andrewlee asks: Birds migrate to sunnier climes with more plentiful food supplies, such as north Africa. So why do they bother to come back
In a nutshell - for the same reason that all us Brits don't all just jet off to Malaga or Mallorca for our hols and decide to stay put. It may be warmer down south, but the UK is home -- and it's where family and food are. (There are differences between birds and us as well. It's never been confirmed that birds return to the UK because they like the beer, a particular football team, Pop Stars: The Rivals or because they have jobs to get back to.)
This pursuit of food, warmth and family life has evolved over millions of years, which probably accounts for the fact that many birds travel far longer than they would otherwise need to, in order to reach specific, traditional breeding or feeding grounds. As many as a third of all the world's birds migrate.
So the prime factor behind migration is...
... the search for food and a place to breed. The difference in temperature between a northern European summer and winter can be extreme - too much for certain land-based animals, who choose to hibernate through the winter (see Why do some animals hibernate and others don't ). For those creatures that that can fly, a journey towards the equator is the preferred option, not least because a good part of their food supply... is in hibernation!
So why come back at all
The British winter may be too harsh but our summer is ideal - long hours of sunlight, plentiful food supplies and the best habitat to breed and bring up baby. Splitting life between two distinct areas has another major advantage: there's no point in a predator evolving that wants to eat you if you're only around for half the year or less.
What triggers the decision to migrate
We don't know for certain. It's likely to be climatic, or some innate sense of what's happening to their food supply. One thing is for certain: they leave before the food runs out. No bird could migrate far on an empty stomach or, to be more accurate, without significant reserves of body fat. And they wait for a following wind.
How does each new bird find its way
This is still something of a mystery. Even the cuckoo, abandoned in the nest of an unsuspecting bird as an egg, grows up knowing where and when to migrate. There's no parental instruction there.
It seems birds use the sun and stars to navigate by, as well as major physical features of the earth, such as rivers, mountains or coastlines. There's some kind of homing instinct at work that allows a bird thrown wildly off course to find its way to its destination.
Some migration facts:
When things go wrong
Changing land use and climate can confuse a bird - even a bad storm can throw an individual off course. When the whole species is threatened, though, mankind has occasionally stepped in to try and fix things. The endangered Whooping Crane is led from Wisconsin to Florida and back every year by conservationists piloting ultra-lite aircraft!
I've developed a twitch.
Either you need to sort out your computer and adjust the height of your chair, or you've developed an interest in birds sufficient to become a 'birder' or 'twitcher' (handle with care: some people take umbrage at the term). Prepare yourself for pre-dawn starts, standing all day knee deep in freezing water, and a penchant deeply unfashionable green anoraks. Don't say we didn't warn you.