A pair of tree swallows in Virginia.
At the Smithsonian Conservation Biology Institute (SCBI) in Front Royal, Virginia, the spring and summer months are a chorus of songbird calls and songs. The loud “pee-a-weeee!” of the eastern wood-pewee and the croaking call of the yellow-billed cuckoo ring out across the grounds. Some birds reside here all year long, while others travel thousands of miles from their warmer winter habitats to breed. Over the course of the summer, they will find mates, compete for real estate, build nests, incubate eggs and care for chicks.
As the breeding season begins, our Department of Animal Programs staff prepare nest boxes for cavity nesters — birds that nest in sheltered holes. Some smaller cavity nesters, including bluebirds and tree swallows, can’t create their own holes. Instead, they nest in spaces they find, like holes in dead trees or those built by woodpeckers. But as land is increasingly developed, these natural nesting cavities disappear. Native birds also face competition from invasive species, such as house sparrows and European starlings. As a result, many cavity-nesting bird populations are declining. Research suggests that setting up artificial nest boxes in areas with good habitat can help bluebirds and other species bounce back.
An artificial nest box at the Smithsonian Conservation Biology Institute. A mesh predator guard affixed to the pole protects the box’s inhabitants from snakes, raccoons and other predators.
In February and March, we installed new nest boxes, fixed predator guards, and cleaned and repaired existing boxes to get them ready for birds. The boxes are spread out across SCBI’s 3,200-acre campus in different habitats where birds might nest: along rural roadsides, in pastures and even on our central campus closer to people.
Biologist Eugene Morton began installing nest boxes shortly after the Smithsonian acquired this land in 1975. He wanted to learn more about native bird behavior, reproduction and population trends. Since then, our nest box trails have grown to include 70 boxes and counting!
Nest boxes can attract several different songbirds, including chickadees, tufted titmouse, house wrens and tree swallows. One of the most common and iconic species in this area is the eastern bluebird (Sialia sialis). Bluebirds are found year-round in much of their range, but they are considered partial migrants because some populations fly south from Canada and the northern U.S. for the winter.
One of the most common and iconic species in this area is the eastern bluebird (Sialia sialis). This female is incubating her eggs inside a nest box at SCBI.
If you live in eastern North America., you have probably seen an eastern bluebird! Males and females both have a blue head, back and wings with an orange chest, though females have a slightly duller color. They are often found sitting on telephone wires, fence posts and nest boxes. Unlike bluebirds, tree swallows (Tachycineta bicolor) migrate long distances. They winter in the southern U.S., Mexico and northern Central America and breed throughout central and northern North America. Nest boxes have helped expand their large breeding range as far south as northern Georgia.
Keeping Up with the Cavity Nesters
We monitor the nest boxes around SCBI weekly from March to the end of August. We check on the progress and health of chicks, note whether nests succeed or fail, watch for signs of predation and address pests (like wasps, ants and blowflies). We also check for harmful invasive species, like house sparrows, and remove their nests before they lay eggs. House sparrows are aggressive and known to outcompete North America’s native birds. They will kill adults and chicks, destroy eggs and then build their nests on top.
When we check a nest box, we first try to figure out which birds are using it. Even if the adults are out collecting construction material, we can guess what species might be nesting there. One indication is the style of nest. Invasive house sparrows typically build messy nests, filling the boxes with grass and bits of trash. House wrens fill their nest boxes with sticks. Tree swallow nests are typically made of dried grasses, with the nest cup full of soft feathers from other birds. Eastern bluebirds build nests of dried grasses and pine needles but don’t usually use feathers.
Eggs inside a nest can also provide clues. Tree swallow eggs are small and white, whereas an eastern bluebird’s eggs are a little larger and rounder. Bluebird eggs are almost always sky blue, though on rare occasions, they can lay white eggs. They usually lay three to five eggs, while tree swallows lay four to seven.
Each time we check a nest box, we record the number of eggs or chicks we see. By using a visual chick-aging guide and recording the date that the first egg was laid, we can estimate the age of the chicks. This helps us understand when the chicks might fledge (leave the nest).
It’s important that we don’t go near the boxes when chicks are in their “fledge range.” Bluebirds leave the nest at about 3 weeks old and tree swallows at 3-4 weeks old. Opening the box during that time could cause the chicks to leave the nest too early, making them vulnerable to snakes, raccoons, domestic cats and other predators!
Eastern bluebird chicks in a nest box at SCBI. Both parents will bring the chicks food until they are old enough to leave the nest.
When the breeding season ends, we send all the data we collected from our nest boxes to the Virginia Bluebird Society as part of their citizen science monitoring project. The Virginia Bluebird Society has been tracking native cavity-nesting birds since 1996 and has recorded more than 70,000 successful bluebird, tree swallow, chickadee and house wren hatches over the years. Last year, our nest boxes at SCBI added 187 bluebirds, 114 tree swallows and five house wrens to the local populations. We’re hoping 2021 will be another great year!
Helping Birds at Home
You don’t have to be a professional researcher to help birds. In fact, you may be able to set up a nest box at home. The first step is to determine if you have the right habitat. During the breeding season, native cavity-nesting birds in the eastern U.S. prefer open grassy land, patchy vegetation, dense tree coverage, spaces close to water, and areas with power lines. With that variety, chances are you have cavity nesters nearby.
Setting up a nest box is fairly simple. The three main components are a wooden box with a small access hole, a pole to mount the box on, and a predator guard to keep other animals out. Nest boxes can be constructed do-it-yourself style or purchased already assembled. It’s best to avoid mounting boxes on trees, fences or other areas where animals can easily break in. It’s also better to position them away from trees or buildings with overhangs, where predators could drop down from above.
If you build a backyard nest box for eastern bluebirds, it may look similar to the boxes we have set up around SCBI.
Setting up a nest box is not the only way to get involved with bird conservation. Check out our Live Bird Friendly page to learn about some simple actions you can take to help birds today. Citizen science projects are often available at local, regional, state, or even national levels, and focus on different plants and animals. If you live in Virginia, the Virginia Bluebird Society regularly recruits volunteers to monitor their nest boxes located across the state. There are citizen science projects out there for all interests and experience levels, so take a look around to see what may best suit you!
This story was featured in the June 2021 issue of National Zoo News. Want animals in your inbox? Sign up for our e-newsletters!