RICHARD COLLINS: House martins take their time

SPRING was late this year.

Wind and rain kept the summer migrants away.

Some intrepid sand martins, ignoring the weather warnings, turned up in early April. With few insects on the wing, conditions were particularly severe for them. Swallows arrived late in the month but house martins are taking their time — I’ve seen none yet. That’s no surprise: the house martin does its own thing.

Often mistaken for swallows, house martins are easily identified. Their conspicuous white rumps are very obvious in flight. Swallows are squatters, nesting indoors, but house martins remain discretely outside. The nest is a neat mud cup, stuck to the wall under the eaves of a building.

Ninety million martins cross the Sahara each spring and autumn. According to The Migration Atlas, over a million had been ringed in Europe up to 2002. Of these, only 20 were found subsequently in southern Africa. Contrast this with the figure for swallows. Of 1.3m swallows ringed in Britain and Ireland, 482 turned up south of the Sahara.

The two species have similar habits; you would expect similar numbers of ringed ones to be found. Why, then, are there so few martin records? The birds’ sleeping arrangements reveal the answer. Songbirds generally travel by night; the daylight hours are spent feeding, laying on fuel for the journey. Swallows and martins catch insects on the wing; they travel and feed at the same time.

This allows them to sleep at night. Locations along the migration routes provide bed and breakfast accommodation for the pilgrims. Areas with reed beds are especially popular for brief stays, as water retains the heat it gathered during the day, such places remain relatively warm at night. Thousands of swallows may roost together. During their sojourn in South Africa, they continue to sleep communally and places where they do so are Meccas to bird-ringers. Thousands of swallows are caught in mist nets close to roosts.

Among them are birds ringed in Europe 10,000km away.

House martins don’t use communal roosts. Ringers, therefore, can’t target them in large numbers. Only ringed birds trapped casually, or found dead by members of the public, come to light and the reporting rate for these is exceedingly low. It’s no surprise the ringing returns are so poor compared to those for swallows.

But, if house martins don’t use communal roosts, where do they spend the night? During the breeding season, parents sleep at, or close to, the nest. Newly fledged youngsters, unlike other small birds, may even return to the nest at night. But what do non-breeding adults do? We don’t know. Nor have we any idea what the birds do on migration or in Africa.

This has led to an extraordinary, but speculative, theory. The Migration Atlas notes that ‘aerial roosting must be suspected’. The evidence is circumstantial. Martins have been seen flying high at dusk and descending at dawn, suggesting that they spend the night on the wing. Ringers have reported catching martins which appeared chilled in the early morning. There is also a physical clue; house martins wear ‘long johns’, their legs are feathered. Birds, such as ptarmigan and grouse, which live in the mountains, are similarly attired. If martins spend the night on the wing, keeping bare parts well covered would help protect them from the cold.

The evidence that swifts roost aloft is compelling, so why shouldn’t martins do likewise? Swifts belong to an ancient order of birds which evolved many curious roosting habits over tens of millions of years. They sleep under waterfalls in South America and in pitch dark caves in Asia. The martins, however, are ‘Johnny-come-latelys’ in evolutionary terms. Could they have developed sleeping arrangements so very different from the tried and tested ones of their relatives, the swallows and sand martins? Is it possible that the martins leaving your house in the autumn, don’t touch the ground again until they return in the spring?