Most birds do not need them, and they only make it easier for predators and House Sparrows or starlings to get into the house. Do not use creosote-treated lumber because the residues are toxic to birds. The outside surfaces of birdhouses can be treated with linseed oil. The house will last longer if it is assembled with brass hardware rather than steel, which rusts. Screws, although more expensive and requiring predrilling, are better than nails since they are less likely to pull out with time.
Some experts recommend using rough-sawn wood, which is easier for birds to grip with their feet.
This is important for birds such as Tree Swallows, which can have difficulty getting out of houses if the inside surface is too smooth. As an added measure for these birds, it is advisable to cut grooves on the inside front panel of the house or to add a small strip of hardware cloth on the inside under the entrance so that the birds have an escape ladder to help them get out. All houses should have drainage holes in the bottom and holes in the side panels near the top for ventilation. See accompanying diagrams for construction details. To be most successful, birdhouses should be constructed for particular species since many species have specific requirements for hole size and inside area.