When you’re ready to build a bird house, one of the most important considerations you must make is the bird house hole size. While the floor space, entrance hole height and roof height are also important dimensions, it is the diameter of the entrance hole that will most accurately determine which birds find the house attractive.
Why Hole Diameter Matters
To make birds feel welcome, safe and comfortable in your bird house, entrance size is critical.
A fraction of an inch too small and your desired residents could get stuck moving in and out of the house, may damage their feathers as they enter and exit or will not fit into the house at all. A fraction of an inch too large and you may find less desirable birds such as house sparrows and European starlings usurping the bird house or even entering and killing other birds or nestlings. A very tiny hole will exclude all cavity-nesting bird species, while a very a large hole is a welcome invitation to squirrels, rats, large snakes, raccoons or other animals to use the house instead of birds.
Resizing a Bird House Entrance
Many bird house kits and novelty bird houses come with pre-drilled entrance holes that may not match the optimum size for birds you want to nest in your backyard. Fortunately, it is easy to increase or decrease the size of a bird house hole and still use the house design you are interested in.
If a hole is much too small for your birds’ needs, use a paddle or spade drill bit to enlarge it uniformly, taking care not to crack or splinter the wood around the hole. Taping around the hole before drilling can help keep the edge smooth and intact, though you may need to sand away any rough edges after drilling.
If the hole only needs to be enlarged a small bit, a round file may be sufficient or you can use a smaller drill bit along the edges of the hole. Be sure to increase the hole size uniformly around the circumference to preserve the circular shape as much as possible, though it does not need to be perfect.
Making a bird house hole smaller is easier than it may seem. For simple bird houses, you can attach a piece of balsa wood or other thin wood with an appropriately sized hole over the original entrance hole with nails or carpenter’s glue. A wooden resizer, however, may only last one nesting season. For a more permanent solution consider using thin sheet of metal that will resist chewing and talon damage from resident birds. Other options include using a thicker wood piece, an old shingle or even a chunk of natural bark for a rustic, somewhat camouflaged entrance.
Optimum Hole Sizes for Common Bird House Nesting Species
Different individuals of the same bird species may vary slightly in size, but that variation is so small that bird house hole sizes can be made to standard dimensions without difficulty. These basic entrance sizes can help attract just the types of birds you want to become backyard residents.
- American Kestrel - 3 inches (7.6 cm)
- Ash-Throated Flycatcher - 1 1/2 inches (3.8 cm)
- Barn Owl - 6 inches (15.2 cm)
- Black-Capped Chickadee - 1 1/8 inches / 2.85 cm)
- Carolina Chickadee - 1 1/8 inches (2.85 cm)
- Carolina Wren - 1 1/2 inches (3.8 cm)
- Downy Woodpecker - 1 1/4 inches (3.2 cm)
- Eastern Bluebird - 1 1/2 inches (3.8 cm)
- Eastern Screech-Owl - 3 inches (7.6 cm)
- House Finch - 2 inches (5.1 cm)
- House Wren - 1 1/4 inches (3.2 cm)
- Northern Flicker - 2 1/2 inches (6.35 cm)
- Prothonotary Warbler - 1 1/8 inches (2.85 cm)
- Purple Martin - 2 1/2 inches (6.35 cm)
- Tree Swallow - 1 3/8 inches (3.5 cm)
- Tufted Titmouse - 1 1/4 inches (3.2 cm)
- Violet-Green Swallow - 1 1/2 inches (3.8 cm)
- Western Screech-Owl - 3 inches (7.6 cm)
- White-Breasted Nuthatch - 1 1/4 inches (3.2 cm)
- Wood Duck - 4 inches (10.2 cm)
Where two bird species share similar hole size preferences and also breed in the same range - such as the downy woodpecker and the house wren - either bird may use the house. Even if other dimensions may vary, if the bird finds the house to be a good nesting spot, they'll be happy to raise a family there. Similarly, birds may often use houses with larger than typical entrance holes, though they can be at greater risk of predators or other threats. Even if a bird nests in an unexpected bird house, however, backyard birders should never attempt to remove a nest unless it is obviously unsafe or the feathered family has left and it is time to clean the house.
By keeping bird house entrance holes to the proper sizes, the houses will be more attractive and backyard birds will be more likely to raise their families safely and securely.