Sand martins are similar to their cousin, the house martin. They are the smallest of these four birds. They are dark brown except for a white underside divided by a brown bar across the chest. They tend to swirl and flap rather than glide. They mainly fly over water, but will also perch on overhead wires or branches. These birds burrow holes into sandy or dry earth banks in sea-cliffs, railway cuttings, gravel pits or high riverbanks, although sometimes they will take advantage of drainpipes poking out of walls and holes in brickwork!
There is an old saying about how swallows can predict the weather. "Swallows high, weather dry." And although it isn't a forecast for endless days of sunshine, it is usually correct for the current day. Swallows fly high in warm weather because all the insects that they eat will fly high too. Only when the weather is cool and overcast do the insects fly low down and the swallows will swoop down too. So keep a look out to check if that saying is true. Happy bird-watching!