Decorative bird house or real nest box?
Many bird houses sold as garden decor look charming and do little for birds. They may have perches that help predators, holes sized for no particular bird, thin walls that overheat, no drainage and no way to open them for cleaning. A genuine nest box is plain by comparison. It is made of untreated wood about three-quarters of an inch thick, has a roof that sheds rain, drains, ventilates and opens for cleaning, and has an entrance hole sized for a particular species.
Cavity-nesting birds in the United States, including bluebirds, chickadees, titmice, nuthatches, wrens, tree swallows and, in some regions, kestrels and screech-owls, nest in holes in dead trees. Where those are scarce, a well-made box can fill the gap. The Cornell Lab of Ornithology's NestWatch program publishes species-specific nest box plans and lets you report what happens inside your box as a citizen scientist.
Entrance holes by species
The entrance hole is the most important dimension in a nest box. Too large and larger competitors or predators get in. Too small and your target species cannot enter. The sizes below are commonly cited; check NestWatch or your state wildlife agency for details on your species and region.
| Bird | Typical entrance hole | Notes |
|---|---|---|
| Black-capped or Carolina chickadee | about 1 1/8 in | Likes wood chips or shavings inside |
| House wren | about 1 1/8 to 1 1/4 in | Fills the box with sticks; common in yards |
| Tufted titmouse, white-breasted nuthatch | about 1 1/4 in | Woodland edges and mature yards |
| Eastern or western bluebird, tree swallow | about 1 1/2 in | Open lawns with short grass for bluebirds; open water for tree swallows |
| Purple martin | about 2 1/8 in | Colony nester; needs open sky around the housing |
| American kestrel, eastern screech-owl | about 3 in | Larger boxes mounted higher |
A 1 1/2 inch round hole is generally small enough to keep European starlings out of a bluebird box. Never add a perch, which helps house sparrows and predators more than it helps birds.
What good construction looks like
- Untreated wood. Cedar, cypress or pine about 3/4 inch thick insulates well. Avoid pressure-treated lumber and paint or stain inside.
- Drainage and ventilation. A few small holes or slots in the floor and near the top under the roof keep the box dry and cool, which matters on the hot Plains.
- Overhanging, sloping roof. Water runs off and the entrance is shaded from rain.
- A side or front that opens. You need to be able to check and clean it without removing the box.
- A rough surface inside below the hole. Chicks need to climb out. Score smooth wood or add small saw cuts.
- No perch and no decorations. Plain is safer.
Where to put a nest box
Habitat matters as much as the box. Bluebirds prefer open ground with scattered trees and short grass, and tree swallows prefer open areas near water. Chickadees and titmice favor woodland edges, and wrens will use boxes in shrubby yards. A box in dense shade or right beside a busy feeder rarely works.
- Mount on a pole, not a tree. Poles with a predator guard are far harder for raccoons, snakes and cats to climb than a tree trunk or fence post. A smooth metal pole or a stovepipe-style baffle is commonly used.
- Height. Common suggestions are about 5 feet for bluebirds and 5 to 10 feet for wrens and chickadees, with higher mounting for larger species. Check species-specific advice.
- Spacing. Bluebirds and swallows are territorial. Boxes are often spread about 100 yards apart for bluebirds, and some trail monitors pair two boxes close together so a tree swallow and a bluebird can each claim one. Treat distances as a starting point and check species-specific advice.
- Orientation. Face the entrance away from prevailing wind, often toward the east or southeast on the Plains, and keep the box open to a clear flight path.
- Distance from feeders. Keep boxes away from busy feeders, where traffic and house sparrows can crowd out the target species.
Timing
Put nest boxes up before the season begins. NestWatch advises having them in place by February in the South and by mid to late March in northern regions. Clean out and repair last year's boxes then too.
Cleaning and monitoring
After the young have fledged and the box is empty, remove the old nest. Wear gloves and a dust mask, brush out the debris, and let the box dry. Some species raise two broods, so a mid-season cleaning between broods can encourage a second nest. Do not use pesticides or strong chemicals. The Migratory Bird Treaty Act protects the nests and eggs of most native species, so only clean a box after the young have left. If you are not sure whether a nest is active, leave it alone. Rules for non-native species such as house sparrows and starlings are different, and your state wildlife agency can explain them.
Monitor gently once a week by tapping, opening the box quickly and closing it. Logging what you see helps NestWatch and your own learning.
Common mistakes
- Choosing by looks. Decorative boxes with perches and tiny holes rarely work.
- Skipping the predator guard. A baffle is cheap insurance.
- Oversized holes. They invite house sparrows and starlings.
- Mounting too late. Birds begin searching before the weather turns.
- Never cleaning. Old nests carry mites and can discourage new tenants.
Frequently asked questions
What size hole does a bluebird box need?
About 1 1/2 inches, round. That is also the size commonly used for tree swallows.
Should I put a perch on a bird house?
No. Cavity nesters do not need one, and it gives house sparrows and predators a foothold.
When should I put up a nest box?
Before nesting begins. NestWatch recommends about February in the South and mid to late March in the North.
Is it okay to clean out a box?
Yes, once the young have fledged and the box is empty. Never disturb an active nest, since native birds' nests and eggs are legally protected.
Do I need to paint the box?
No. Untreated wood works best. If you want to reduce heat in a hot climate, a light exterior paint on the outside only, with no paint inside or on the entrance, is sometimes used.
