Researchers at the University of New Mexico have found that the mysterious deaths of hundreds of thousands, if not millions, of migratory birds in the Western U.S. were primarily caused by a cold blast that killed off edible insects and induced hypothermia.
Around August 20, a large number of bird carcasses were found in the U.S. Army White Sands Missile Range and White Sands National Monument. Researchers thought the case was isolated, but hundreds of more birds were discovered in other regions.
Since the die-off was first reported, the event caused a stir on social media and among scientists, who considered wildfires and drought as some of the factors. The birds reportedly behaved unusually prior to their deaths-- some that are commonly seen in shrubs and trees were found chasing bugs on the ground.
"I am no stranger to dead birds, but I had never seen anything like this. This was a lot of dead birds. It really is sad to see something like that," said UNM ornithology Ph.D. student Jenna McCullough, one of the researchers who examined the samples.
McCullough, along with colleague Nick Vinciguerra, collected 305 samples of bird carcasses-- most of which were violet-green swallows-- and took them to UNM's Museum of Southwestern Biology in Albuquerque.
The researchers documented a pattern of atrophied breast muscles and a lack of fat stores, indicating that the birds suffered dehydration and starvation.
After conducting tests, the researchers said a cold front that swept through the Rocky Mountains possibly killed off insects eaten by the birds. The cold weather also likely induced hypothermia.
The birds seem to be in relatively good condition, except that they are extremely emaciated. They have no fat reserves and barely any muscle mass. Almost as if they have been flying until they just couldn’t fly anymore. (5/9) pic.twitter.com/tTPlDxwraf— Allison Salas (@salasphorus) September 13, 2020
Birds keep hitting windows in Markham. Seven days ago I found 76 birds from 22 species from a handful of buildings. Today, I found 79 birds from 22 species from the same locations - a total of 30 species for both days! This MUST stop! pic.twitter.com/bgDYThKF64— UrbanBirdman (@UrbanBirdman) September 18, 2020
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 @NVbirds and I collected in Velarde this week. pic.twitter.com/BuKrPlWakF— Jenna McCullough (@Jenna_Merle) September 18, 2020
If a lack of food contributed to the mortality event, birds would have less fat and no protection against hypothermia. Furcular fat, a sign of a resource-rich migrant, was completely lacking. Flight muscles were atrophied, which points to starvation & dehydration. pic.twitter.com/fKuaOS3ZM4— Jenna McCullough (@Jenna_Merle) September 18, 2020
U.S. Forest Service Wildlife Program leader Leslie Hay noted that a wide range of bird species was involved in the mass death. According to the Southwest Avian Mortality Project, observers reported 150 species of birds found dead across the Western U.S. and Mexico.
"Migration ties us all together. A bird might fly 8 000 km (5 000 miles) from Canada to Costa Rica, so what's happening in a forest here in New Mexico is important to our partners elsewhere," Hay remarked. "We're asking for help to collect as much data on these dead birds as possible because that data helps us generate theories about what's happening."
She added that bird migration across New Mexico is heaviest in September to October, so it is possible that another mortality event may occur in the coming weeks as West Coast wildfires could be a factor in adjusting migration routes.
"It's like if you're used to driving to Denver and have your typical pit stops. Migratory birds follow the same coastlines or mountain ridges and stop in the same forest. We're very concerned the fires are interrupting their typical stopping points."
Meanwhile, tissues from the specimens collected by McCullough and Vinciguerra will be extracted and frozen. "We will have a complete molecular record of all the pathogens and parasites being carried by these swallows," said Museum of Southwestern Biology Director Chris Witt. "The bright side of the tragedy is the carcasses present rich scientific opportunities."
Featured image credit: Jenna McCullough
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