As autumn progresses and the temperatures drop in Britain, we see the arrival of hundreds of thousands of birds migrating south from the Arctic to winter in the relative warmth of the British Isles. In wetland areas you can see huge flocks of ducks, geese and other shorebirds visiting from the Arctic throughout the winter months. These birds have navigated thousands of miles to reach the wintering grounds. However these journey pale in comparison to some bird species; the Arctic tern travels between the Arctic and Antarctic and back again every year! But how do birds find their way across our planet?

Scientists have long puzzled over what cues and "compasses" birds use to both locally navigate and migrate long distances. The sun is one obvious cue that could be used by birds to navigate. Researchers have shown homing pigeons misinterpret the sun's information and so hinder their ability to navigate when released by subjecting them to artificially altered light/dark regimes. But what about on cloudy days? It is thought birds are able to see polarised light. The way light waves are rotated and reflected by atmospheric particles depends on the sun's position above the horizon and it's thought birds - and some other animals including insects - can use this a kind of compass even on overcast days.

Not all birds migrate by day; moving at night can reduce the risk of predation and avoid the heat of the days in lower latitude regions. The moon is not a reliable navigational cue due to its change in shape throughout the lunar cycle and orientation across the world. It's thought that nocturnally migrating birds use star patterns to navigate. For example Indigo Buntings raised so that they could only see the night sky, and no landmarks, were still able to migrate, and further experimental evidence suggests the North Star was a crucial cue.

The Earth's magnetic fields have long been considered an important cue for migrating animals by land, sea or air. However, until relatively recently, how animals are able to detect and use these weak magnetic fields to navigate has been a mystery. It has been shown that magnetic navigation is light dependant and the discovery of the light dependant protein cryptochrome in photoreceptive cells in the brains of birds (along with many other organisms from plants to mammals) has brought the understanding of magnetic navigation a step closer. Photo-induced electron transfer between components of the cryptochrome protein results in radical electron pairs being formed. Magnetic fields are capable of altering the spin of such radical pairs. Electron pair spin is the only known mechanism by which the Earth's weak magnetic field could be detected biologically. As cryptochrome is found in photoreceptor cells of birds, it is suggested that they may be able to even "see" the Earth's magnetic fields.

Learning can also be an important aspect for some migrating bird species. For example migrating adult starlings are capable of compensating for being displaced during migration. However, juvenile birds on their first trip cannot fully compensate if displaced, although they will continue to fly south. This demonstrates their migration behaviour is in part innate but relies on the learning of a map during their first migration. In some species the innate ability to migrate thousands of miles is quite remarkable - for example British cuckoos chicks, who are not raised by their parents but by another species altogether, are capable of migrating south to certain areas of Africa entirely unaided.