Houses

Nest Box Hole Sizes and Dimensions by Species

By Beak & Branch Editorial · Published August 20, 2026

A nest box is mostly a box. The one part doing real work is the entrance hole, and it works as a filter: at 1 1/2 inches an eastern bluebird passes and a European Starling does not (1 9/16 in is equally safe). Drill that hole an eighth-inch generous — the spade bit wandered, 1 5/8 looked close enough — and you have not built a bluebird house. You have built a starling house with a bluebird’s name on it. Most other numbers in a birdhouse plan are comfort and drainage. The hole diameter is the biology.

Nest box dimensions by species

Which source this table follows. Entrance diameters, floor sizes and mounting heights come from Cornell Lab’s NestWatch species pages in All About Birdhouses except where a specialist or state-agency source is named in the cell; the departures are set out below. Bluebird hole sizes follow the North American Bluebird Society (NABS), the specialist authority for that genus. The “hole above floor” column mostly does not come from NestWatch at all — NestWatch publishes total cavity depth instead — so those figures follow the classic state-agency chart as published by the Oklahoma Department of Wildlife Conservation, cross-checked against NABS for bluebirds and the Purple Martin Conservation Association for martins. Where a cell carries two figures with sources named, those sources genuinely disagree and both are set out below. The ranges are ranges on purpose: anywhere inside them is fine.

Why hole diameter is the whole mechanism

1 9/16 inches or smaller is the starling gate. European Starlings are cavity nesters that will evict and kill bluebirds, and a round entrance of 1 9/16 in (40 mm) or smaller excludes them (per NABS and sialis). That is why both accepted bluebird sizes — 1 1/2 in and 1 9/16 in — are starling-proof. Art Aylesworth was unable to find documented cases of starlings using a 1 9/16-inch hole unless it had been enlarged, and there are reports of dead starlings stuck in one. Larger round holes admit them, and so does any hole enlarged or gnawed wider: after starlings were found using 1 5/8-inch holes in 1984, Mackintosh and Aylesworth fitted 1 9/16-inch patches over them and shut the starlings back out. That is the single most valuable dimension in backyard nest box design, and why NABS treats bluebird hole size as a specification rather than a suggestion.

1 9/16 inches for mountain and western birds. Mountain Bluebirds are the largest of the three and need the bigger opening. NABS recommends the 1 9/16-inch hole anywhere Mountain Bluebirds occur, and anywhere their range overlaps Eastern or Western Bluebirds; NABS’s own nestbox recommendations factsheet goes further and gives Western Bluebirds 1 9/16 in outright, a split set out under the source disagreements below. Going up from 1 1/2 in to 1 9/16 in costs nothing in starling protection, because both sizes shut starlings out — so in overlap country the larger hole is simply the better fit for the bigger bird, with no trade to weigh.

Enlarging the hole destroys the protection, permanently. The commonest way a good box goes bad is a well-meant repair: a chewed or weathered hole gets “cleaned up” with a larger bit. A hole cannot be un-enlarged, which is why a hole-restrictor plate — a metal faceplate screwed over the opening with the correct diameter cut into it — is standard bluebird-trail hardware. It also defends against squirrels gnawing an entrance wider, which is how most boxes lose their hole size in the first place.

And hole size cannot solve House Sparrows — in a bluebird box. This is the honest limit of the approach. House Sparrows fit through any hole a bluebird fits through, so at bluebird diameters no hole admits your target bird and excludes them. Smaller boxes are a different story: a 1 1/8-inch round hole does exclude House Sparrows, so a chickadee box built at 1 1/8 in excludes House Sparrows in entry tests, provided the hole is cut exactly to size. Frank Navratil’s entry tests found 1 1/4-inch diameter still allows entry while 1 1/8-inch stops it, and NestWatch likewise notes House Sparrows squeezing through 1 1/4-inch holes — so 1 1/4 in and larger, which is titmouse, nuthatch, downy and every bluebird size, is not sparrow-proof.

On a bluebird box, then, nothing you drill will fix a sparrow problem. It is handled by monitoring, by siting boxes away from buildings and livestock feed, by fitting a sparrow spooker once the first egg is laid, and by removing sparrow nests. Identify the occupant before you touch anything. House Sparrows and European Starlings are non-native and are not protected by the Migratory Bird Treaty Act, so their nests and eggs may be removed — the US Fish and Wildlife Service publishes the list of species the Act does not apply to. Every native cavity nester you will actually find in a box — tree swallow, chickadee, titmouse, house wren, nuthatch, and the owls — is federally protected, and removing an active native nest is illegal. That includes the House Wren, which punctures bluebird eggs and is protected all the same. Competitor and predator defense is a separate job from hole sizing, covered in our nest box predator guards guide.

