Wildfire Smoke Disrupts Bird Migration in the West

Wildfires have increasingly coincided with the beginning of fall migration for many bird species. This greater frequency of fall wildfires is thought to be primarily the result of frequent drought conditions and climate change, and the impacts of these fires on migrating birds are not yet well documented. Since 2018, researchers from the USGS Western Ecological Research Center have marked tule geese (a subspecies of the greater white-fronted goose that breed in Alaska and northern Canada and winter in Washington, Oregon and California as well as Texas, Mexico and Central America) with GPS-tracking devices. This is part of a larger program to study the movements and behavior of migratory waterfowl in the Pacific Flyway in real-time.

In September 2020, four GPS-marked tule geese encountered dense wildfire smoke while flying through the Pacific Northwest in the first part of their migration. By matching up the GPS tracks with a three-dimensional model of the smoke during the time of the migration and comparing them with migration tracks from 2019 (a year without heavy smoke), researchers were able to assess how the wildfire smoke disrupted the geese's flight paths.

"Everything coincided such that we could watch this unfold in almost real time," said Cory Overton, a USGS wildlife biologist and lead author on the study. "It's virtually impossible to see this type of event without preparedness and good fortune - you can't design a study to look at an unprecedented fire season. And then to have all four individuals survive long enough to get the data to us? It's pretty incredible."

The geese responded to dense smoke by pausing their migrations, altering the direction or altitude of flight, or both. Some geese stopped their flight for two to three days until the smoke cleared. Geese that flew through smoke or directly over fires had disorganized flight paths, sharp increases in altitude to fly over the smoke plume, and stopovers in non-traditional habitats far from traditional migratory pathways. All of the marked geese eventually arrived at their destination of Summer Lake, Oregon, but the average migration in 2020 took twice as long as in 2019 (nine versus four days) and covered an additional 470 miles (757 km).

These longer and farther flights resulted in much higher energy expenditure than a typical migration. The study estimates that the geese would need several extra days to recover from the resulting caloric deficits. These energy deficits could lead to increased mortality or lower reproductive rates, suggesting that smoke disruptions could ultimately put vulnerable migratory bird populations at greater risk.

The tule goose, a Species of Special Concern in California with a population of fewer than 10,000 birds, is especially susceptible to disruption to its migration. Tule geese exhibit strong site fidelity, the tendency to use the same migration paths and return to the same stopover locations year after year, and they fly in formation which conserves their energy by reducing wind resistance. These strategies normally make migration more efficient and help maintain social cohesion for many migratory birds, but can also make it more difficult to unexpected obstacles like wildfire smoke. Tule geese are at additional risk because of their small population size.

"With a significant proportion of the species migrating together, the population level risk from these types of events has the potential to be extremely impactful," says Michael Casazza, USGS research wildlife biologist and co-author on the study.

Wildfire smoke has the potential to impact migratory birds at a geographic scale far beyond the bounds of the fires that generate it. At ground level, smoke concentrations can reach a level that impacts bird behavior across a geographic extent more than 27 times the area burned by wildfires. Up in the sky, at the altitudes where the tule geese migrated, smoke can spread even farther-an area up to 44 times larger than the wildfires themselves. In September 2020, this encompassed 64 percent of the area in California, Nevada, Oregon and Washington.

The study indicates that wildfire smoke from expanding wildfire seasons and geographies may increasingly disrupt migration for Pacific Flyway birds into the future, especially for species with strong site fidelity like the tule white-fronted goose.

USGS provides science for a changing world. For more information, visit www.usgs.gov.

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