Can birds solve the longitude problem?

As everyone knows, migratory birds are formidable long-distance travellers. Some of them cross entire oceans non-stop, and can find a small target at the end of their voyage. Not just an astonishing feat of endurance but also an amazing piece of navigation!

How do they know which way to go? How can they allow for cross-winds that may blow them far off course?

It used to be thought that migratory birds relied on a simple ‘clock-and-compass’ mechanism when they set off on their first journey.

The idea was that some kind of compass sense enabled them to select the right course to reach their goal, and that a clock mechanism told them to stop when they had travelled for long enough to reach it.

Rough-and-ready, to say the least!

Having made the journey once, it was supposed that the birds had access to new information (perhaps a learned sequence of landmarks) that would enable them to repeat the journey in subsequent years.

We know that birds have access to as many as three different compass systems - based on the sun, the stars or the earth’s magnetic field. It’s also clear that they can keep track of the passage of time.

But the ‘clock-and-compass’ model can’t explain everything.

Not only do birds often find their way to very precise locations after flying for thousands of miles, but they also stop in quite specific locations to rest and refuel en route.

How is this possible?

It looks very much as if these migratory birds have access to a ‘map’ of some kind.

But what kind of map could this be?

Well, believe it or not, the latest research suggests that it’s based on the birds’ ability to distinguish between the direction of true north and magnetic north!

These two poles lie in different places. The ‘true’ or geographical poles (that mark each end of the earth’s axis of rotation) are stationary, while the magnetic poles drift around, sometimes quite rapidly. The latter can be separated from the geographical poles by thousands of kilometers.

It’s long been known that migratory birds learn the direction of the northern geographical pole by observing the stars. Unlike us humans, they don’t pay attention to any particular star (like Polaris). Instead they somehow locate the still, central point of the rotational pattern of the stars that lie around the celestial pole (the point in the sky vertically above the geographical pole).

Their magnetic compass sense - on the other hand - gives them the direction of the magnetic pole (exactly how it does so remains uncertain and is the subject of intense research - see earlier post).

The angular difference between the geographical and magnetic poles (known as magnetic declination) varies depending on where you happen to be. In some places, the two poles line up, so the declination is zero. In other places it may have a large value.

If birds can measure magnetic declination, they would - at least in some parts of the world - be able to detect changes in their longitude. Now it looks as if they can indeed do so.

Truly astonishing!

If you’re interested in the technicalities, follow this link.