Habitat
Nest Boxes: Sizing and Siting
One number on a nest box matters more than all the others: the diameter of the entrance hole. Get it right and you choose your tenant. Get it wrong and you have built a house sparrow apartment.
The short answer
- Entrance hole diameter selects the species: about 1 1/8 in (2.9 cm) for wrens and chickadees, 1 1/2 in (3.8 cm) for Eastern bluebirds, 1 9/16 in (4 cm) for Western and Mountain bluebirds.
- A box with a perch below the hole is a worse box. Cavity nesters do not need one, and it hands predators and house sparrows a grip.
- Untreated wood, drainage holes in the floor, ventilation gaps under the roof, and a panel that swings open for cleaning. No paint inside, no metal roof, no plastic.
- Mount on a smooth metal pole with a baffle, not on a tree or fence. A box on a tree trunk is a raccoon buffet.
- Face the hole away from prevailing wind and driving rain, and away from the hottest afternoon sun in southern yards.
The entrance hole diameter is the single most important number on a nest box. It decides which species can get in and, just as usefully, which cannot. An eighth of an inch is the difference between a box that raises chickadees and a box that raises house sparrows.
Everything else on the box is real but secondary: untreated wood, a floor that drains, air at the top, a side that opens, a pole with a baffle under it. Those choices decide whether the brood survives. The hole decides whose brood it is.
This guide gives hole diameters and mounting heights by species, the construction details worth insisting on, why a perch is a defect rather than a feature, and how to stop raccoons, cats and snakes from turning a nest box into a feeding station.
Entrance hole diameter and mounting height by species#
Cut the hole with a hole saw or spade bit to the exact diameter and resist the urge to sand it wider. Precision is the entire point.
| Hole diameter | Species | Mount height | Floor size |
|---|---|---|---|
| 1 1/8 in (2.9 cm) | House wren, Carolina wren, Black-capped and Carolina chickadee | 5 to 10 ft (1.5 to 3 m) | 4 x 4 in (10 x 10 cm) |
| 1 1/4 in (3.2 cm) | Chickadees, Tufted Titmouse, White-breasted Nuthatch | 5 to 15 ft (1.5 to 4.5 m) | 4 x 4 in (10 x 10 cm) |
| 1 1/2 in (3.8 cm) | Eastern Bluebird, Tree Swallow, Downy Woodpecker | 4 to 6 ft (1.2 to 1.8 m) | 5 x 5 in (13 x 13 cm) |
| 1 9/16 in (4 cm) | Western Bluebird, Mountain Bluebird | 4 to 6 ft (1.2 to 1.8 m) | 5 x 5 in (13 x 13 cm) |
| 2 1/2 in (6.4 cm) | Northern Flicker, Hairy Woodpecker (2 in / 5 cm) | 8 to 20 ft (2.5 to 6 m) | 7 x 7 in (18 x 18 cm) |
| 3 in (7.6 cm) | Eastern and Western Screech-Owl, American Kestrel | 10 to 30 ft (3 to 9 m) | 8 x 8 in (20 x 20 cm) |
The hole should sit 6 to 8 inches (15 to 20 cm) above the floor in a small songbird box, and much higher in a flicker or owl box, so that a young bird cannot tumble out and a reaching paw cannot get down to the eggs.
For species-specific plans with exact panel dimensions, Cornell's NestWatch program publishes free downloadable plans for dozens of North American cavity nesters, along with a tool that tells you which species your yard can realistically host. It is the reference to build from.
The eighth of an inch that matters
A 1 1/8 in (2.9 cm) hole excludes most house sparrows while letting chickadees and wrens through comfortably. Open that hole to 1 1/4 in and sparrows start getting in. Open it to 1 1/2 in for Eastern Bluebirds (Sialia sialis) and you accept sparrows as a management problem you will have to handle actively.
Why a nest box should not have a perch#
Almost every decorative birdhouse sold has a dowel under the hole. It exists because people think birds need somewhere to land. Cavity nesters do not. They grip the wood, hook their claws on the lip of the hole and go in.
