Explaining the recent mass mortality of western birds

In mid-September, a mass die-off of migratory birds in New Mexico and other western states occurred as wildfires were raging across the west. One expert said hundreds of thousands of birds may have died.

Many observers suspected that smoke from the fires may have contributed to their deaths, but an article published on the American Birding Association’s website pointed to a lack of food and hypothermia as the likely cause.

The writer, Jenna McCullough, a PhD student at the University of New Mexico who studies avian evolution and systematics, noted that a severe early-September windstorm dropped temperatures from the mid-90s to 40°F and brought several inches of heavy wet snow.

When the die-off made news, she and a fellow grad student collected hundreds of carcasses and found that “none had fat stores on their bodies. Furthermore, many birds also showed signs of breast muscle atrophy, which points to starvation and dehydration.”

Of 305 birds they collected, 85 percent were Violet-green Swallows, which feed by catching insects on the wing. The storm likely led to a sudden decline in prey availability for the birds. McCullough wrote that the mass of the birds they found was about two-thirds the weight of normal swallows, another sign that “these birds were starved and succumbed to hypothermia.”

Trouble for aerial insectivores

The tragedy is also notable because populations of avian aerial insectivores — birds that catch insects on the wing — have been declining for decades. Research shows that aerial insectivores have declined by an estimated 32 percent across North America.

“This is the most severe decline of any group of birds, and translates into the loss of more than 160 million individuals across Canada and the United States,” say the authors of an editorial published in June in Avian Conservation & Ecology. “Nine of the 31 species of flycatchers, nightjars, swallows, and swifts are currently listed in Canada under the federal Species at Risk Act.”

The editorial, written by ornithologists from universities and agencies across Canada, offers an “urgent” roadmap for the conservation of aerial insectivores. The authors identify five broad steps and numerous action items that will help. They include strategies to inform the public about the importance of insects, reducing pesticide use, strengthening wetland protection, and many others.

A version of this article will be published in the November/December 2020 issue of BirdWatching. Subscribe.