Biologists in New Mexico have sounded the alarm after thousands of song birds were found dead in recent days.
New Mexico State University Professor Martha Desmond told local media that the reason for the mass die off is a mystery but could be tied to smoke from wildfires, or the recent cold weather.
The number of birds that have died may now be in the “millions”, she said.
Scientists have reported North American bird populations have declined massively in recent years.
State biologists have asked people to report sightings of dead birds for further research to be conducted.
Some birds – including migratory warblers, swallows and bluebirds – were seen acting strangely before their deaths, according to witnesses.