The perch does three unhelpful things. It gives a house sparrow (Passer domesticus) a comfortable place to sit and intimidate an occupant, and sparrows will destroy eggs and kill nestlings to take a cavity. It gives a raccoon or a jay a brace point while it reaches an arm inside. And it gives a European starling somewhere to work from at a hole that would otherwise be marginal.
If your box has one, snap it off and plug the hole with a dowel and glue. The birds will not miss it.
Wood, drainage and ventilation#
Use untreated softwood or cedar, 3/4 in (2 cm) thick. Thin wood cooks. Pressure-treated lumber, plywood with exterior glues and any interior paint or stain are all off the list, because the box holds a featherless nestling against bare wood for two weeks.
Four details separate a working box from an ornament:
- Drainage. Four 1/4 in (6 mm) holes in the floor, or the four corners of the floor panel trimmed off at an angle. Rain will blow in through any hole facing any direction.
- Ventilation. Gaps or drilled holes near the top of both side panels. A sealed box in full sun becomes an oven, and overheating kills more nestlings than cold does.
- A rough interior below the hole. Saw kerfs, a strip of hardware cloth, or a scored surface, so fledglings can climb out. Smooth plywood traps them.
- An overhanging roof. Two inches (5 cm) of overhang past the entrance keeps rain off the hole and shades it.
The front or a side panel should pivot on two screws so you can open the box in a second. A box that requires a screwdriver and a ladder does not get cleaned, and an uncleaned box accumulates parasites and old nest material.
Do not add a metal roof or a plastic body
Metal and dark plastic absorb heat fast and hold it. In a July heat wave a metal-topped box on a south-facing post can exceed the temperature nestlings can survive. Wood, light color on the outside only, and shade after midday.
Mounting: pole, baffle, and away from the tree#
Mount the box on a smooth metal pole, not on a tree trunk or a fence line. A tree gives raccoons, squirrels and rat snakes a direct climbing route to the hole, and a fence gives them a highway. It is the same logic that governs feeder poles.
Under the box, fit a stovepipe or cone baffle at least 24 inches (60 cm) long, mounted so its top sits about 4 feet (1.2 m) off the ground and it wobbles when pushed. A rigid baffle that a raccoon can grip is much easier to defeat than one that tips.
A metal or wooden hole guard, a second thickness of the same diameter fixed over the entrance, stops squirrels from gnawing the hole wider and stops raccoons from reaching straight through to the back of the box. It costs almost nothing and prevents the most common takeover.
Face the hole away from prevailing weather. Across most of North America that means east or southeast, with north acceptable and west the aspect to avoid. In the hottest southern yards, avoid any aspect that takes full afternoon sun.
Spacing, habitat and realistic expectations#
Boxes are not interchangeable with habitat. A bluebird box works in open ground with short grass and a perch to hunt from, which is why bluebird trails are set along pastures and mowed edges rather than in woodland. A chickadee box wants trees within a short flight. Read how to attract bluebirds before deciding your yard is bluebird country.
Space boxes for the same species at least 100 yards (90 m) apart where you can, because most cavity nesters defend a territory and a second box inside it stays empty. Boxes for different species can be much closer. Pairing two boxes 10 to 20 feet (3 to 6 m) apart is a standard trick where Tree Swallows compete with bluebirds: the swallows take one, defend it, and leave the second to the bluebirds.
If a box sits empty for a season, the usual reasons are the wrong habitat, too much traffic nearby, or existing natural cavities doing the job already. Leaving snags and dead wood standing is the version of this that requires no carpentry at all.
Managing house sparrows and starlings#
Starlings cannot enter a hole of 1 1/2 in (3.8 cm) or smaller, which solves that problem for every songbird box on the list. House sparrows can, and a sparrow that claims a bluebird box will kill the occupants.
Monitoring is the answer, not tolerance. Check boxes weekly, and remove house sparrow nests (a messy dome of grass, trash and feathers) as soon as they appear. Both house sparrows and starlings are introduced species without federal protection in the United States, so removing their nests is legal, while touching a native bird's nest is not. The broader tactics are in controlling house sparrows at feeders.
