Why birds migrate?

A migrating bird uses up quantities of energy equivalent to a large proportion of its body weight. It flies over vast areas of ocean, desert and other hostile environments in which it cannot pause to rest and will certainly not find any food. It may be blown off course by winds, thrown out of the air by storms and pursued by predators. That many migrating birds die en route to their destinations is a certain fact, though not one that can be backed up with precise numbers. Migrating birds generally return to the same areas to breed, so we can work out that as few as half of them may return from one breeding season to the next. However, it is not known how many of these die on migration and how many die in their winter quarters, for the simple reason that we only have a vague idea about where most of them go.

So why do they do it? The short answer is that they migrate because it profits them to do so. In the autumn, migrating birds escape the cold and the food shortages that follow in winter's wake by fleeing to the abundance of the tropics. The spring sees them returning to take advantage of longer days and wider spaces, in which they can more easily raise their families. Sometimes a bird returning to breed in an area for the second or third time will nest in exactly the same place as before. More frequently, however, birds will upgrade to one of the more desirable residences left empty by neighbours who failed to return. Very occasionally, a bird will return to breed with a previous partner. But after a bird has travelled across the world and back again, the circumstances will usually have changed sufficiently that it will find itself in a new territory, with a new partner.

Before setting off to do it all again.

Mark Wilson