3/4 inch wooden boards or PVC pipe with attachable wooden roofs are commonly used for bluebird boxes. Peterson boxes often use 2 x 4 inch boards.
Do not use pressure treated wood because they include toxic compounds.
Paper milk carton style or corrugated cardboard boxes are unacceptable.
Woods such as redwood and cedar are long-lasting even when left natural.
Eastern Bluebirds use 1-1/2 inch round holes, 1 3/8 x 2 1/4 inch vertical oval holes, or 1-1/8 inch horizontal slot entrances.
Western and Mountain Bluebirds use 1-9/16 inch round openings.
Where the ranges of the species overlap use 1-9/16 inch round openings.
Oval holes should only be used in eastern bluebird boxes with moderate to small dimensioned boxes to reduce the possibility of starling use.
Eastern Bluebirds: floors in wooden boxes should be approximately 4 x 4 inches or 5 x 5 inches (Peterson style boxes are somewhat smaller), floors of circular boxes (such as PVC pipe) should be approximately 4 inches in diameter.
Western or Mountain Bluebird boxes should be at least 5 x 5 inches or 5-1/2 x 5-1/2 inches to accommodate larger clutch sizes.
It is imperative that all bluebird nest boxes open readily from the top, side, or front to facilitate box monitoring and cleaning.
If box sides or front pivot to allow access to the box, they should do so at as high a point as possible to ensure that you can observe tall nests without the door obstructing your view.
A screw or angled nail in a pre-drilled hole should be provided to ensure that mammalian predators can not readily open the nest box.
Natural wood is acceptable.
If painted or stained, use light colors to minimize having the box overheat during warm weather in areas where overheating is likely.
Drainage holes must be provided in the box bottom to allow any rain entering the box to drain from the box and to provide air circulation to keep nesting material dry.
The box should be water-tight.
The roof should provide sufficient overhang beyond box entrance or vent holes to minimize possibility of rain entering these openings.
The roof should cover top edge of the box back unless other features eliminate any possibility of rain entering the joint between back and roof of box even if the wood warps.
Vents providing cross ventilation should be present near the box peak. These openings should be protected from rain by having the box roof overhang a sufficient amount to minimize precipitation entering the box.
Dark colors should be avoided to minimize overheating.
It should be possible to plug or cover vent holes during cold weather periods early in the nesting period.
Long roof overhangs minimize the possibility of sun, rain, or snow entering the box.
The box should be easy to mount on a predator-resistant post in areas with raccoons or cats.
A 5 inch roof overhang above the entrance hole reduces the possibility of raccoon or cat predation.
Wooden guards placed over the entry hole are not effective in eliminating raccoon predation.
Boxes mounted on heavily greased pipes or on waxed metal electrical conduits will deter many climbing predators.
Mounting boxes less than 5 feet from the ground increases the opportunities for climbing or jumping predators to raid the nest.
Wooden posts, un-greased pipes, PVC pipes are readily climbed by nest predators such as raccoons.
Boxes should be designed so that they may readily and securely be mounted on a support post such as water pipe or electrical conduit.
Fence posts are acceptable mounts in areas where raccoons are rare.
Having the back extend beyond the main box body below or above the box will allow you to attach the box with screws, nails, pipe clamps, wires, or u-bolts.
Perches should never be used on any bluebird boxes because they are not needed by bluebirds and only facilitate harassment by non-native species such as House Sparrows.
Interior walls should not be painted or stained.
The front wall below the entrance hole should feature a rough surface to facilitate chicks climbing to the entry hole.
Nest boxes with raised screen floors may reduce blowfly infestations but this has not been proven conclusively.
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