This, the smallest in our three swallows, is usually the first to reach, often before the end of March. All three species are drawn to the vicinity of water for feeding purposes, but as the sand martin's nesting-requirements-cliffs or banks of sand or earth-are frequently related to water, one has a tendency to think of it as the most water-haunting of the three.
Just like the swallow and house martin have thrived with the multiplication of nesting-sites made by man's building activities, so, less directly, has the sand martin. The digging of the recycleables for brick and concrete make available the earth-faces of innumerable clay and gravel-pits, which, wherever the softer layers prove penetrable, are readily colonised.
Other fairly natural sites developed by man are available in railway cuttings through suitable soils, including soft chalk, or perhaps in the thick mortar between the stones of medieval masonry; more purely artificial sites, relieving the bird of the labour of tunnelling, are supplied by drainage pipes in retaining walls in sites varying from riverside gardens to railway stations well from water. It appears strange those of all the British types of perching-birds, this, one of the most aerial in habits, and never apparently endowed with highly specialised tools for the job, should have grown to be the only excavator of tunnels in the earth.