Most of the casualties are thought to be songbirds that feed on insects, flying south from north-western US or Canada to spend winter in central or southern America.
Andrew Farnsworth, a senior research associate at Cornell University's ornithology lab, called the die-off a "major, major event".
"The hope is that this is not going to be something that continues in the future, but impacts from climate change are wreaking havoc in different systems, and this is unfortunately one of the kinds of things that we might see more and more of."
The migratory birds, which include flycatchers, warblers and swallows, have also been found dazed on the ground in large numbers.
Jon Hayes, executive director of the conservation nonprofit The National Audubon Society's New Mexico office, said he had encountered dozens of birds while on a bike ride last week in Albuquerque, the state's largest city.
"There were actually dazed exhausted birds all along the path, most all of them swallows in this case, and a number of them were dead as well. I actually had to stop multiple times and help a couple of them off the trail," he said, adding that the experience was "really troubling."
"I've never seen anything like this. You get high mortality rates during migration. It's a particularly hard time for all bird species, but you don't ever see any more than just one dead bird."
"I don't know what else we can do to raise the alarm, when it seems like there's alarm bells everywhere," Hayes added.
Usually, die-offs are caused by disease or a specific event, such as an oil spill. But the unprecedented scale of this event has led scientists to believe that environmental factors are to blame.
Many of the birds would flown through areas in Oregon, Washington and California that have been affected by wildfires this year.
Wildfires began months before the usual October fire season and have already burned almost five million acres across the three west coast states, following unseasonably dry weather believed to be caused by climate change.
The biological impacts of breathing wildfire smoke are not well-understood, but birds are thought to be particularly vulnerable to particulates and polluted air.
Extremely high summer temperatures across the entire western US are also thought to have impacted birds' ability to survive, and dry conditions in New Mexico have impacted the populations of the insects they feed off.
Conservationists warned that a repeated trend of high heat, wildfires and extreme weather events could threaten the survival of the migratory bird populations and disrupt ecosystems which rely on them to cull insect populations and spread seeds.
Unexplained songbird mortality has wildlife biologists in NM very concerned. We are hoping Citizen Science can help --...
Posted by U.S. Forest Service-Santa Fe National Forest on Friday, 11 September 2020
The state also experienced an unseasonable cold snap and 95mph winds last week, which would have exhausted many birds already running low on fuel and energy after migrating for thousands of miles.
Residents have been asked to photograph and report sightings of dead birds so that scientists from the US Wildlife Service and New Mexico State University can catalogue them.
