How to tell the difference
Always on the wing - nests in holes and nest boxes high up on buildings. Can often be spotted in groups wheeling over roofs and calling to each other with high-pitched screams.
Nests inside buildings/barns. Often perch up on wires in small numbers, or in larger groups as they prepare to migrate.
Can be spotted around our towns and villages, where they build mud cup nests beneath the eaves of houses.
Nests in burrows, usually dug into sandy banks. They nest in colonies, with many pairs nesting close together at suitable sites. They're often seen over water.
Photographs from the RSPB