Many birds nest in holes. But modern forestry methods, and safety concerns in towns, mean that there are fewer old trees with hollows where birds can make their nests. And compared with older buildings, new houses are less likely to have spaces in or around their roofs, where birds like Common Swifts and House Sparrows can raise their young.
Nest-boxes are excellent substitutes, and more than 60 bird species are known to have used them. Regular residents include Blue, Great and Coal Tits, Eurasian Nuthatches, House and Tree Sparrows, Starlings, Spotted and Pied flycatchers, Robins, House Martins, Common Swifts, Common Kestrels and Tawny Owls.
The links below will take you to instructions for building different kinds of nest-box for different kinds of bird.