Over the past few weeks hundreds of Dark Bellied Brent Geese, among thousands of other birds have made their long journeys both to and away from British shores. It was the geese above that inspired me to write this article, watching them flock in to the Essex coast, their home for the winter months. They are particularly handsome birds with white dog collars set against their dark plumage and tufts of white at their posterior end. At least 4,000 species of bird are regular migrants – about 40 per cent of the world’s total. In the far north, including Canada, Scandinavia and northern Russia the vast majority of birds migrate whereas in a temperate climate such as the British Isles about half of the birds move on. In tropical regions such as on the equator few species migrate since the food is more plentiful all year round. Similarly birds will migrate in different ways. Some will migrate from high ground to low ground, some will only move on when population outstrips food supply (irruptions – such as the Waxwing population in Scandinavia moving to Britain occasionally), some will use a place as a ‘stop off’ for a few weeks before continuing a journey whilst others are tied by the moult (passage migrants – such as Shelducks which lose all their flight feathers for a while and so have to stay put).
I will always find it a great wonder as to how all birds know when to move on. In many ways however it is simply down to biology. Each year, around the same time, glands in the bird’s body will secrete hormones resulting in the bird spending as much time as possible eating as much as possible, to prepare it for the long journey ahead. Smaller species can gain 3-4 per cent of their body weight a day during this time; a Sedge Warbler almost doubles its weight from 10g to 20g in just three weeks, giving it sufficient energy to fly to Africa. Birds will also begin to move in to flocks – evident especially with the starlings moving from rooftop to rooftop. Many birds will wait for high pressure before they set off – low atmospheric pressure causes wind, cloud and rain and therefore gives extra risks for the migration. Birds of one species do not all migrate together. With many wading birds, the females set off first, leaving the males to stay behind and raise the chicks. The males leave as soon as the chicks can fend for themselves. Dunlins from Greenland reach the UK in three waves: first the females in July, then the males in August, and finally the young in September.
Most migrating birds usually fly at a height of between 200 and 1,500 metres above sea level with many not touching down during the flight, especially when crossing water, unless a sea bird species. Navigation remains the biggest mystery of migration but we assume that they make use of various geographical and built landmarks. However, young birds who have never made the journey before are still able to make the migration which goes against this theory. The sun is probably the most important signpost for the bird to give it the general direction of flight. Scientists found that over 80 per cent of common terns from one colony returned to nest within eight metres of their previous year’s nest – having traveled thousands of kilometres in between. Our knowledge of the natural world continues to grow with much observational data now available to us. However, there is much that we still need to learn and volunteers can do their bit as part of citizen science programmes. Why not get in touch with your local wildlife trust group or visit a site such as Nature’s Calendar, ebird or wildlifelog.
Far more information on migration can be found at the RSPB’s website. I acknowledge this site as the source of much of the information above.