entire migration are starting to allow us to look at what challenges the bird faces during its migratory journey and understand more about the cues it may be exposed to and use,” says Holland, who works with small birds such as robins and reed warblers.

Last summer, British scientists fitted satellite-tracking devices to five cuckoos, a declining migrant in Ireland and Britain. Two birds arrived back this month and were the first to have their African migration route mapped. They had wintered in the Congo rainforest – with transmitter backpacks. The cuckoo "Lyster" left Ghana and crossed the Sahara to Algeria in April, and flew 1,880km north in less than five days. He was spotted not far from where he was tagged in the Norfolk Broads this May (details of the project are at bto.org/cuckoos).

Birds are not the only migratory creatures here. The red admiral is a butterfly that is “around every year and usually in substantial quantities”, says David Nash, co-author of Ireland’s Butterflies. “It is perhaps the most reliable and regular migrant,” he says, originating around the Mediterranean.

The painted lady is another butterfly migrant, which originates in Morocco. A Spanish scientist found huge numbers in thistle fields near Marrakech in Morocco and they were tracked moving up through Europe.

“Butterflies tend to expand out when numbers are large and if winds are right they can appear anywhere in Ireland,” says Nash. Millions came to Ireland in 2009; it was thought their progeny succumbed to our winters.

But a British-Spanish team discovered that painted ladies attempt an autumn migration southward. “This return migration is not as noticeable as the spring influx for some reason, but the answer may be because a large proportion of the emigrants fly at high-altitudes, where they are not visible to the naked eye,” explains Jason Chapman, a scientist involved in the study at Rothamsted Research in the UK.

Butterflies flown in a simulator without a sky showed a random scatter of flight headings and less-directed flight tracks; the painted ladies apparently use a sun compass to keep a constant flight path. Another study by Chapman showed that migratory moths ride seasonally-favourable tailwinds to maximise distances travelled and migrated at similar speeds to songbirds.

How insects navigate is a quandary. A study of monarch butterflies, which migrate between Mexico and the eastern US, showed they get lost if you paint their antennas black. The antennas, once thought to be primarily for odour detection, can sense the light of the sun and have a built-in clock – essential to adjust for sun position. The jury is out on whether butterflies also use a magnetic compass, says Chapman.

There are also mammalian aviators. Holland says he recently demonstrated that bats are able to use the Earth’s magnetic field and detect it for a compass. It’s territory ripe for further exploration. “We have almost no information at all on bat navigation,” says Holland.

Migrating birds: the comings and goings

In the last few weeks, brent geese departed our shores for an Icelandic stopover, where they refuelled before flying on to the high Arctic to breed.

Meanwhile, our summer migrants have been arriving from Africa: sand martins first, then swallows and cuckoos, and now house martins and swifts, explains Brian Caffrey of Birdwatch Ireland. Children and adults can track and log arrivals on a website, springalive.net.

Other summer migrants are less noticeable. “The likes of the willow warbler, chiff-chaff and black cap would be coming from Africa,” says Caffrey. The eight-gram willow warbler, an apparently weak flyer, will have undertaken an arduous flight without food over the Sahara and Mediterranean.

The corncrake is an unlikely long-distance flier from south of the Sahara. “They are a farmland bird in our meadows and tend to arrive around mid-May,” says Caffrey. They are found in particular in west Donegal. “Some find it hard to believe that these birds migrate that distance. They are secretive birds and don’t like being exposed, so they appear to be bad fliers and unlikely to migrate any distance.”

The longest journeys involve seabirds, with some Arctic terns migrating each year between the Arctic and Antarctic. Ireland hosts the largest roseate tern colony in Europe, on the island of Rockabill off north Dublin.

Chicks are given an identifying ring, Stephen Newton of Birdwatch Ireland says. They accompany parents on their maiden flight to west Africa, where they spend their first few years.

When they are three years old, they come back to Ireland and look for somewhere to nest,” Newton says. Their ID rings have been recovered from Ghana and Senegal.

“I have a colleague in Belgium who catches the birds in Senegal and every year he will send me a list of all the birds he has recaptured there. In April, just as the birds are leaving to come back up to Ireland, he can capture a bird and two weeks later it is back on Rockabill,” Newton says. His work with University College Cork revealed last year that Kerry puffins migrate to North America during our winters.