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N.M. scientists work to understand 2020s mass bird die-off and prevent another one

A variety of dead migratory birds collected from White Sands Missile Range and sites in Dona Ana County, New Mexico, were examined by researchers at Knox Hall at New Mexico State University before being sent for necropsy in 2020.
Climate change likely to blame for the sudden death of thousands of migrating birds in the state, researchers find

As summer turned to fall in 2020, people from Taos to Las Cruces reported unusual clusters of dead songbirds. Golden warblers, iridescent swallows, pale flycatchers and others were found scattered on riverbanks, huddled under barn eaves and strewn on playing fields.

The birds perished during their long migration from summer breeding grounds in the U.S. and Canada to wintering grounds in southern New Mexico, Mexico, and Central and South America.

The stakes are high for western birds, which are particularly vulnerable during migration. An October report from science and conservation organizations warns of the “widespread loss” of birds across the U.S., including western forest birds, which declined by almost 20% since 1970.

New Mexico scientists are looking for ways to protect migrating birds from another mass die-off. Scientists at the Los Alamos National Laboratory researched the factors that led to the unprecedented 2020 event, and released a 2022 study examining drought impacts on birds.

Patricia Cutler is a wildlife biologist at White Sands Missile Range who fielded numerous reports of dead birds from August through October of 2020. She collected over 400 dead birds from the range, and received reports of even more that she was unable to collect.

Though at first they didn’t know what caused the deaths, she knew it was “abnormal.” She and other scientists studied bird carcasses found on the range and elsewhere in southern New Mexico, counting 59 different species. Of the birds they could analyze, nearly all had zero fat.

“Migration is really stressful for birds in the first place,” Cutler said, and it’s a time they lose weight on their arduous journeys. “But to find this many birds with no fat, that was concerning.”

Some researchers believe migratory birds are being found dead throughout Southwest Colorado because of wildfire smoke in California and Oregon, which birds are particularly susceptible to. (Courtesy of David Porter)
The likely culprit

Federal labs that examined birds collected in 2020 found evidence they died of starvation and hypothermia, with many birds “severely emaciated.”

Though a storm front and cold snap preceded many of the bird deaths, scientists say the real culprit was climate change. The summer of 2020 saw widespread drought throughout New Mexico, part of a 20-year, regionwide megadrought.

And the Rio Grande, an important migratory corridor, has seen dramatic impacts from climate change in recent decades, with miles-long stretches of the river drying each year.

One outcome of long-term drought is fewer food resources, both seeds and insects, Cutler said. And critical stopover habitat, where birds can rest and refuel, has been lost across the West, she added. Smoke from the record-setting California wildfires in 2020, Cutler said, might have caused birds to migrate early or detour around smoke plumes.

Given the hundreds of birds collected on White Sands Missile Range, Cutler guessed the total number of bird deaths statewide was in the tens of thousands to potentially hundreds of thousands.

And while there have been no documented mortality events since 2020, she notes that a lack of deaths doesn’t mean human activities aren’t impacting birds. “Anytime they’re distracted off of their normal migration pathways,” she said, they use up energy and fat critical for migration.

David Porter, who lives in the Animas Valley north of Durango, said he found five dead birds on his property in six days in 2020. (Courtesy David Porter)
Help from the public

While scientists gathered bird carcasses to examine, they also turned to the public. Researchers from New Mexico State University set up a project on the online nature identification platform iNaturalist to collect citizen observations of dead birds.

Hundreds of people shared photos and descriptions of the birds they found, contributing to a picture of widespread die-offs across New Mexico and other southwestern states.

Ecologist Neeshia Macanowicz was surveying for plants in the Jornada Basin north of Las Cruces when she started encountering dead birds. In 12 years of fieldwork in the area, she said she’d never seen anything like it – not the number of dead birds nor the variety of species.

How to help

At home, people can reduce threats to birds by keeping cats inside, turning exterior lights off at night, and making windows bird-safe with cords that hang down and break up reflections, Stanek said. Growing native plants and avoiding pesticides is also critical.

Consumer choices like buying shade-grown coffee and avoiding single-use plastics support healthy bird habitat. And Stanek recommends sharing bird observations on websites like eBird and Project FeederWatch, or joining an in-person bird survey like the Christmas Bird Count.

She posted the photos to iNaturalist, and asked her coworkers to look for birds. She felt sad at the time but said the discovery ended up raising awareness of the status of birds at her worksite.

Scientists from LANL and other federal agencies analyzed 11 years of data from migrating birds captured and banded in and near Los Alamos, in order to understand the relationship between drought and the health of migrating birds.

Jenna Stanek, lead author, found that in years of more severe drought, birds were more likely to be unhealthy with very low fat reserves, while in wetter years, birds were more likely to have fat to support their migration. She also found that young insectivore birds were least likely to have enough fat, suggesting they’re less resilient to drought.

Stanek said birds are more at risk now due to increased climate variability. “Before, if an extreme weather event happened, they had the fat stores to be able to make it through it.”

Other factors

Cutler said she was surprised to collect so many dead birds at White Sands Missile Range and hypothesizes that artificial lighting may have disoriented and drawn in birds. Songbirds migrate at night, using star patterns, landmarks and the Earth’s magnetic field to guide them. So Cutler started a study to determine how birds are affected by lighting on the missile range.

She said there’s no downside to designing lighting to lessen impacts on birds and other wildlife.

“Birds are just getting hit from such a wide variety of impacts,” she said, “that I think anywhere that we can make improvements is important.”

As fall migration peaks this year in New Mexico, warblers, swallows and flycatchers are again winging South. While the other three North American migration flyways receive greater numbers of birds, many Western bird species migrate via the Central Flyway, which includes New Mexico. Stanek said the Rio Grande corridor in particular is an important refuge for birds as well as butterflies and bats migrating through the arid Southwest. “It’s one of New Mexico’s greatest living treasures.”

Identifying important stopover sites and enhancing and protecting them is one critical way to help birds on the move. If steps are taken to help birds, Stanek said, maybe there won’t be another bird die-off.