BTO: April sees surge in arrival of migrants

Sand martin (Riparia riparia). Picture: John Harding/BTO ANL-150330-114648001
Sand martin (Riparia riparia). Picture: John Harding/BTO ANL-150330-114648001
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As we roll over into a new month so we see an upturn in the arrivals of spring migrants. One of the first birds to arrive back in any numbers is the sand martin, the earliest of our breeding hirundines (swallows and martins) to make its return – see www.birdtrack.net

This rather plain looking bird winters in West Africa, where its fortunes are influenced by the availability of invertebrate food. In those years when the rains fail, the drought conditions greatly reduce the availability of the insects on which the martins feed. Sand martin populations crashed following the Sahelian droughts of 1968-69 and 1983-84, though numbers have since recovered.

Sand martins nest in colonies, taking advantage of soft sandy banks in which they can excavate their short nesting tunnels. Suitable sites may form where river meanders cut into their banks but many more birds utilise the sites created by quarrying activities. Over recent years conservation organisations have developed artificial sand martin banks on nature reserves, many attracting sizeable sand martin colonies and allowing researchers to monitor the birds. The more advanced artificial banks have nesting chambers that are accessible from inside of the bank, something that enables nest recorders to record the numbers of eggs and chicks produced, and to ring the nestlings while still in the nest. By collecting information over several years it is possible to examine various aspects of the martin’s ecology, providing information that can inform and direct conservation action.

The sand martin is less well-known than its relatives, the swallow and the house martin, and many people remain unaware of its presence here as a summer visitor. It has, however, been well-studied by researchers and remains a firm favourite with birdwatchers because it is one of the first returning migrants to be encountered. As a breeding species, the sand martin is most abundant in the north and west of Britain but there are breeding colonies scattered across Suffolk, particularly within the Brecks and along the coastal fringe.

Breeding is highly synchronized within sand martin colonies and the first young will have fledged by the end of May, leaving the adults to make further nesting attempts. Departures are a more protracted affair and many young birds wander widely within Britain ahead of their autumn migration. Late summer sees large numbers of sand martins congregate in communal roosts, most of which are located within areas of reed-bed, which presumably offer some protection from would-be predators.

It is back in spring, however, that the sight of these long distance migrants is perhaps most rewarding. Early arrivals will gather over waterbodies, where they can find the small flies and other flying insects needed to replenish reserves expended on their annual migration.