The sand martin (Riparia riperia), a swallow-like bird with a brown back and white underparts, is a fairly common breeding bird throughout mainland Britain and Ireland. A summer migrant to the UK, sand martins arrive between mid-March to mid-April to breed, before moving south in September to winter in sub-Saharan Africa.

Sand martins are aerial invertebrate feeders, and are often closely associated with areas productive for insects, near to open water. They nest in colonies that may contain 100 or more pairs, excavating horizontal nest chambers in dry, sandy, vertical banks in sand and gravel pits, railway cuttings, riverbanks and exceptionally in drainpipes in walls, holes in brickwork near to reservoirs, canal banks and similar areas of open water. Increasingly, however, sand martins have been found to make use of the sandbanks of both working and redundant mineral extraction pits. They will also utilise specially designed artificial 'nest-boxes' and banks.

Reedbeds are an important pre-migration habitat for sand martins, where large numbers of the birds can often be seen feeding over nearby open water, especially in the early evening.

The sand martin can be found in a number of locations in London where suitable breeding banks lie in close proximity to open water. 11 colonies were reported in the 1998 London Bird Report, representing a total of some 130-140 breeding holes. Of these colonies, 7 were within the Greater London boundary. The cumulative London Bird Report data can be considered to be a low estimate of the total population, since some sites will inevitably not be visited every year unless specific survey is being carried out.

A comparison of recent records with those from 30 years ago suggests a general decrease in population in the London area. This has been particularly marked in West London, though this has been partially offset by an increase in East London and Essex.

The sand martin population is also affected by circumstances in their wintering grounds. Droughts in the Sahel region of Africa in 1968 and 1985 seriously impacted the UK population. The movement of colonies, or transfer of birds between them can mean that new colonies are overlooked. There is therefore incomplete knowledge of the London population.

The Wildlife and Countryside Act (1981, as amended) protects sand martins from killing and catching, being held in captivity, and the wilful destruction of their nests.

The Environment Agency places sand martin holes in new flood defences. The London Wildlife Trust, BTCV and the Lower Lea Project have formed a partnership to seek funding for habitat work in the London Boroughs of Tower Hamlets and Newham. A sand martin barrel has been set up at Beddington Sewage Works in Sutton.

Monitoring of sand martin colonies is carried out at a national level through the Waterways Bird Survey (WBS) and Breeding Bird Survey (BBS).