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Nature Notes: Nest Boxes for Birds

Ed Robinson
March 20, 2019

Winter passes slowly for wildlife and for humans, but the longer days tell us that spring is only a few weeks away. Soon your favorite species will emerge from hibernation or return from long migrations to warmer climates. Better weather will trigger the mating season for many creatures and finding a home for rearing their young will be a priority. You can lend a helping hand by erecting nesting structures selected for your favorite species.

I can remember in my youth seeing all kinds of elaborate multi-unit bird houses erected on poles to attract purple martins. Drive along any highway and you will see wooden boxes placed to attract Eastern bluebirds. Many tourist shops sell bird houses in all shapes, sizes and colors, although most of them are more decorative than they are useful for birds. There are houses for a wide variety of species including mallards, butterflies, bats, squirrels and even opossums. In this article I will focus on nesting boxes for birds.

Some birds are known as cavity nesters, meaning they require a hole in a tree or other structure to build their nests. The cavities are often created by woodpeckers and/or the gradual decomposition of old trees. Standing dead trees are important habitat. Birds can be highly selective in the size and shape of the chosen cavity or opening, and the location is also very important. To supplement natural cavities, you can put up a bird box in hopes of attracting new neighbors.

John Berry photo

The first photo accompanying this article shows an Eastern bluebird emerging from a newly installed nesting box at the Curtis Farm Preserve. Since people became concerned about declining numbers of bluebirds in the 1960s millions of these boxes have been installed across the country, with positive results. Depending upon their locations, the boxes are also popular with other birds such as the graceful tree swallow. I have installed nearly two dozen bluebird boxes and have been pleased to see many of them occupied year after year.

The second photo shows (from left to right) a bluebird box, a bat house, a box for screech owls and kestrels and a wood duck box. Notice the differences in size, shape and entrance openings. All of these boxes were made out of rough cut pine, though other woods like cedar, hemlock and fir are suitable. The type of wood and the construction must be suitable for safely rearing young birds and to help them exit the nest when ready. I use boards that are ¾-inch to one-inch thick for insulation purposes. Durability is important over time so some boxes are made of a wood/concrete mixture called “woodcrete.” Metal houses can be used for purple martins, and I have had success with plastic houses for wood ducks. Pressure treated or painted wood is not recommended.

The placement of boxes is important. For example, it is recommended to place two bluebird boxes within five feet of each other, five to six feet above the ground. It is likely that tree swallows will quickly occupy one of the boxes. They will not tolerate other tree swallows so close but they will accept bluebirds as neighbors. I install bluebird boxes on smooth metal poles to keep mice and squirrels out of them and the boxes face northeast for optimum ventilation and temperature.

Wood duck boxes are generally placed over or near suitable bodies of water, although there have been reports of ducks nesting up to ¼-mile away from a pond or lake. I place mine 12-15 feet above ground using four inch by four inch posts topped by detachable two by four boards to allow servicing the boxes each year (cleaning then adding new wood chips). It took four years before the first successful wood duck nest appeared. Screech owl/kestrel boxes are placed 10-12 feet high on the north side of forest areas, overlooking a field. If I can keep the squirrels out of the boxes screech owls will readily use them but it took eight years before I hosted my first kestrel clan.

Other factors are important for success. Design and construction must allow for ventilation to avoid overheating of eggs or young birds. Drainage holes help to keep the boxes dry in bad weather conditions. Whether mounted on posts, poles or trees, it is helpful to install predator guards such as aluminum sheeting to make it difficult for raccoons, squirrels and snakes to snatch eggs and chicks.

While some birds will remove old nesting materials before making a new nest, species like bluebirds will only use an empty box. Thus it is important to clean the boxes each year by removing everything in the box, spraying the inside with a mix of biodegradable soap and bleach, then scrubbing with a brush. I do this in the late autumn or winter when no birds are around, making sure there are no bees or wasps in the box before opening. Some people plug the entrance holes during the winter to keep the boxes free of squirrels, chipmunks or mice.

This is a great time of year to build or to purchase a nesting box for your favorite birds. There are lots of plans available on the internet or in books if you are capable of simple woodworking projects. The next step up is to purchase a kit so you can assemble the bird box without having to cut large pieces of wood. Finally you can purchase finished boxes at a wide variety of outlets. Perhaps the best source is The Cornell Lab of Ornithology, but you can also check out sources like the National Wildlife Federation, Ducks Unlimited, Audubon, Wild Birds Unlimited, Amazon or eBay. It is ideal to have your boxes installed by mid to late March, but some birds will nest again later in the season or simply check out your box as a possible home for the 2020 season.

Nesting boxes represent a modest investment in labor and dollars but they can pay big dividends for the wild creatures who use them, and for your viewing pleasure. If you are unable to observe the coming and going of your new neighbors, consider installing a trail camera to snap photos of their activities. Once nesting birds are active in your box, try to avoid disturbing them by using binoculars to observe them at a distance. It is always exciting to see young birds emerge from a box you have installed, giving you a sense of having helped the birds to prosper in a challenging environment.