As autumn draws to a close and winter comes into the fore, many of the world’s migratory birds are travelling far and wide in preparation for the season ahead. Visiting osprey and swallow have left the UK for the warmer climes of Africa, while fieldfare, brambling and widgeon arrive in their thousands.
Migration is a wondrous natural phenomenon, and true testament to the extraordinary capabilities of birds. 40% of all birds in the world are migrants, the long distance travellers amongst them being the most well-known. So you may be surprised to learn just how many other birds make up this vast and varied group – even the blackbirds and robins that inhabit your local park may be seasonal visitors from Eastern Europe.
Just imagine having to find your way from one country to another with no map, satnav or compass to guide you. Exactly how birds are able to navigate so effectively over such long distances remains the greatest mystery of migration, although we can be fairly sure of the main contributing factors.
Understanding the position of the sun (or the stars and moon for night-flying migrants) is a crucial skill for any migrating bird. A bird flying north knows to keep the sun behind it, and understands that in order to stay on course the sun must be over its right shoulder in the morning and over its left shoulder in the evening. Amazingly, birds also know that the height of the midday sun gets lower the further north they travel, and they use this information to help them determine when they have reached their destination.
Even when the sun is hidden by clouds in bad weather, birds can detect its whereabouts using polarised light, ultra-violet rays invisible to the human eye.
An internal compass
It’s also possible that birds have an internal ‘compass’. Scientists have found tiny grains of manganite in the brains of some birds, a mineral that detects the earth’s magnetic field. It may be that this helps birds to navigate by finding the position of true north.
Knowing when you have reached your destination is just as important as finding it. Some small birds use their body fat as an indicator – before setting off, they gain just enough body fat to last them the journey. Once the fat is used up, they know they have arrived.
As remarkable as they are, birds are not perfect, and do sometimes lose their way. But for the most part, they arrive exactly when and where they mean to. So, if ever someone calls you a birdbrain, take it as a well-earned compliment.