Is Wildfire Smoke Causing Birds to Tend to Empty Nests?

Early last summer, dull orange horizons replaced blue skies and hid city skylines in New York and Washington, D.C. Even the midday sun was dialed down to an ominously weak glow. People in the western half of North America have become all too familiar with the eerie scene that often accompanies a big wildfire. Thick smoke often blankets cities like Boise and Seattle. But the morning of June 7, 2023, was different. Smoke season had come east, heavier than many people there had ever seen before.

But humans weren’t alone in dealing with that sudden smoke and its health effects. Wildlife across the Northeast, including millions of breeding birds, felt it, too. New research suggests the smoke may have more impact on birds than we ever realized, harming them both during their nesting season and throughout their lives. These studies are part of a new understanding of smoke, not just as a side effect of wildfire, but as a threat to wildlife and ecosystem health on its own.

Just as that historic smoke was about to arrive in the White Mountains of New Hampshire last summer, two tiny songbirds were building their nest. A pair of black-throated blue warblers, striking birds with a blue back, a black mask and a bright white belly, had finished that season’s nest on June 4. The cup-like structure was constructed of bark, pine needles and other materials from around the forest. But just when the next cycle of life for these little birds was about to start, something unusual happened. The female of the pair started to sit in the nest vigilantly, as if incubating her eggs. But no eggs were there. She was incubating an empty nest, and nearby, someone was taking notes.

These warblers didn’t build their nest in just any woods, but in Hubbard Brook Experimental Forest, a 7,800-acre ecological research station that has hosted scientists and students of multiple fields for over 60 years. Hubbard Brook is well known in certain scientific circles and has produced a wealth of studies over the decades, but it is most famous as the site where acid rain was first discovered and studied in the 1960s. The only reason this nest was being monitored at all was because the male of the pair had previously been captured and marked with colored leg bands. A student named August Davidson-Onsgard noticed a nest being built in that banded bird’s territory and started watching it. Davidson-Onsgard happened to work for an ecologist and self-described naturalist with just the right experience and interest in the details of bird ecology to recognize how unusual, and potentially important, this observation could be.

Ecologist Sara Kaiser is a research associate at the Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI), and she directs the Cornell Lab of Ornithology’s field study program at Hubbard. When Davidson-Onsgard mentioned the empty nest incubator to Kaiser on her next visit to the site, it immediately got her attention.

For a breeding bird, incubating a nest is always a gamble. Female black-throated blue warblers sit on their eggs for about 12 days straight, only leaving briefly to feed. During that time, they are more vulnerable to predators looking to make a meal of the bird, her eggs or both. Without the payoff of eggs and future offspring to keep her at the nest, the bird should not, theoretically, make the investment to stick to the nest so closely.

“I said, you know, I haven’t really heard of anything like this before,” says Kaiser. She encouraged her student to follow up and search the scientific literature for examples of birds sitting on empty nests as if they had eggs. They found over 200 records of birds of 11 species, all in Europe, showing this strange behavior, and in over 80 percent of these accounts, the authors had noted environmental pollution as a possible cause.

Researchers can’t say for sure if smoke is directly causing nesting birds like the warblers to fail in such a particular way, but the new observation as well as the literature review suggest a potential link. Kaiser and colleagues at Cornell and NZCBI recently published their findings in the Wilson Journal of Ornithology, with Davidson-Onsgard as the lead author. The study is the first systematic review of this particular kind of nest failure.

---

We are in an age of megafires. Fire has always been an important part of the ecology of the West, but climate change and a century of forest mismanagement have amplified the scale and severity of wildfires beyond anything in modern history. A 2020 study showed that the average acreage burned in a wildfire almost tripled between 1950 and 2019. Through mid-September this year, over seven million acres have burned in the United States. Three times in the last decade, that number has topped seven million acres per year. In 2023, Canada’s fire season made even those numbers seem small. Forty-five million acres burned in Canada last year, an area larger than the entire state of Florida turned to smoke in one season.

Wildfire smoke contains chemicals like carbon monoxide, sulfur dioxide and even lead. It also carries particles that scientists call PM2.5, meaning they are less than 2.5 microns, or 1/400th of a millimeter, in size—which makes them deadly. Because they’re so tiny, these particles can fit into the smallest crevices of the lung. A global study published in the Lancet found that four million people died as a result of PM2.5 pollution in 2019, more than twice the death toll of Covid-19 in 2020.

According to Kaiser, the effects on birds could be even more severe.

“It’s because of their respiratory system,” says Kaiser. “They’re moving a lot of air, much more volume of air than other animals. They have very efficient lungs, and so any air that has this particulate matter could be problematic for these birds.”

In clean air, bird lungs have evolved to be incredibly efficient, but in smoky conditions, that same adaptation works against them.

Nesting isn’t the only time that birds are susceptible to wildfire smoke. Until recently, smoke season rarely overlapped with the breeding season of most North American birds. But as fire seasons become longer and more severe, the chances increase of big smoke events like the one in 2023 that catch birds right as they are nesting.

On the other side of the country from the woods of New Hampshire, another research team recently showed another set of effects from PM2.5 pollution and wildfire smoke on birds across the summer and fall, not just when they’re on the nest.

Olivia Sanderfoot is an ecologist at the La Kretz Center for California Conservation Science at the University of California, Los Angeles. In a recent paper in Ornithology, Sanderfoot and colleagues found that birds in the San Francisco Bay Area lost body mass, a key measure of health that harms their ability to migrate, when they were exposed to wildfire smoke during the Bay Area’s July to November fire season. Scientists had theorized about that kind of effect but had not been able to establish and measure it before.

“Knowing how big of a deal wildfire smoke is for people, it just made sense to me that birds would be similarly impacted,” says Sanderfoot, “because they are so highly sensitive to air pollution, and because, unlike people, they can’t take refuge indoors when the air is hazardous.”

Like the New Hampshire warbler study, the San Francisco paper was led by a student, Anna Nihei, then an undergraduate majoring in computational and systems biology at the University of California, Los Angeles.

Sanderfoot also helps oversee Project Phoenix, a community science effort that lets anyone in a study area in California, Oregon and Washington help monitor birds during wildfire season. She sees the warbler observations around the smoke event in 2023 as a meaningful development in understanding the effect of smoke on birds, and says the study is really important and powerful.

“I think that considering smoke to be a disturbance in and of itself, outside the concept of fire, is a brand-new way of thinking about air pollution impacts on wildlife,” says Sanderfoot.

Researchers are only now starting to understand some of the long-term effects of wildfire smoke on humans, and they still have a long way to go to measure its effect on birds and other wildlife as well. But evidence is mounting that smoke is another climate-related stressor, like heat and drought, that can have serious repercussions for birds, off and on the nest.

As Sanderfoot explains, the warbler nesting observations around the smoke event in 2023 are a meaningful development in understanding the effect of smoke on birds. “It is not the norm that when we see these intense smoke events, the timing overlaps with the breeding season,” she says. “I have had conversations with many colleagues in which we have anticipated that should these two things align, the impact on bird populations would be big.”

Ian Rose | READ MORE