Thousands of migratory birds have recently been found dead in New Mexico, causing alarm among biologists.

New Mexico State University professor Martha Desmond explained that the mass die-off is a mystery but could be related to smoke from wildfires, or the recent cold weather.

She also estimated the number of bird deaths to possibly be in the millions.

"It's devastating. I don't think I've ever seen anything this horrible in my life," Desmond said.

Some birds — including migratory warblers, swallows and bluebirds — were seen acting oddly before their deaths.

Smoke from wildfires is thought to have affected the birds or forced them to change migratory routes. Recent snows in Colorado could also be responsible, Desmond said, adding that more will not be known until the animals’ carcasses can be studied.

Colorado, Texas and Arizona have also reported an increase in birth deaths.