Last updated: August 21, 2026
The entrance hole is the only part of a birdhouse that decides who gets in. Everything else, the roof pitch, the wood, the little painted shutters somebody glued on, changes almost nothing about occupancy. Get the hole right and the box has a real chance of being used by the species you had in mind. Get it wrong by a quarter of an inch and you have built a house sparrow apartment.
The numbers below come from NestWatch, the Cornell Lab of Ornithology’s nest monitoring project, which publishes box specifications per species in its Right Bird, Right House resource. They are not our estimates, and where the published figure is a range rather than a single number, we have kept it as a range.
This article contains affiliate links. If you buy through them, we may earn a commission at no extra cost to you. Learn more.
Quick Answer
What size hole does a birdhouse need?
- It depends entirely on the species. NestWatch publishes a figure per bird, and the useful range for common backyard boxes runs from about an inch and an eighth up to about an inch and nine sixteenths.
- Eastern Bluebirds: NestWatch gives a round hole of 1 1/2 to 1 9/16 inches.
- House Wrens and chickadees: 1 1/8 inches, which is small enough that little else fits.
- To keep European Starlings out, NestWatch guidance is to keep the hole smaller than 1 9/16 inches.
Bottom line: pick the bird first, then cut the hole to the published figure for that bird and nothing else.
Why a Quarter Inch Decides Everything
Cavity nesting birds choose holes they can just fit through, because a snug entrance is protection. A hole larger than the bird needs lets in competitors and predators, and in North America the two competitors that matter most at backyard boxes are the House Sparrow and the European Starling, both introduced and both aggressive occupiers.
That is why the published sizes are exact rather than approximate. NestWatch guidance is that a hole smaller than one and a quarter inches will exclude House Sparrows, and that essentially only House Wrens and chickadees fit through an opening that small. Keeping the hole under one and nine sixteenths of an inch is what excludes starlings.
The Table
These are the entrance hole figures published by NestWatch for common backyard species. Use the exact figure for the bird you are targeting; do not round up because your drill bit set stops short, and do not enlarge a hole to hedge your bets.
| Species | Entrance hole | Keeps out | Notes |
|---|---|---|---|
| Eastern Bluebird | 1 1/2 to 1 9/16 in, round | Starlings | Open ground with scattered perches |
| Mountain Bluebird | 1 9/16 in | Starlings, marginally | Western open country |
| House Wren | 1 1/8 in | House Sparrows and starlings | Happiest near shrubs and cover |
| Black capped Chickadee | 1 1/8 in | House Sparrows and starlings | Wooded yards, wants wood shavings inside |
| Tufted Titmouse | 1 1/4 in | Starlings | Wooded and suburban yards |
| Tree Swallow | 1 3/8 in | Starlings | Open ground, often near water |
| Downy Woodpecker | 1 1/4 in | Starlings | Wants a box packed with wood chips |
Source for the entrance hole figures: NestWatch, Cornell Lab of Ornithology, Right Bird, Right House. Their species pages also give floor dimensions and box depth, which matter and which we have deliberately not paraphrased here.
Species Not in the Table
Purple Martins, screech owls, kestrels and Wood Ducks all use boxes, and all of them need specifications well outside the small box range above. Purple Martins in particular are a specialist project involving a colony house or a gourd rack rather than a single box, and the entrance geometry has its own literature.
For any of those, go to the NestWatch species page and follow the published plan rather than scaling up a bluebird box. Our guide to the purple martin house covers what buying one involves, and it is a different undertaking from hanging a wren box on a fence.
Boxes Sold Without a Species
A lot of decorative birdhouses have a hole cut to whatever looked right, often around an inch and a half on a box far too small for a bluebird, or a two inch hole on a box that then fills with starlings. If a box does not state which species it is built for, measure the hole and match it to the table before you hang it.
Round Holes, Slots and Guards
Round is the safe default. NestWatch has published on the round versus slot entrance question, and the round hole remains the standard recommendation for the common backyard species. A slot entrance is a variation used by some bluebird trail managers, not a general improvement.
Hole guards are a different matter and worth knowing about. A wooden or metal plate over the entrance thickens the wall at the hole, which makes it harder for a squirrel or a woodpecker to chew the opening wider. Once a hole has been enlarged, the box is open to whatever fits the new size, so a guard is cheap insurance on a box you want to keep for years.
Cutting or Correcting a Hole
A hole saw or a spade bit at the published size, drilled square through the front panel, is all that is needed. Sand the inside edge smooth so it does not catch feathers, and leave the wood below the hole rough or add shallow grooves on the inside so young birds can climb out when they fledge.
If a hole has already been chewed oversize, the fix is a hole guard plate with the correct opening screwed over the front rather than a new box. That restores the exact size and reinforces the spot at the same time.
Frequently Asked Questions
What hole size keeps house sparrows out? NestWatch guidance is that a hole smaller than one and a quarter inches excludes House Sparrows, and that at that size essentially only House Wrens and chickadees will fit. Bluebird sized holes cannot exclude sparrows by size alone.
What hole size keeps starlings out? Keeping the entrance smaller than one and nine sixteenths of an inch is the figure NestWatch gives for excluding European Starlings. That is why bluebird boxes sit right at the edge of that range.
Can one box suit several species? Only loosely. A one and a half inch hole is usable by several small cavity nesters, but each species also wants a particular floor size, depth and habitat. Building for one bird and accepting whoever moves in is more realistic than designing a universal box.
Does the box need a perch under the hole? No, and most guidance advises against it. Cavity nesters do not need a perch to enter, and a perch gives competitors and predators something to hold on to.
How high off the ground should the hole be? That is a placement question rather than a hole question, and it varies by species. Our guide on where to hang a birdhouse covers the heights and orientations NestWatch publishes.
Where do I check a species not listed here? The NestWatch Right Bird, Right House tool lets you filter by region and habitat and gives the published box specifications per species, including the ones too specialized to cover in a backyard table.
The Bottom Line
Choose the bird, then cut the hole to the exact figure NestWatch publishes for it, and add a hole guard if squirrels are around. Everything else about a birdhouse is secondary to that one measurement. If you are buying rather than building, our bluebird house guide and the wren house guide cover boxes that already have the right opening, and where to hang a birdhouse covers the placement half of the job.