Epic migration – birds do it, bees do it, even educated gastropods do it

http://www.theguardian.com/commentisfree/2014/may/19/migration-birds-bees-snail-whale

Version 0 of 1.

The news that snails have a homing instinct – which crafty gardeners can overcome by moving them more than 20 metres away from their home patch – may come as a surprise to some. But perhaps not to those of us who have long marvelled at the navigational skills of wild creatures.

Ever since our prehistoric ancestors first looked up into the skies and wondered at the annual return of swallows in spring and geese in autumn, the seasonal migrations of wild creatures have been the subject of human curiosity and awe.

The ancient Greek philosopher Aristotle was one of the first to speculate on these annual movements, proposing that the redstart – a summer visitor to Europe from Africa – transformed each autumn into the robin, and that swallows hibernated under water or in caves.

The latter theory persisted for at least two millennia. Even the great 18th-century naturalist Gilbert White clung to this mistaken belief, unable or unwilling to believe the truth: that a bird weighing less than an ounce is able to travel thousands of miles each year to spend the winter in southern Africa; and then, even more incredibly, return to the very place it was born the following spring.

We now know that birds use a range of tools to find their way on these epic global journeys: from the Earth's magnetic field and the position of the sun and stars, to physical landmarks such as mountain ranges and rivers as they get nearer to their destination. They are adaptable too: scientists discovered that the proverbial avian navigator, the homing pigeon, has learned to follow roads in order to find its way home.

Bird migrations vary enormously in distance and complexity. At one extreme, the Arctic tern travels up to 35,000km from the Arctic to the Antarctic and back each year, while the bar-tailed godwit was recently discovered to fly from Alaska to New Zealand – a journey of 11,000km across the Pacific Ocean – in a single hop.

Less spectacularly, the shortest known bird migration is that of the mountain quail in North America, which simply descends a few hundred metres from its mountaintop summer home to spend the winter in the valleys below – a journey it undertakes on foot. Presumably it has no need to employ complex navigational skills to do so.

Most people are well aware that birds migrate, but may not realise that many other creatures do too. Some of the longest global journeys are made by marine mammals such as grey whales, which do an annual round trip of up to 19,000km from their winter breeding grounds off Mexico to their summer feeding areas in the Arctic Ocean.

Even small and delicate butterflies are also great travellers. Five years ago Britain experienced an invasion of tens of millions of painted ladies, which had flown from North Africa to breed here. Hummingbird hawk-moths do the same journey each spring and summer, bringing a touch of the exotic to our flower borders as they hover to feed on nectar.

Honeybees, too, employ complex navigational skills to find their way to and from distant sources of nectar and pollen. When performing their famous "waggle dance", they even use an inbuilt clock to make allowances for the shift in the position of the sun during the time elapsed since they have flown back to the hive.

Back home, if you do move a snail the full 20 metres from your garden, how long will it take to return to munch on your prize begonias? Although proverbially slow, a typical garden snail can travel at between one and four metres an hour – so it will arrive back in your garden well within a day. Maybe you should simply tolerate the presence of snails – and rather than waste your energy relocating them, trust to your local song thrush the control of their numbers.