How fast does time fly for migrators?

THE songbirds of Europe are well studied, but some aspects of their lives are shrouded in mystery. Most small birds migrate at night, when we can’t see them; their flocks appear on radar screens.

Roosting places would be impossible to find in the dead of night in unfamiliar terrain, so birds probably fly non-stop, using the stars as a guide.

How far can a small bird travel in a single flight, and, after a long journey, does a bird take the next night off? Carrying extra fat adds to a bird’s work-load, and, as with humans, being overweight is dangerous; a fat bird is slower, less manoeuvrable and more easily seized in a hawk’s talons. A migrating bird might only carry enough fuel for one flight. Being as light as possible and refuelling at each stopping point may be the best option. But is it?

Wind and weather can upset travel plans and migrants often get lost. Nor can a bird know, in advance, whether food will be available at staging points along its route. Shortages, or a run on food when too many birds arrive at a location, may present problems for the lean flier. Carrying extra fat is a chore but a necessary one. Ornithologists catch birds, weigh them, and put numbered rings on their legs. More than 35,000,000 birds have been ringed in Britain and Ireland, with 700,000 of the rings being reported subsequently. A similar scheme is operating in Scandinavia, where K. Susanna, S. Hall-Karlsson and Thord Fransson, of the Swedish Museum of Natural History, have tried to answer bird travel questions. Their findings appear in the current issue of Ringing and Migration, a journal published by the British Trust for Ornithology.

The Swedish team says small songbirds carry 20% to 30% additional fat when migrating. This will ensure that birds have extra fuel in case they get lost or encounter food shortages at staging posts. Migrants carry an even greater load when crossing a major obstacle. Before heading out over the Sahara, a bird may put on 40% to 70% body weight in additional fat. Wheatears regularly fly from Greenland to France and Iberia, a distance of 2,500km. They may have to fly non-stop for three days and nights.

Small birds can eat a prodigious amount in a short time, refuelling quickly. A waxwing in Norfolk, in 1957, ate 390 berries, its own weight in food, in two and a half hours. Most birds could put on enough fat to fly for several nights in a row without taking a break. Do migrating birds press on to their destination or take time off along the way? The Swedish ornithologists concentrated on birds known to have travelled at least 50km within a week of being ringed. There were 419 of them. Thirty-six birds covered more than 200km each day. Journeys varied from 50km to 1,978km, with a median of 254km. Two sedge warblers flew for three nights. An ortolan bunting, ringed in Gotland, was found 1,978km away in France six days later. A garden warbler travelled 1,562 km to Italy in seven days.

Birds returning to Sweden in the spring travelled faster than those leaving the country in the autumn, 77km per day compared to 56km per day. A bird facing into the breeding season must secure a territory after arriving at its destination; there may be better territories available to birds that arrive early.

The team concluded that “the distances covered by migratory birds during a flight stage can vary considerably, but long-distance passerine migrants may regularly fly several stages in a row, with short rests in between, before refuelling, even if there are no major barriers ahead.”

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