Nature: Goodbye to all that

In April we look forward to the arrival of the cuckoo and the swallow – the most notable of our summer migrants – to indicate that summer is on the way. But nature has other, perhaps less obvious, ways of demonstrating that the most welcome season is imminent. Summer can also be heralded by the departure of visitors that have been here all winter. Migration is a global phenomenon – a complete movement northwards of birds (and indeed mammals too, in Canada and in northern Europe and Asia).

Why does this happen? Why can't everything stay where it is? Surely there are enough insects in Africa for swallows to feed on?

With the advent of summer, the days become longer and there is more time to feed. Wildfowl like geese and swans are herbivores and feed on grasses and water plants. They cannot do this in winter in the high Arctic, as their feeding grounds are frozen hard and covered with snow. But why do they bother putting all that hardship on themselves, flying back there from Ireland for the summer? Doesn't grass grow wonderfully well here? Haven't we loads of lakes with plenty of waterweed vegetation? It doesn't get that warm here in summer. So why do they go?

It's length of day that counts. These birds feed in the bright, and grass is poor enough fare. Loads of it must pass through a goose's digestive system to give it the energy to breed and to nourish the young growing birds. In fact a goose does a dropping every seven minutes when its digestive system is in full flow. So it needs to feed continuously during long hours of daylight, and that is exactly what they get in the high Arctic regions – tundra grass and continuous daylight. This phenomenon has been happening since the end of the last Ice Age. Much of northern Europe and Canada became available when the ice melted and any creature that could exploit this was at a breeding advantage.

The flocks of wild geese and Whooper swans are stimulated by the lengthening day, so by the Equinox, our swans were already off, back to Iceland. Geese go further north – the White-fronted and Barnacle geese go to Greenland and the Brent to Arctic Canada. That's very far north, so they are still hanging around fattening up on Irish wetlands, and no doubt sneaking any look they can get at Met Éireann's weather forecasts.

The Brent are still on Bull Island posing for photographs, but any day now, when they sense a favourable wind, they'll be off in great skeins, taking the last of the winter and the cold with them, we hope. Their departure is just as much a sign of summer as the much over-hyped first cuckoo.