Fattening swallows prior to migration

THERE’S an interesting paper in the current edition of the journal Ringing & Migration.

In ‘Fattening Strategies of British & Irish Barn Swallows prior to Autumn Migration’ Lisette Coiffait, Robert Robinson, Jacquie Clark and Bridget Griffin examine the weights and ‘fat scores’ of 543 swallows caught at 35 locations in Britain and six in Ireland. Prior to migration, swallows spend their nights in communal roosts, close to which they can be caught using mist-nets. Each bird is ringed and a range of measurements taken.

Small birds must fatten up before migrating; they need extra fuel for the journey. At this time of year, most of the day is spent feeding and there’s little time left for anything else. The only option for most avian travellers is to forego their beauty sleep and ‘fly by night’. It’s safer to do this in any case; an overweight bird is sluggish in the air. Less able to wriggle out of danger, it’s a sitting duck for predators.

Swallows, however, are an exception to the rule; they spend the night sleeping and travel by day, insects providing the in-flight meals. But to live on the wing and catch individually targeted insects as swallows do, a bird must be slim and trim. Carrying surplus fat is not on. Daylight flying in unfamiliar territory means running the gauntlet of the local hawks and falcons, another reason to be light. Swallows are seldom far from a supply of insects when flying over land, but on sea crossings no food is available. There can also be shortages over the sandy wastes of the Sahara, although insects are abundant near pools at oases.

In 1997, Andreas Pilastro and Ariele Magnani showed that Italian swallows put on extra weight before crossing the Mediterranean. A similar fuelling pattern was found in Spain. Some swallows increase their body mass by as much as 30 to 40%. According to one estimate, an extra gram of fat may fuel up to 400kms of travel. Our swallows must cross the Irish Sea, the English Channel or the Celtic Sea. Do they put on extra weight for the journey? The Ringing & Migration authors found that British and Irish swallows lay on fat at this time of year and that both adults and juveniles do so. The adults tended to gain more mass than the youngsters. However, deciding whether birds fatten up to help them migrate, or do so for other reasons, is not as straightforward as it seems. Bird weights fluctuate during autumn in any case. Adults are light after the demands of nesting and raising young. They must also renew their tattered worn-out flight feathers. The moult is a drain on resources and, until it’s completed, birds will tend to remain light. Weight gains are to be expected as normality returns. A similar regime applies to juvenile birds. They take a while ‘to get to know the ropes’ after leaving the nest.

The swallows in the south of Britain tended to be heavier than those caught further north, suggesting that birds were laying on fat for the sea-crossing. However, northern birds which have completed their moult and restored their body weight may tend to move south earlier, increasing the average weight recorded there.

The authors examined all of these possibilities and subjected their data to the statistical pyrotechnics which scientists use on these occasions. They claim to have found ‘the first compelling evidence that, despite the first stage of their autumn migration consisting of a short crossing to the continent, British and Irish swallows build up moderate fat reserves prior to departure’.

Perhaps the most extraordinary finding is that swallows here put on just enough weight for the sea-crossing to Europe with some to spare in the event of adverse weather or food shortages. When they prepare to cross the Mediterranean later, they will take on a much greater load. The birds seem to know in advance how far they will have to fly and adjust their feeding rates accordingly.