Is it safe to use pressure-treated wood to build a nestbox?
QUICK TIPS: Because we don't have information about its effects on the occupants, I think it's safer not to use nestboxes made of pressure treated lumber.
The North American Bluebird Society's nestbox recommendations (June 2012) state "Do not use pressure treated wood because it includes toxic compounds." If you want my personal opinion on the subject, I agree. Although the risk is probably low, we simply do not have enough information about how pressure treated wood can affect nestbox occupants. I would, therefore, err on the side of caution and avoid using a box made with pressure treated wood. There are other choices.
Just because a bird might use a box made with pressure-treated wood doesn't make it safe.
Just because pressure treated wood may be declared safe for framing a human's house doesn't mean it's safe for to house a cavity nesting bird.
Some of the newer types of pressure treated wood are significantly less toxic than the older type, but less toxic doesn't mean non-toxic.
Cavity nesting birds spend considerable time inside a nestbox. Brooding parents are inside for hours and days on end, in close proximity to wood walls. Eggs are laid and the embryos develop inside the box. Growing nestlings are confined to the box for up to three weeks, depending on the species. This type of exposure differs from human exposure to pressure-treated wood.
To my knowledge, no studies have been conducted to evaluate the effects of pressure treated wood nestboxes on nesting adults and developing embryos and hatchlings. That is why I think it is more responsible to use boxes made of untreated, natural wood.
MORE ABOUT TREATED WOOD
Treating wood by to make it more durable is nothing new. Wood was brushed, sprayed, dipped or soaked with something that repelled water, or preserved the wood, or had insecticial properties, or all of the above. Ancient Greeks soaked wood in olive oil to make it last longer. The Romans brushed ship hulls with tar. Creosote (made from coal-tar) has been used commercially since 1838 to preserve railroad cross ties, utility poles, marine pilings, fence posts, etc.
Later on, wood was pressure-treated by applying pressure or using a vacuum to force deeper penetration of a liquid preservative into wood fibers.
Chromated copper arsenate (CCA or CCA-C) pressure-treated lumber was patented in 1934. It contains chromium, copper and arsenic. Unweathered CCA wood has a greenish tint. Most wood sold for outdoor use in the U.S. between 1975 and 2003 was CCA-treated.
Because of concerns about arsenic leaching from wood, the U.S., wood treatment industry voluntarily stopped treating residential lumber with CCA on December 31, 2003. CCA wood can still be used for some industrial applications like docks and guardrail posts.
Newer pressure-treated lumber does not contain arsenic, but does contain compounds like amine copper quat (ACQ) or copper azole (CA or CuAz).
Micronized copper particulate is also being used as a biocide. (Sold under trade names like MicroPro and Wolmanized).
Coal-tar creosote is toxic to fungi, insects and marine borers. Creosote is considered a human carcinogen.
Arsenic is toxic. Humans are advised to wear gloves when handlign CCA-treated wood, and not breathe in sawdust or burn it.
ACQ is a fungicide and an insecticide. Note: ACQ-treated lumber is also extremely corrosive to regular nails and screws.
The safety of nanoparticles like those used in micronized wood is still a subject of debate.
Note: Contact with water can cause preservatives to leach out of treated wood.
Consider that the birds are not EATING or sawing the wood - although woodpeckers that roost in a box often excavate the entrance hole and interior.
WHAT CAN I DO INSTEAD TO HELP MY NESTBOXES LAST?
Making nestboxes from trees that cavity nesting birds use in nature should be about as safe as it gets. But eventually, after exposure to weather, etc., wooden nestboxes will deteriorate. Boxes that are split or leaking expose occupants to the elements, and need to be repaired or replaced.
Nestboxes can be constructed from wood that is naturally resistant to rot - for example, Western Red Cedar pr Eastern Red Cedar. (Eastern Red Cedar dimensional lumber is not widely available in some areas.) Black locust (Robinia pseudocacia) wood is hard and durable, but is also heavy and would need to have holes pre-drilled for screws, as the wood is prone to splitting. Red pine (Pinus resinosa) is long lasting, but I don't know about its availability. Some people even worry about using these woods too. For example, cedar and some of its derivatives (e.g., cedrol which is distilled from junipers) has insecticidal properties. That's a good thing if you want to keep bugs out of your boxes, but could it affect the birds?
Some people use boxes made of plastic or PVC. Plastic does not split, chip or rot. It resists moisture, and requires little maintenance. However, we are now realizing that certain types of plastic (like Bisphenol A [BPA]) have the potential to affect reproduction and development when used to package food or drink.
Some non-pressurized wood treatments are available. Be sure to allow any treatment like this to dry thoroughly before making a nestbox available for use.
Neem Seed oil (which is considered "ecofriendly" and biodegradable) can be warmed, fluidized and then painted on wood to repel insects.
Linseed oil is a water repellent used to preserve wood fences, log cabins and wood furniture. Some people use it on the exterior of nestboxes.
Some people use non-oil based deck preservative.
Some people paint the exterior of boxes with a non-oil based paint in a lighter color (to avoid overheating.)
We will never know everything we need to know. Nothing can ever be 100% safe. But when we invite birds to use our nestboxes, it is wise to consider the risks and try to minimize them.
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