Last week, the Rocky Mountain states experienced a strong storm that brought with it snow, near hurricane force winds, and unseasonable record-breaking cold temperatures. In Albuquerque on September 8, it was sunny and a record-high 96ºF. The next afternoon, a severe windstorm tore through the region. The Albuquerque airport measured windspeed of over 70 mph, and temperatures plummeted to historic lows. Albuquerque broke a 100-year record low temperature when the mercury dropped to 40ºF. While snowfall was heaviest in the northern Rockies from Montana to Colorado, New Mexico received several inches of heavy, wet snow as far south as the Sandia Mountains east of Albuquerque.

Migratory bird casualties in Velarde, NM on 13 Sep 2020. Image from video posted on Twitter by Austin Fisher.

My colleagues and I spent the morning of Thursday 10 September picking up dead birds in the Sandias. We found several dead Empidonax flycatchers of three species, a Vesper Sparrow, and a Townsend’s Warbler. Some birds were wet from the overnight snow, but others were completely dry, huddled in the corners of buildings. A Dusky Flycatcher sat dazed in the parking lot.

We first thought little of it: mortality is expected for migratory birds, and we didn’t find more than a handful of carcasses. But social media told a grimmer story that night. We read reports of widespread mortalities across the state: dead swallows along a bike path in Albuquerque, a half-dozen Empidonax flycatchers and swallows in one park in Clovis, and a local news report of 300 carcasses recovered by researchers from New Mexico State University and nearby White Sands Missile Range. It was soon apparent that a significant mortality event had occurred.

But one video on Twitter recorded by local journalist Austin Fisher stood out to me: several dozen swallows dead in an arroyo in Velarde, approximately 40 miles north of Santa Fe. It was only when I reached out to Austin for the purposes of this report that I realized the video wasn’t taken the week before during the cold snap, but rather the previous night, on 13 September. To see it for myself, fellow ornithology grad student, Nick Vinciguerra, and I drove the hour and a half north that night.

When we arrived at midnight, we found a macabre scene. 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. These proportions are similar to what was reported by researchers at NMSU.

Nick Vinciguerra collecting Violet-green Swallows on the banks of the Rio Grande River at midnight in Velarde, NM.

Several hypotheses are emerging to explain this mass mortality event in New Mexico. Recently, heightened attention has been given to the possibility that historic wildfires across western North America are to blame, and wildfires certainly pose a major disruption to migratory birds. For instance, a wildfire could cause birds to flee an area before they’ve replenished their fat stores. Indeed, anecdotal reports from banding stations suggest that wildfires contribute to unusual migrant influxes into areas that are free from fire. Michael Hilchey, a volunteer bander at the Rio Grande Bird Research Station in Albuquerque, noted a significantly higher volume of migrants over the past two weeks than has been over the last 10–15 years. Smoke is covering nearly all of the lower 48 states, and while we experienced heavy smoke in Albuquerque the night before the storm arrived, fires are not new or unexpected during the height of fall migration. Indeed, wildfires are common and increasing in frequency.

There is, I believe, a much more plausible reason for large numbers of birds to die during migration: lack of food.

The 55–60ºF temperature swing observed in New Mexico combined with hurricane force winds and with wet snow very likely caused hypothermia in some birds, especially juveniles. Furthermore, cold temperatures also affect the food supply for insectivores, as insects (which become dormant or dead) are then covered by snow. Certainly, they are not flying through the air, as swallows and pewees need. Dave Leatherman, a former entomologist for the state of Colorado, noted marked behavioral differences in foraging insectivorous birds during the week’s snowstorm. In addition, a 2007 study by Ian Newton found that unseasonably cold weather can have a negative effect on migrating birds. While cold temperatures and snow cut off the food supply for naïve migrants, resident birds not stressed by migration typically have both fat reserves and local knowledge of where to find shelter.

Notably, and understandably, this type of die-off commonly affects swallows. In several documented cases of swallow mortality events (Newton 2007), a sudden drop in temperatures caused insects to become dormant (and stop flying). In Kazakstan during the fall of 2000, cold and snow killed thousands of Barn Swallows (Berezovikov and Anisimov 2002). Severe cold snaps in 1931 and 1974 killed “hundreds of thousands, possibly millions” of swallows and martins in central Europe (Alexander 1933, Ruge 1974, Bruderer and Muff 1979, Reid 1981, Newton 2007). Specifically, Newton (2007) states, “When short of food in cold weather, swallows and swifts often seek shelter in buildings, huddle together for warmth, and may suffer from hypothermia and starvation. Other migratory insectivores also die in such conditions, but less conspicuously.”

The 305 individuals laid out at the Museum of Southwestern Biology that Nick Vinciguerra and I collected from Velarde, NM on 14 Sep 2020. All individuals will be deposited as specimens in the museum’s Bird Division for future research and education.

Sudden and dramatic unavailability of food caused by a historic and drastic cold snap is, I believe, a more parsimonious explanation than a widespread, smoke induced, mass mortality event. While we do not have data on how fast smoke inhalation would kill birds hundreds of miles away from the fires themselves, what we do have are data from the 258 Violet-green Swallows that Nick and I collected in Velarde this week.

Satellite imagery showing smoke from wildfires in the western U.S. on 9 Sep 2020. Image © NOAA.

If a lack of food contributed to the mortality event, birds would have less fat and no protection against hypothermia. Indeed, of the hundreds of birds we assessed, none had fat stores on their bodies. Furthermore, many birds also showed signs of breast muscle atrophy, which points to starvation and dehydration. The average mass of an adult male Violet-green Swallow is 14.4 g; females are slightly lighter at 13.9 g. In addition, I used an open-access museum collections database, Vertnet, to find data on thirty specimens collected July–September, and their average weight was 15 g. We weighed 234 swallows which showed only minor signs of decomposition, and their average mass was dramatically lighter: 9.5 g, or about two-thirds the weight of normal birds. Though we have yet to perform any toxicology analyses or inspect their lungs for signs of smoke inhalation, I think it is safe to say that these birds were starved and succumbed to hypothermia. When USFWS autopsies of other birds are reported in the coming weeks or months, we suspect they will reveal a similar cause of death.

Christopher Witt, Professor at UNM and Director of the Museum of Southwestern Biology, waxed poetical with me this week about how fall 2020 has brought a spectacular array of fall migrants to Albuquerque, noting that it’s been the “Best I’ve seen in years.” As a birder myself, I also benefitted from this better than average migration with my lifer Blackpoll Warbler on the University of New Mexico campus this week. Our influx of migrants may or may not have been due to wildfires, but I have no doubt that they were affected by the extreme cold and high winds in New Mexico. Though the fires and extreme weather events are influenced by human-induced climate change, it is unlikely that the wildfires alone caused the death of thousands of birds in New Mexico.

A comparison of body mass from the birds we salvaged on 14 September 2020 with that of other Violet-green Swallows collected during fall migration across the North America, downloaded from Vertnet, an open access biodiversity database. Both outlier points on the right refer to specimens that had little to no fat stores.

Jenna McCullough is a 3rd year PhD student at the University of New Mexico and the Museum of Southwestern Biology, where she studies avian evolution and systematics of birds in the South Pacific. Jenna is a third-generation female birder and regularly birds across the western U.S. and internationally. She can be reached on Twitter @Jenna_merle or via email at mcculloughj@unm.edu