Findings by UMass-Amherst led team uncovers critical effects of weather-induced mass mortality events on seemingly healthy populations
AMHERST, Mass. — Thousands of birds, including beloved purple martins, died in "The Great Texas Freeze" of 2021. Thanks to a recent study published in Nature Ecology and Evolution led by biologists at the University of Massachusetts Amherst, we now know not only the extent of the die-off—up to 27% of the birds' breeding population in Texas and Louisiana—but that recovery may take decades, and that we can expect weather-driven mass mortality events, increasingly common in the era of global climate change, may increasingly wreak havoc on animal populations.
For nine days in February 2021, two back-to-back deep-freezes gripped the Gulf Coast, dropping large loads of snow, sending temperatures plummeting, knocking out the Texas power grid and earning the nickname "The Great Texas Freeze." It also resulted in the death of thousands of purple martins, a beloved migratory bird that annually arrives from its South American wintering grounds to the Gulf Coast in early February, just as the temperatures dropped.
"I hate the cold," says Maria Stager , assistant professor of biology at UMass Amherst and the paper's lead author, "but I love birds, which is probably why I'm especially drawn to studying the physiology and evolution behind how they survive the winter."
Purple martins are one of the earliest arriving migratory birds to the U.S. Gulf Coast each year, with adults beginning to return in January and early February. But such an early arrival can make them especially susceptible to the historically rare winter storms that can sweep through the area. In the era of climate change, weather patterns are becoming more erratic and storms more violent and unpredictable.
Yet, while it's obvious that major mortality events, such as The Great Texas Freeze, would have major effects on local species, it has been difficult for biologists to study such events in the past because of their unpredictability.
Enter the citizen scientists.
Stager and her co-authors worked with the Purple Martin Conservation Association (PMCA), which was founded almost 40 years ago and has members across North America; and Louisiana State University's Museum of Natural History, which has one of the world's preeminent collections of birds from the southeastern U.S., to create a baseline historical scenario against which deaths associated with the Great Freeze could be compared.
"People in the Gulf States put up houses for the purple martins and look forward to their return every year," says Stager. "When they saw the returning birds dying, they reached out to the PMCA asking what to do."
"The purple martin may be one of the most beloved and closely monitored backyard birds," says Joe Siegrist, president and CEO of the PMCA and one of the paper's co-authors. "When we recognized this unprecedented research opportunity, we were able to mobilize our army of martin lovers across Texas and Louisiana to increase monitoring of storm mortality and preserve samples for collection. Folks saddened by the loss of their birds were eager to turn this disaster into a contribution for the greater good of the species."
With this unique trove of civilian-gathered data, the team discovered that the storms killed adult purple martins at up to 52% of the breeding sites monitored by citizen scientists across Texas and Louisiana.
Those martins that survived the cold delayed their reproduction and hatched fewer chicks in spring 2021 than they would have in normal conditions.
Furthermore, the effects continued to be felt long after the storm. During the 2022 migration season, martins arrived at their breeding grounds two weeks later than normal, and they differed genetically from those that had died the year before—in some ways, they were more like individuals from martin populations found further north.
By 2023, the migration pattern was returning to normal, and the population could fully recover in six to seven years…if there are no more deep freezes in the near future. The decrease in nesting success, however, could have ripple effects lasting generations. And purple martins are already declining more rapidly in Texas and Louisiana than anywhere else in their range.
Taken together, the findings indicate that even though large and diverse populations should be resilient to individual mass mortality events, even seemingly healthy populations may become vulnerable to repeated die-offs.
"People always ask me, 'if you're interested in climate change, why are you studying the cold?'" says Stager. "But if birds are going to have a future, we need to know more about how they survive newly unpredictable conditions, which can include surprisingly cold temperatures."