Where the sources genuinely disagree

Anyone comparing charts will hit these, so they are worth stating plainly.

Eastern Bluebird, 1 1/2 in vs 2 1/4 in. At the time of writing, NestWatch’s Eastern Bluebird page displays an entrance hole size of 2 1/4 inches, while its own Western Bluebird page shows 1 1/2 inches and NABS specifies 1 1/2 inches. Note that 2 1/4 inches is also the height of the accepted oval bluebird entrance (1 3/8 in wide by 2 1/4 in tall), so it may be an oval spec shown without its width. Unlike a round hole, an oval or slot is not inherently starling-proof — NABS warns that ovals not cut precisely to 1 3/8 in by 2 1/4 in can admit smaller starlings, and slot results against House Sparrows are mixed. Either way: a 2 1/4-inch round hole admits starlings. Build 1 1/2 inches.

Western Bluebird, 1 1/2 in vs 1 9/16 in. NABS’s own Nestbox Recommendations factsheet says Western and Mountain Bluebirds prefer a 1 9/16-inch round opening (or a 1 3/16-inch slot), and Bluebirds Across Nebraska, a NABS affiliate, likewise gives Western and Mountain Bluebirds 1 9/16-inch round openings; nestboxbuilder.com lists Western Bluebird at 1 9/16 in. Sialis’s summary of the NABS recommendation, by contrast, gives 1 1/2 in for Eastern and Western Bluebirds, with 1 9/16 in reserved for Mountain Bluebirds and overlap zones. Nothing is risked by taking the larger figure: 1 9/16 in (40 mm) also excludes starlings, so both sizes are starling-proof and the choice costs nothing in protection either way.

Carolina Wren, 1 1/2 in vs 2 1/2 in. Both are right, for different boxes. NestWatch’s Carolina Wren design is deliberately a “pseudocavity” with a 2 1/2-inch opening, because Carolina Wrens favour partly enclosed sites — the instinct that puts their nests in flowerpots and coat pockets. The traditional enclosed box uses 1 1/8 to 1 1/2 inches. Pick the design first, then the hole.

Purple Martin, round vs crescent. The Purple Martin Conservation Association gives 2 1/8 inches as the standard round entrance, with 1 3/4 to 2 1/4 inches acceptable, and a starling-resistant crescent entrance of 3 inches wide by 1 3/16 inches tall, whose bottom edge should be flush with the compartment floor or not more than 1/4 inch above it. Raise it further and you have given up the starling resistance that is the only reason to cut a crescent instead of a round hole. Compartment size is argued about too: 6 x 6 inches is PMCA’s stated minimum and the common commercial room, but PMCA reports that research has determined deeper compartments — 7 inches wide by 12 inches front to back — offer greater protection from predators and the elements and keep nestlings more comfortable.

Purple Martin mounting height, 10–15 ft vs 12–20 ft. NestWatch gives 10 to 15 feet; PMCA, the specialist authority for the species, recommends a pole height of 12 to 20 feet. The two overlap between 12 and 15 feet, which is the safe place to land if you want to satisfy both.

Bluebird mounting height, 4–6 ft vs 5–10 ft. NestWatch gives 4 to 6 feet; the Oklahoma Department of Wildlife Conservation chart gives 5 to 10 feet for Eastern Bluebird. Bluebirds Across Nebraska, a NABS affiliate, is blunter about the lower bound: mounting boxes less than 5 feet from the ground increases the opportunities for climbing or jumping predators to raid the nest. Since the whole point of the hole-to-floor depth is to keep a reaching predator short of the nest cup, 5 feet is the sensible floor for the pole as well.

Cavity depth for flickers. NestWatch lists 24 inches of interior depth for a Northern Flicker box where older agency charts say 16 to 18. Deeper is the safer error for large species, provided the hole-to-floor distance stays inside the range in the table.

The dimensions people skip

Depth below the hole. The gap between the bottom of the entrance and the floor is a predator specification. The NABS nest box specification factsheet puts the top of the floor 5 to 7 inches below the bottom of the entrance hole in a bluebird box; the Oklahoma agency chart gives a flat 6 inches for Eastern Bluebird. The point of the gap is that a raccoon or a cat cannot reach down to the nest cup. Shallow decorative boxes — hole three inches off the floor — are why “something took the babies” is such a common first-season story.

Floor area. Roughly 4 x 4 up to 5 1/2 x 5 1/2 inches covers almost every small songbird in the table. Too small crowds a late-stage brood; too large invites oversized nests. Bigger species scale up hard: 7-inch square for flickers, 8 inches and up for screech-owls, and a wood duck box is essentially furniture.

