Appliance of Science: How do birds know where to go when they migrate?

The swallows and house martins are back and have started building their nests and the distinctive sound of the cuckoo fills the air during many country walks.

These birds mark the arrival of the seasons and the passing of time in cycles.

Of course, not all birds migrate, but for those that do, come autumn we will watch them flock together before taking off on another epic journey back to warmer climates for the winter months.

But how do they do it? How do birds know when to go and how to get there?

ENVIRONMENTAL CUES

The migration of certain birds is probably triggered by a number of environmental cues such as changes in food supply, daylight hours and temperature. These parts are easier to understand; but the idea that they can all suddenly take to the skies and fly thousands of kilometres over many weeks is a little more baffling.

HOW DO THEY NAVIGATE?

Migrating birds appear to navigate using mental maps of the terrain, the position of the sun, moon and stars and the earth’s magnetic field. This last form of navigation is called magnetoreception.

MAGNETORECEPTION IN BIRDS

Magnetoreception is a sense that allows an organism to detect a magnetic field and orientate themselves according to it. Scientists have found small amounts of a substance called magnetite in the beak of many birds. Magnetite is a magnetic mineral. It is thought that the magnetite can align to the earth’s magnetic field, just like a compass, helping migrating birds navigate these long travels. Birds may have a second method of navigation using magnetic fields, specialised cells have been discovered in birds’ eyes that may make magnetic fields visible to them.

IT’S NOT JUST BIRDS

Many other animals use the magnetic field for orientation and navigation. Fish (like salmon and trout), butterflies, deer, dolphins, whales, molluscs and even certain bacteria are thought to use the earth’s magnetic field in some way. Small amounts of magnetite have been found in many different animals.

Salmon and trout are thought to have tiny traces of magnetite in their olfactory system (their smelling system) that allows them navigate back to their spawning areas to breed.

Even certain single celled organism, called magnetotatic bacteria, appear to navigate using the earth’s magnetic field. They contain little parcels of magnetite, called magnetosomes, each forming a chain along an axis that effectively turns the bacterial cell into a type of magnet itself, with a north and south type pole.

WHAT ABOUT HUMANS?

These magnetic detection senses may seem like super powers to humans but it is interesting to learn that small amounts of magnetite has also been found in bones in our noses and even in our brains.

Do we have some untapped ability to navigate by the earth’s magnetic field too?

Or perhaps, as some studies suggest, magnetite in the brain may play some part in the process of setting down long-term memories.

Some studies suggest a link between magnetite in human brains and a predisposition to diseases such as Alzheimer’s.

To answer all these questions it might help to first determine how the magnetite got into our brains in the first place.

Is it there naturally, or, as recent studies suggest, is it merely a product of pollution, inhaled from our environment?

It is probably best not to ditch the compasses and GPS systems just yet.