Most bluebird enthusiasts don’t build their own nest boxes. They tend to buy them from their favorite wild song bird store, “big box” store, Farmers Market, or perhaps from a volunteer organization that produces low-cost nest boxes.
Regardless of where you obtain your bluebird nest box, the most important thing is to ensure the nest box is SAFE for bluebird habitation.
In order to be safely inhabited by eastern bluebirds, the nest box should have the following minimal characteristics.
– Correct entrance hole dimension. A number of different entrance configurations are in use today. Probably the most common configuration is a 1 1/2” diameter circular entrance hole.
– Correct nest cavity dimensions. Although there is an ongoing somewhat “spirited” discussion regarding dimensions, most pundits would agree that a nest box with a 4” W X 4” L floor and 10″ H is adequate. The inside height from the floor to the bottom of the entrance hole should be approximately 6”. The Gilbertson style nest box is considerably smaller but is safe and attracts bluebirds.
–The nest box should have provisions to allow it to be opened for monitoring and cleaning purposes.
–Adequate ventilation should be provided to help ensure the safety of the birds during hot weather.
The North American Bluebird Society web site www.nabluebirdsociety.org provides detailed nest box specifications including dimensions for eastern bluebird nest boxes.
Quality of Construction (QOC) is very important. A number of volunteer organizations build nest boxes. Unfortunately, some of these organizations concentrate on production volume and sacrifice quality of construction. The result is that some percentage of the nest boxes will be UNSAFE for bluebirds. One of the more common types of production problem is poor aim with a nail gun. The digital photo included below shows a nest box produced by a volunteer organization that left two sharp nails protruding into the nesting cavity. Obviously, a lack of quality assurance allowed this nest box to enter the marketplace.
Sharp pointed nails protruding into the nesting chamber
Another common problem is unacceptably wide gaps between the access door and the sides of the nest box that allow rain water into the nesting chamber. Wet nests from cold rains are potentially dangerous for bluebird eggs and young nestlings who have no capability to regulate their own temperatures. Wet nests also tend to attract unwanted visitors such as ants and moisture seeking insects. The digital photo included below shows the same nest box with the door closed. Take note of the rather significant gap on the left side between the door and the side.
Gap on left side of door that will allow rain water into the nest cavity
Although nest box entrance hole enlargement by woodpeckers and gnawing varmints isn’t a particularly common event, it does happen. When it does, the result is a potentially unsafe nest box. The enlarged entrance hole will allow larger aggressive bird species such as European Starlings to enter the nest box and harm the occupants. The photo included below is the same nest box showing a 1/4” gap between the entrance hole and the metal flashing around it that serves as a hole guard. Woodpeckers and gnawing varmints have unobstructed access to the wood around the entrance hole. One has to wonder why the organization that produced this nest box even bothered to include a hole guard.
1/4” gap between hole guard and entrance hole
The “message” in this article is to always carefully examine every nest box you plan to acquire for defects and design flaws before you accept it. If you find a significant defect in a nest box, be sure to bring it to the attention of the organization that produced it.
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