Ventilation and drainage. NestWatch’s build specification is concrete and worth copying: two 5/8-inch ventilation holes near the top of each side wall (four in total), at least four drainage holes of 3/8 to 1/2 inch in the floor or the floor corners cut off, the floor recessed at least 1/4 inch up from the bottom edges, walls a minimum 3/4 inch thick, and a sloped roof overhanging 2 to 4 inches at the front and 2 inches at the sides. Heat kills more nestlings than rain does; an unvented box in full sun in July is a genuine hazard.

A rough interior, or kerf cuts. The inside face below the entrance has to be climbable. Nestlings fledge by hauling themselves up that wall; on smooth planed pine or slick plastic they cannot, and they die in a box that is otherwise perfect. Score shallow horizontal saw kerfs like a ladder, or rough the wood with coarse sandpaper. For deep duck boxes, staple 1/4-inch mesh hardware cloth from floor to entrance — ducklings leave within a day of hatching, and a 24-inch smooth interior is a trap.

No perch. Ever. A dowel under the hole is the tell of a decorative birdhouse. Cavity nesters grip the entrance and go straight in; the perch only gives House Sparrows somewhere to sit and harass, and predators a handhold at exactly the wrong spot. NABS and NestWatch both say to leave it off. If a box you like has one, snap it out before hanging it.

Building versus buying

Buying is not automatically the safer option: a large share of retail “birdhouses” are ornaments with wrong hole sizes, perches, no ventilation and no way to open them for cleaning. Reading the numbers above against a product listing is the whole skill, and it is the same check applied in our bluebird house guide.

If you are building, you do not need to buy plans. NABS publishes free nest box plans as PDFs, NestWatch hosts downloadable plans on each species page, and state wildlife agencies — Tennessee’s TWRA and Oklahoma’s Department of Wildlife Conservation among them — publish cut lists under woodworking-for-wildlife programs. One 1 x 6 or 1 x 8 board of untreated cedar, pine or cypress makes a box. Never paint or treat the interior, and never use pressure-treated lumber.

Getting the box right is half the job. The other half is where it goes — covered in bluebird house placement.

Frequently asked questions

What size hole for a bluebird house? 1 1/2 inches in diameter for Eastern Bluebirds. For Western Bluebirds the sources split: NABS’s own nestbox recommendations factsheet and Bluebirds Across Nebraska give 1 9/16 inches, while sialis’s summary of the NABS recommendation gives 1 1/2 inches. Either is safe, because a round hole of 1 9/16 in (40 mm) or smaller excludes European Starlings, so both sizes are starling-proof. Use 1 9/16 inches for Mountain Bluebirds, and anywhere Mountain Bluebirds overlap the other two species.

What size should a bluebird house be? An interior floor of about 5 x 5 to 5 1/2 x 5 1/2 inches, with the bottom of the entrance hole 5 to 7 inches above that floor per NABS — the Oklahoma agency chart gives 6 inches. Mounting height is disputed: NestWatch says 4 to 6 feet, the Oklahoma agency chart says 5 to 10, and Bluebirds Across Nebraska warns that under 5 feet makes it easier for climbing or jumping predators to raid the nest — so treat 5 feet as the lower bound. Depth matters as much as width: it is what keeps a raccoon’s reach short of the nest cup.

Will a bigger hole attract more birds? It will attract more species, and most are ones you do not want. But not every sixteenth of an inch matters: 1 1/2 in and 1 9/16 in both exclude starlings, and the starling risk begins materially above 1 9/16 in — starlings were documented using 1 5/8-inch holes in 1984, a superseded historical spec rather than a current option — and with any hole enlarged or gnawed wider. Squirrels are not a hole-size problem you can drill your way out of either: flying squirrels will occasionally den in a standard 1 1/2-inch bluebird box and adults can enter a 1 1/4-inch hole, while gray squirrels gnaw an existing hole wider rather than needing a large one to begin with. Match the hole to the target species and accept that the box then belongs to that bird.

Should a nest box have a perch? No. Cavity nesters cling to the entrance without help, and a perch aids House Sparrows and predators. Remove any dowel before mounting. And if a hole has already been chewed wider by squirrels, fit a metal hole-restrictor plate rather than living with the enlarged opening — enlarging is irreversible.

Figures follow Cornell Lab NestWatch’s All About Birdhouses species pages, North American Bluebird Society nest box recommendations, Purple Martin Conservation Association housing standards, and state wildlife-agency nest box charts; disagreements between those sources are called out in the text rather than averaged away. Nest box dimensions are welfare-critical, so this page is re-checked against them on a rolling basis — last reviewed August 2026. See how we test.