Birdhouse entrance hole sizes by species, and why they matter

The one number on a birdhouse listing that decides who moves in is the diameter of the hole, and it is the number most decorative houses get wrong. A hole that is a sixteenth of an inch too big lets in a species that will evict the one you built for. A hole that is a little too small gets ignored. Here are the sizes, in the fractions of an inch that nest box plans use, and the other dimensions that decide whether a box is used for one season or ten.

If you are buying for the small birds most yards get, a box with a 1 1/8 inch hole, a 4 by 4 inch floor and a side that opens is the house that works for chickadees and wrens. For bluebirds, 1 1/2 inches in the east and 1 9/16 in the west, on a 5 by 5 inch floor.

Hole sizes by species

Chickadees and house wrens: 1 1/8 inch. This is the tightest common size. It admits both of those species and keeps out house sparrows, which need about 1 1/4.

Tufted titmouse, white-breasted nuthatch, Carolina wren: 1 1/4 inch. House sparrows can now enter, so these boxes need placement away from buildings where sparrows gather.

Eastern bluebird and tree swallow: 1 1/2 inch. The round hole is traditional; an oval 1 3/8 by 2 1/4 inch opening also works and some trail monitors prefer it.

Western and mountain bluebird: 1 9/16 inch. The extra sixteenth matters for the larger western birds and is still too small for a starling.

Great crested flycatcher: 1 3/4 to 2 inches.

Northern flicker: 2 1/2 inches, on a tall box 16 to 24 inches deep, often filled with wood shavings the bird excavates.

Screech owl and American kestrel: 3 inches, on a box with a 7 to 8 inch floor and 15 to 18 inch depth.

Purple martin: 2 1/8 inch round, or a starling-resistant crescent opening about 1 3/16 inches tall and 3 inches wide, in multi-compartment houses or gourds.

Wood duck: 3 by 4 inch oval, on a large box near water.

The starling and sparrow lines

European starlings cannot pass a hole of 1 1/2 inches or smaller, which is why every bluebird box is built to exactly that size and not a hair more. House sparrows are smaller and fit through 1 1/4 inches. Both species take over boxes and neither needs help.

So a box with a 1 1/8 inch hole is the only common size that both species are shut out of by the hole alone. Every larger size relies on where the box is and on checking it during nesting season. Listings that promise a “sparrow-proof” bluebird house are overselling.

Floor, depth and the drop to the hole

Floor size sets how many chicks fit. Four by four inches suits chickadees and wrens. Five by five is the bluebird standard. Six by six for flycatchers and flickers, seven or eight square for owls and kestrels.

Depth from the hole to the floor matters for safety from reaching predators and for keeping chicks in until they are ready. Six to eight inches for the small species, eight to twelve for bluebirds, more for the large cavity nesters. A shallow decorative box with the hole an inch above the floor is a box a raccoon can reach into.

The panel that opens

Nest boxes need cleaning out between broods and at season’s end. A side or front panel on a hinge or a pivot screw lets you do that in a minute. A roof that lifts off also works. A box with no opening at all cannot be cleaned without tools, and most owners stop bothering after the first year, which is when the box stops being used.

Drainage, ventilation and the roof

Four small holes or clipped corners in the floor drain rain. Slots or holes under the roof overhang vent heat in summer. A roof that overhangs the hole by two inches or more keeps rain off the entrance and shades it. These three features are what separate a box built from a plan and a box built to look like a house.

Wood, paint and metal guards

Untreated cedar or pine three quarters of an inch thick is standard, insulates, and weathers without paint. If a box is painted or stained, the inside should be bare and the outside a light colour. Dark boxes overheat. A metal hole guard, a plate screwed around the entrance, stops squirrels and woodpeckers enlarging the hole, and is a cheap addition to any wooden box.

Perches, and why to skip them

A perch below the hole is a decorative habit. The birds that use cavities land directly at the entrance. Starlings, sparrows and climbing predators use the perch. Houses that come with perches are built for the garden centre, not the bird. Pull the perch out or pass.

Who should not buy a box

Anyone whose yard has no open ground or a sheltered edge for a pole, since a box screwed to a house wall mostly draws sparrows. Anyone who will not open and clean it at least once a year. And anyone buying for a species not found locally; a bluebird box in a dense city centre is a sparrow box.

How we choose houses

When we recommend nest boxes in our houses, baths and poles section, every pick lists the hole diameter, the floor size, the depth, how it opens for cleaning, the wood and thickness, and the mounting type, because those are the facts that decide whether a species uses it. We make no claims about bird health. Our full process is on the how we choose page.

Final verdict

Pick the species first, then the hole: 1 1/8 inch for chickadees and wrens, 1 1/2 for eastern bluebirds, 1 9/16 for western. Insist on a panel that opens, a floor that drains, a roof that overhangs, and no perch. Mount it on a pole with a baffle, as our feeder placement guide describes, and clean it every autumn.