Cut back on what draws sparrows
Sparrows concentrate where cheap seed is scattered. Switching your feeders off mixed grain and onto black oil sunflower or safflower seed thins the local sparrow flock, which lowers the pressure on every box in the yard.
The yearly cycle: what to do and when#
- Late winter. Clean out old nests, check hinges and screws, re-seat baffles, patch chewed holes. Boxes up by late February in the South and mid-March farther north catch the earliest prospecting.
- Spring and summer. Check weekly. Note what is nesting, watch for wasp nests in the roof (a smear of bar soap on the underside of the lid discourages them), and keep the pole clear.
- After each brood fledges. Scrape the box out. Many species raise two or three broods, and a clean box gets reused within days.
- Fall. Leave boxes up. Chickadees, titmice, nuthatches and wrens roost in them through cold nights, which is reason enough not to store them.
Feeding continues alongside all of this, though the food that matters most during nesting is protein: see feeding during nesting season for what to offer while there are nestlings to supply.
A box that will actually be used#
- Correct hole diameter for a species your yard can realistically host, cut clean, no perch.
- Untreated wood 3/4 in (2 cm) thick, floor drainage, side ventilation, roof overhang.
- Side or front panel that opens on two pivot screws.
- Smooth metal pole, wobbling baffle beneath, metal hole guard over the entrance.
- Hole facing east or southeast, 4 to 10 feet (1.2 to 3 m) up depending on species.
- Weekly checks in season, cleaned after every brood.
Then treat the box as one piece of a larger setup. Boxes fill fastest in yards that already offer cover, water and insects, which is the argument laid out in the bird-friendly yard checklist, and they fail fastest where free-roaming cats hunt, which is covered in cats and birds. Safe material for the nest itself is a smaller question with a few sharp edges: see offering nesting material safely.
Frequently asked questions#
What size hole should a birdhouse have?
It depends entirely on which bird you want. Use 1 1/8 in (2.9 cm) for wrens and chickadees, 1 1/4 in (3.2 cm) for chickadees and titmice, 1 1/2 in (3.8 cm) for Eastern bluebirds, and 1 9/16 in (4 cm) for Western and Mountain bluebirds. Cut the hole cleanly and do not round it out any wider, because a quarter inch of extra diameter can let in a starling or a house sparrow.
How high should a bird house be mounted?
Bluebird boxes go 4 to 6 feet (1.2 to 1.8 m) up, wren and chickadee boxes 5 to 10 feet (1.5 to 3 m), flicker boxes 8 to 20 feet (2.5 to 6 m) and screech-owl boxes 10 to 30 feet (3 to 9 m). Height matters less than what the box is mounted on: a baffled metal pole beats a tree trunk at any height.
Should a nest box have a perch?
No. Cavity-nesting birds climb in and out using their feet and tail and have never needed a perch. A dowel below the hole gives house sparrows a place to sit and harass the occupants, and gives a raccoon or a jay something to brace against while reaching inside.
Which way should a nest box face?
Away from the prevailing wind and driving rain, which in most of North America means facing east, southeast or north rather than west. In hot southern yards, avoid a westerly aspect that bakes the box through the afternoon. If you can, aim the hole at an open flight path with a shrub or low branch 10 to 20 feet (3 to 6 m) away for fledglings to land in.
When should I clean out a nest box?
After each brood fledges, and again in late winter before the season starts. Scrape out the old nest, brush the box dry, and leave it open to air. Old nests hold parasites and hold moisture, and a box packed with three seasons of material sits the next clutch dangerously close to the entrance hole.
Do nest boxes need drainage and ventilation?
Yes to both. Drill four holes about 1/4 in (6 mm) in the floor, or cut the corners off the floor panel, so rain that blows in can drain. Leave gaps or drill holes near the top of the side panels so hot air escapes. Boxes without ventilation can reach lethal temperatures in full sun.