To Paint, or Not to Paint?
NestWatchers often ask if it is safe to use paint or stain to preserve a nest box, so we recently published a new FAQ article about this at NestWatch.org. The controversy lies in the fact that there are no conclusive studies that determine whether residual fumes from paint, stains, or pressure treatment can harm the birds. Some builders argue that it takes longer to paint a box than it does to build a new one, and that paint must be reapplied every few years to be effective. Others point out that trees can be spared if nest boxes are made to last longer. And there are many who claim that the risk from birds overheating in an unpainted box is greater than the risk of toxic fumes.
Without peer-reviewed studies on this topic to guide us, we recommend using untreated, unpainted wood to construct boxes because it most closely resembles what the birds would have used before the advent of nest boxes. A well-constructed house should last 10–15 years on its own (cedar, spruce, white pine, and yellow pine are good rot-resistant choices for lumber). Pressure-treated wood has been saturated with a combination of pesticide and fungicide, and therefore, should be avoided as nest box material. Alternatively, you can extend the life of your nest box by gluing all the seams of the non-opening sides before nailing them. Recess the floor about ¼″ to reduce deterioration from moisture. Use only durable materials, especially for the roof, which deteriorates faster than the other panels. You can further protect the box by placing the roof panel such that the growth rings bend down in a “frown” rather than up in a “smile.” When the roof begins to warp in the direction of its growth rings (as all solid lumber will do), then it will warp downward and still protect the box from rain. Angled roofs will last longer than flat roofs, but flat-roofed boxes can be mounted at a slight downward angle to shed rain.
We do recognize that in hot climates, where daytime temperatures regularly exceed 95°F, some nest box monitors choose to paint the exterior of boxes so that they stay cooler. Proper ventilation and a roof that extends two inches over the sides will help shade the box and protect it from the elements, reducing the need for paint. However, some additional cooling may be gained by painting the roof and exterior walls a light color (white is preferred for Purple Martin houses, but opt for tan, gray, or dull green for other cavity-nesting species as these are less conspicuous to predators). If paint is deemed necessary by the monitor, then it should only be applied to the exterior, never the inside. Even zero- and low-emissions latex paint formulas or oil-based stains can release fumes for months, so if you paint, plan to do so in the fall, which will give fumes time to dissipate throughout the winter as the paint cures.