A nest box is an artificial nesting place for birds, or a roost for bats. In construction the term usually means not the wooden box on a tree but the built-in unit: a woodcrete or comparable block that is bedded into the masonry or fitted behind the cladding, and is all but invisible once the work is complete. The best known example is the swift brick.
Why it appears in building projects
Modern walls and roofs are sealed. Gaps under roof tiles, openings behind fascias and access into the cavity — the places where swifts, house sparrows and bats have always nested or roosted — disappear when a building is insulated or newly built. Breeding sites and resting places of these species are protected in law even when unoccupied at the time of the works. Renovation and insulation therefore require an ecological assessment, and nest boxes are the standard mitigation.
Types and dimensions
- Swift box or swift brick — a horizontal entrance about 65 mm wide and 30 mm high. Fit high in the wall, right under the eaves, with a clear flight line
- House sparrow box — a round entrance of about 32 mm. Sparrows are colonial nesters, so fit them in groups
- Bat box — a shallow crevice unit with a narrow slot on the underside, generally 15 to 20 mm high
- Surface mounted versus built in — one is screwed to the wall, the other bedded into the wall face
Built-in units are usually set back 20 to 30 mm so that brick slips can be fitted in front, leaving only the entrance visible. Fit several boxes with at least 500 mm between them, horizontally as well as vertically.
Points to watch
- Orientation. Avoid a full south wall: a box behind dark masonry in direct sun overheats. North to east suits most species better
- Height. Swifts need a clear drop, at least about 4 m up with nothing in front
- Never over a door or window. Droppings below the entrance are the reason boxes get removed again
- Keep the insulation continuous. A box set within the insulation layer must be thermally broken, or it becomes a cold bridge
- No perch on a bird box; a perch mostly helps predators
- Specification. Woodcrete lasts for decades and resists woodpeckers; thin plywood does not
- Timing. Carry out demolition, insulation and roof work outside the breeding season, and install replacement boxes well before the works so that animals can find them
Related terms
- Ecological building
- Green roof
- Natural value
- Retrofit insulation
- Cavity wall
- Fascia board
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