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).