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In July 2023, biologist Felicia Wang was conducting surveys on nesting Tree Swallows in a nature preserve outside of Woodland, California. Under the warm morning sun, she approached a nest box on the edge of the marsh and peered at the contents inside. Instead of peeping, she was greeted with four dead nestlings, still and silent.

As the field season went on, Wang and her team continued to find casualties. By August, more than half of the nests in the preserve had one or more dead chicks—a much lower breeding success rate than what Wang typically sees.

“When you get the first few nestlings that are dying in the box, you're like, oh, you know, that's unfortunate,” Wang says. “But when you start getting more and more every week, suddenly it's not just a coincidence anymore. There's something happening here.”

Wang says that these deaths are likely related to heat. In the second half of the 2023 field season, temperatures around Sacramento averaged a searing 90 degrees Fahrenheit. Although hot, dry summers are typical in California, deadly heat waves are becoming more extreme there and throughout North America. June 2024 marked the of record-breaking global heat. Tree Swallow nest boxes, like the ones Wang surveys, are also often in direct, blistering sunlight.

Soaring temperatures can turn nest boxes into miniature ovens. These human-made structures are often not temperature controlled, says Katie Stumpf, a professor at Alma College in Michigan. She notes that it can be more than 6.3 degrees Fahrenheit hotter inside of a nest box than the outside air, driving temperatures well above 107 degrees Fahrenheit on hot days—a level that is lethal to nestlings. Even if these dangerous levels are not surpassed, prolonged exposure to high temperatures can cause dehydration, heat stress, and impair or stop the development of an egg or chick, says Stumpf.

Thankfully, there are adjustments that can make nest boxes more heat-resilient. Although these interventions only help offset one of the many impacts, making nest boxes more habitable is one small step to help them survive a warming world.

Avoid direct sun 

Sunlight, especially in the late afternoon, can easily bake boxes to uncomfortable temperatures. One of the most straightforward ways to make nest boxes cooler is to pay attention to their location. 

Emerson Keith Bowers, an ornithologist at the University of Memphis, has seen firsthand how poor nest box placement can have consequences. He studies Carolina Wrens and Prothonotary Warblers on a site near Memphis. Most of the 625 nest boxes he monitors stand in shaded areas in the mature forest, but three are, inadvertently, in direct sun. “That in itself has caused the failure of some nests, unfortunately,” he says.

If you decide to install your own nest box, keep the sun in mind. Shaded areas in forests are much cooler than the open field, and can provide homes for birds like wrens, warblers, or chickadees (For species like swallows and bluebirds, which prefer their nest boxes out in the open, try adapting the box itself with the tips below). The direction the nest box faces can also make a difference. Try to orient the opening of the nest box toward the north and east, away from the afternoon light.

Bowers also recommends putting out more than one nest box, if possible. Cavity nesters can’t build their own nests, so they often have to take what they can get. “Let's give our birds a chance to choose what they like,” he says.

Mind the gap

What do you do when it gets too hot in your home? You might crack open a window. Similarly, openings such as holes and slits in a nest box top can help nestlings keep cool.

Stumpf uses modified nest boxes with a gap between the box’s roof and the top of its walls. As hot air rises, it escapes out the slits. “I would imagine that without that vent it's going to be significantly warmer inside,” she says. “Just because there isn’t a way for that hot air to get out.”  

Bowers has another method to add ventilation to a nest box. For his nest boxes, he makes the entrance a long and narrow opening, rather than a small round hole. These “slot” nest boxes allow more airflow while minimizing the danger of predators or parasites. Plus, when chicks get large enough, more than one can gather at the entrance for a breather—or dinner.

Adding a double roof to insulate a nest box or shields to reflect the sun can also help offset heat.  With a double roof, insulation slows the transfer of heat, keeping hot air out and cool air in. It can make “a significant difference,” says Bet Zimmerman Smith in an email, an environmental scientist who wrote an . To make this additional structure, the North American Bluebird Society using a slab of wood or Styrofoam. For a shield, place a sheet of white or reflective material (polystyrene works well) above the roof, using spacers to leave a gap for airflow.  You can also use heat shields on the sides of boxes.

Paint it white

For one easy way to cool down your nest boxes, all you need is a fresh coat of paint. White pigment, specifically, can reflect incoming sunlight, reducing the temperature within.

Stumpf is currently studying the white-paint effect by putting temperature loggers inside Eastern Bluebird nest boxes. She has already found that white-painted nest box interiors stay up to 8 degrees Fahrenheit cooler than unpainted wooden boxes—a magnitude she called a surprise. “That's a big difference,” she says. “None of our white nest boxes ever reached the lethal temperature.”

Whether an extra coat of ivory can boost nesting success, however, is unclear. Stumpf is currently working to quantify these benefits. Wang also painted all the Tree Swallow nest boxes exposed to direct sun at the nature preserve she works at. She's currently monitoring the boxes for breeding productivity this year and will have a better idea of their effectiveness at the end of the summer."

“The ultimate goal of providing a nest box is to have a home for a bird who wants to nest,” Wang says. “We want to make sure that, to the best of our abilities, we’re providing good suitable nesting habitat for birds.”