Correct Entrance-Hole Sizes by Bird Species

The hole is the gatekeeper

Every other nest box dimension is comfort. The entrance hole is selection. It determines which species can physically enter, and — critically — which invasive species are locked out. European Starlings need roughly 1 5/8 inches; anything at or below 1 9/16 inches excludes them completely. House Sparrows squeeze through 1 1/4 inches, so holes of 1 1/8 inches and below keep them out in most situations.

Both species are non-native and unprotected in the United States, and both routinely kill native cavity nesters to take a box. Sizing the hole correctly is the cheapest, most effective intervention available to a backyard host.

Reference dimensions

Use round holes drilled with a sharp hole saw or forstner bit, cleanly deburred. Hole height above the floor should be six to eight inches for most species — deep enough to frustrate a reaching raccoon, shallow enough for fledglings to climb out with the help of interior grip cuts.

• House Wren — 1" to 1 1/8" round; floor 4" x 4"; mount 5–10 ft in cover.
• Black-capped Chickadee — 1 1/8"; floor 4" x 4"; fill with wood shavings.
• Tufted Titmouse — 1 1/4"; floor 4" x 4"; 6–15 ft near woodland edge.
• White-breasted Nuthatch — 1 1/4"; floor 4" x 4"; 12–20 ft on a mature trunk line.
• Eastern Bluebird — 1 1/2" (1 9/16" in the deep south); floor 5" x 5"; 4–6 ft in the open.
• Tree Swallow — 1 1/2"; floor 5" x 5"; open ground near water.
• Purple Martin — 2 1/8" round or a crescent 1 3/16" x 3"; gourd or compartment house 10–20 ft up.
• Downy Woodpecker — 1 1/4"; floor 4" x 4"; 12–20 ft, packed with wood chips.
• Northern Flicker — 2 1/2"; floor 7" x 7"; 10–20 ft, packed with wood chips.
• Screech-Owl / American Kestrel — 3"; floor 8" x 8"; 12–30 ft with a chip bed.

Why oversizing is the common mistake

Decorative birdhouses sold as garden ornaments frequently ship with holes of 1 3/4 inches or larger, a perch below the hole, no drainage, no ventilation, and no way to open them. That combination is close to a worst case: it admits starlings, gives predators a handhold, drowns or overheats nestlings, and makes cleaning impossible.

If you already own one, a hole reducer plate — a metal or hardwood faceplate with the correct diameter drilled through — converts it cheaply and adds valuable depth against raccoons. Remove the perch while you are at it; no native cavity nester needs one.

Crescent and slot entrances

Purple Martin landlords often prefer a crescent entrance roughly 1 3/16 inches tall by 3 inches wide. Martins pass through it easily while European Starlings, whose body shape differs, are largely excluded. It is the single most effective starling control available to martin colonies.

Slot entrances — a horizontal opening about 1 3/16 inches tall across the top of the front panel — are used on some bluebird boxes. They let a pursued adult escape more readily and may reduce House Sparrow interest in some regions, though results vary and round holes remain the standard.