The annual and very welcome aerial invasion by some of our earliest summer visitors, the sand martins, has begun in Dorset. Despite their tiny size, they make the amazing journey of thousands of miles from southern and western Africa.

Weighing in at around half an ounce, about the same as six teabags, sand martins (Riparia riparia) are the smallest of the three species of swallows and martins, known as hirundines, that summer in Britain and Ireland. They’re a uniform, sandy-brown above and whitish below but with a distinctive brown band across the breast and a short, forked tail.

They’ve been spotted at the our reserves at Arne and Radipole Lake, where last year, some of them appeared to be checking out the artificial nesting wall specially built for them there.

Overlooked by a birdwatchers’ hide, the sand martin wall has 60 specially constructed nest holes, and cameras will send the images back to the refurbished visitor centre when it re-opens in May. So if they do breed, people will be able to watch the whole life history of this lovely little bird unfold, from courtship, nest-building and egg-laying to feeding, fledging and flying.

They can be seen hawking for insects over open water, sometimes even picking them off the surface. They’re very gregarious, nesting in large colonies by excavating long tunnels in sandy, dry, vertical banks. All of which makes Radipole Lake a popular destination for this charismatic little bird.

Sadly though, they are of conservation concern. Their numbers have declined across Europe and in Great Britain. There is also evidence that their numbers are falling in Dorset with the loss of their favoured breeding habitat in sandy banks.

It may take a few more years before the artificial wall at Weymouth Wetlands is fully occupied as the colony builds. At that point, it could house between 10-20 per cent of the Dorset population of sand martins.

Usually next to arrive are the house martins (Delichon urbica) from late March, then swallows (Hirundo rustica) from early to mid-April. Although superficially similar in looks and lifestyle to swallows and martins, swifts (Apus apus) belong to a completely different family. They are among the last visitors to arrive in May, heralding summer rather than spring. They’re also the first to leave in late August, early September.

The RSPB run a range of events and guided walks at both Arne and Weymouth Wetlands. Check out the website at rspb.org.uk or the blog at www.rspb.org.uk/weymouthwetlandsblog