Condensed by Paula Dittrick, TMNCPC blogmaster, from American Birding Association column by North American Birds Associate Editor Jenna McCullough
Hundreds of birds were found dead in New Mexico earlier this month, prompting researchers to question whether wildfires or severe weather (cold and high winds) were to blame. The deaths came amid a record high influx of migrants around Albuquerque this year compared with the last 10-15 years.
A recent column by Jenna McCullough, ABA North American Birds associate editor, discusses what might have happened. McCullough is a third-year PhD student at the University of New Mexico and the Museum of Southwestern Biology. She studies avian evolution and systematics of birds in the South Pacific.
McCullough and colleagues picked up dead birds in the Sandia Mountains on Sept. 10. News media reports several days later indicated a significant mortality event, initially attributed to historic fires in California, Oregon, and Washington or a historic drought in the Southwestern U.S.
On Sept. 14 north of Santa Fe, McCullough and fellow ornithology grad student Nick Vinciguerra found, “Several hundred Violet-green Swallows were strewn across the bank of the Rio Grande. Dozens of birds had stuffed themselves into the few natural cavities, and many more were dead amongst the vegetation. In total, we found 305 individuals of six species, all of which were insectivores: 258 Violet-green Swallows, 35 Wilson’s Warblers, six Bank Swallows, two Cliff Swallows, one Northern Rough-winged Swallow, a MacGillivray’s Warbler, and two Western Wood-Pewees.”
Several hypotheses are emerging on the causes of this mass mortality. One suggestion centered on historic wildfires, which do pose a major disruption to migratory birds.
But McCullough believes it is more likely that cold temperatures and a lack of food caused those deaths. Just before the recorded deaths, there was a significant cold snap and heavy snow across the northern Rockies from Montana to Colorado. New Mexico received several inches of wet snow as far south as the Sandia Mountains east of Albuquerque.
New Mexico’s temperature swing of 55–60º F, high winds, and wet snow probably caused hypothermia in some birds, especially juveniles.
McCullough believes the mortality event stems from lack of food for migrants caused by the drastic cold snap. Insects go dormant or die before getting covered by snow. Unseasonably cold temperatures and snow cut off the food supply for migrants while resident birds typically have more fat reserves and know where to find shelter, she said.