Animal magnetism behind navigation
EVERY year birds migrate to our shores from as far away as Africa and Russia.
Amazingly, very few birds seem to take a wrong turn on their journey, with some even returning to the exact same spot every year.
So with neither road signs nor a computer GPS to guide them, how on Earth do birds navigate from one place to another? It turns out that some birds have tiny grains of a magnetic mineral called magnetite in their heads. This mineral can detect the Earth’s magnetic field.
To test if birds do use magnetic fields, an experiment in the 1960s took some robins and placed then in cages with artificially-created magnetic fields. They found that the birds orientated themselves according to the new magnetic fields and that the new direction corresponded with the route they would have taken had they been migrating naturally. We know that birds follow the same migration routes each year, but whether birds learn a ‘map’ of their immediate surroundings via remembering specific landmarks on their journey is hard to prove. However, the humble starling has offered some insight into this issue. Just as they were about to start their migration, thousands of starlings were captured in the Netherlands and then released in Switzerland.
The juvenile starlings set off in the direction that would normally have taken them to their wintering grounds had they still been in the Netherlands. The adults, however, corrected for their new location and nearly all reached their normal wintering grounds. It seems that birds with experience of a location can re-find it and that they must to some extent have ‘map sense’ to know where they are in relation to their home, based solely on their current location.
For birds that migrate at night, star positions are important in helping them get to where they are going. Birds put in planetariums where the star patterns had been changed became confused, while on overcast nights there tends to be less recorded migration.
That birds use the sun to navigate has been known for more than 50 years. And it’s thanks, again, to starlings.
In experiments that changed the sun’s direction, it was shown that starlings corrected their flight paths to take account of the sun moving. Lastly, we come to good old-fashioned learning. For those species who migrate as families (e.g. geese and cranes) it seems that the young learn the routes from their more experienced parents.
Once learned, younger birds are then able to travel the route successfully themselves.
Ancient books show how people in early times noted the comings and goings of birds. In the Iliad, written in the Eighth Century BC, the Greek poet Homer described the Trojan army as being ‘Like the cranes which flee from the coming winter and sudden rain’. In the Bible, the prophet Jeremiah says: ‘Yea, the stork in the heaven knoweth her appointed times; and the turtle (turtle dove) and the crane and the swallow observe the time of their coming’ (Jeremiah 8: 7).
Thousands of years ago, migratory birds were an important source of food. No wonder people noticed when they disappeared! The ancient Greek scientist and philosopher Aristotle made the first serious attempt to explain migration. He even spotted that birds get fatter while migrating. But he didn’t get everything right.
For example, he thought that many birds, including swallows, storks and kites, hibernated in trees for winter. He even believed in transmogrification, which is the idea that some species magically change into others – such as redstarts becoming robins for winter.
Some early explanations for migration seem pretty bizarre today.
When a swallow was found dead in a reed-bed, people thought this proved that swallows hibernated in the mud at the bottom of ponds. Believe it or not, people even thought that barnacle geese hatched from barnacles.
Perhaps this was because of the colour of barnacles or because their ‘feet’ (called cirri) look a little like feathers. In fact, barnacle geese migrate to Greenland to breed, but it was not until 1891 that a European scientist arrived there and actually saw one of their nests.
All around the world, people had their own ideas. The Cree Indians of North America thought that small birds, such as warblers, migrated on the backs of larger ones, such as sandhill-cranes.
The Laughing Badger Gallery, 99 Platt Street, Padfield, Glossop
●● Cranes following their migration route