Several birds have come to depend upon buildings when looking for a suitable nesting site. These are birds which have originally built on cliffs or in hollow trees, and each species chooses a position on the building which corresponds with its original nesting habits.

In most cases birds are regarded as welcome guests, and many give great pleasure. It should, however, be remembered that birds’ nests may be the source of certain parasites and of some pests of textiles (see pp. 36, 41, 43, 45, 49 and 94).

This familiar bird is very dependent upon buildings. It may nest inside a building, for instance, under the roof of a stable where there is continuous access to the outside, or it may build under the eaves.

The nest is in the form of a hemispherical saucer of mud mixed with earth and a little straw. Swallows are territorial, so there is usually a fair distance between the individual nests. They often return to the same nest year after year, and in Britain they usually start to lay eggs in the middle of May.

They feed on insects, mainly flies, gnats, midges, which are caught in the air.

In the wild house martins build on cliffs but many make use of buildings. They always nest on the outside of buildings, often in small colonies, but occasionally there may be hundreds together. They build high up, rarely less than 3 m from the ground, and the nest which is constructed of mud is completely enclosed with a small entrance hole at the top.