Why hole size matters
The entrance hole is the single most important measurement on a birdhouse. A hole that is the right size lets your target species in while making it harder for larger competitors and some predators to get inside. A hole that is too large can invite house sparrows, European starlings, or squirrels that may take over the box or harm the eggs and young.
Different cavity-nesting birds have different body sizes, so there is no all-purpose hole. Pick the species you want to help first, then size the house around it.
Hole size chart
The sizes below are typical recommendations from conservation groups and nest box guides. Sources differ by a small amount, so treat them as a starting point and check a regional guide such as NestWatch, which is run by the Cornell Lab of Ornithology, before you build.
| Bird | Typical entrance hole diameter |
|---|---|
| House Wren | 1 to 1 1/4 inches |
| Black-capped and Carolina Chickadee | 1 1/8 inches |
| Tufted Titmouse | 1 1/4 inches |
| White-breasted Nuthatch | 1 1/4 inches |
| Downy Woodpecker | 1 1/4 inches |
| Carolina Wren | 1 1/2 inches |
| Eastern and Western Bluebird | 1 1/2 inches |
| Mountain Bluebird | 1 9/16 inches |
| Tree Swallow | 1 1/2 inches |
| Great Crested Flycatcher | about 1 3/4 inches |
| Purple Martin | about 2 1/8 inches (often a round or crescent opening) |
Larger birds such as American Kestrels, screech-owls and Wood Ducks need much bigger openings and boxes, and plans for those are best taken from a species-specific source.
Keeping out house sparrows and starlings
House Sparrows and European Starlings are introduced species that compete aggressively with native cavity nesters. Hole size helps, though it is not a guarantee.
- Starlings generally cannot fit through holes much smaller than about 1 9/16 inches, so the 1 1/2 inch holes used for bluebirds and the smaller holes for chickadees and wrens help exclude them.
- House Sparrows can squeeze into holes around 1 1/8 to 1 1/4 inches or larger, so hole size alone does not reliably stop them. Some people use sparrow-resistant box designs or monitor boxes and remove sparrow nests where it is legal to do so.
Native birds are protected in the United States, while House Sparrows and starlings are not, but check your local rules before removing any nest. For feeder-side tactics, see how to deter sparrows from feeders.
Beyond the hole: floor, depth and height
The hole is only part of the picture. Each species also prefers a certain floor size, interior depth and placement height. As a rough guide:
- Small species like chickadees and wrens do well with a floor of about 4 by 4 inches.
- Bluebirds and tree swallows are commonly given a floor of 4 by 4 to 5 by 5 inches.
- The hole should sit well above the floor, often several inches, so that nestlings are less reachable from outside.
- Mounting height ranges widely, from a few feet for bluebirds to higher for chickadees and martins.
If you want to build a box for bluebirds specifically, see our bluebird nesting box plans.
What to do if your hole is too big
If a house you bought has an oversized opening, you can fit a metal hole guard or a wooden block with the correct diameter over the entrance. Metal guards also protect the wood from squirrel chewing. Choose an untreated wood block, and make sure the opening is smooth so birds do not scrape themselves.
Where not to put a bird house
A good site matters as much as a good hole. Many guides suggest avoiding:
- Spots in full, all-day sun in hot climates, where the box could overheat.
- Locations directly beside busy feeders, where traffic and conflict are higher.
- Places easy for cats, raccoons or snakes to reach, such as near a fence a predator can climb.
- Spots with no clear flight path to the entrance.
Mounting on a metal pole with a baffle is generally safer than nailing a box to a tree.
Common questions
What birds use birdhouses? Cavity-nesting species such as chickadees, wrens, bluebirds, nuthatches, titmice and tree swallows. Many birds, including robins, prefer open platforms and ignore enclosed boxes.
What about gourds? Hollowed gourds can work for some species if the hole is sized correctly. See what birds use gourd birdhouses.
Do I need to clean the box? Yes, in most cases, after nesting is complete. See how and when to clean bird houses.