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What killed the birds in New Mexico?

Updated: Aug 24, 2021

Headlines from media across the Rocky Mountain region and much of North America, in recent weeks, indicate a catastrophic mortality event of migratory bird species in the southwestern United States. While these headlines are certainly worrisome and emotion-provoking, there is very little information that is certain at this time.

Bank Swallows were found deceased, packed into crevices in New Mexico
Bank Swallows were found deceased, packed into crevices in New Mexico

This 'die-off' event, originally thought to include over 5 states and Mexico, may not be a region-wide issue as indicated by some outlets. The majority of observations, specimen collections, and reports are centered around northern New Mexico and southern Colorado. The species most affected are swallows and other insectivores that feed almost exclusively on flying insects. The species outside of this dietary group that have been found deceased are likely a result of other mortality issues such as window collisions, free-roaming cats, car collisions, wildfires, etc.

Update #1 (08/01/2021): The most recent updates indicate that the majority of these birds starved to death, likely due to insect unavailability from the snowstorm.

What was the cause of death for this large group of migratory birds?

Currently, the leading theory for the cause of death is starvation and hypothermia. How would this happen? A winter storm capable of crushing our power grids, 100-year-old trees, and hopes for wearing shorts in September. That's right, a snowstorm not only kicked our collective human butt, but it also took a flyswatter to hordes of migratory birds. The combination of a drop in temperatures over 60°F, 70+ mph winds, and wet snow likely grounded and/or killed most flying insects needed by swallows, warblers, and flycatchers.

However, nothing is guaranteed at this time. So, here are some facts and some 'less-than-facts':


  • Mortality Event - There are reports of a significant number of birds that have been reported as dead across northern New Mexico & southern Colorado.

  • Migration - There were significant migration events (movements) in the nights prior to these discoveries.

  • Insectivores - A large percentage of this event seems to be insectivores, though other species are being found as well.

  • Necropsy - Birds are being sent off to various labs for necropsy, the number of samples has not been provided

  • Time - It will take weeks to get answers from these labs.

Violet-green Swallows were one of the species with significant mortalities recorded
Violet-green Swallows were one of the species with significant mortalities recorded


  • Cause of Death - The official cause of death won’t be known for a while. However, the possibilities are numerous. 

  • Any guess at this point would be pure speculation, but speculation is what is being asked from the bird community. 

  • There is likely no SINGLE answer. 

  • Migration Event - Combine a 3-4-day period of large migration movements with wildfires, a winter storm, and habitat loss, and we can begin to see how an event like this could occur.

  • Numbers – Despite the headlines, numbers are not known. You will see quotes from Martha Desmond, but no other group has offered any real numbers, yet. The number is likely not as high as the "hundreds of thousands or millions" often quoted.

  • Location – Outside of New Mexico and Colorado, we do not know how many other dead birds are tied to this event. Arizona, Texas, California, and Nebraska are also being included, but that is leaving large gaps of information unaccounted for.

  • Continuance – Will this continue to happen? We do not know. It is impossible to know if this is a singular phenomenon or the new norm.

Western Wood-Pewees also suffered significant mortalities
Western Wood-Pewees also suffered significant mortalities

Does smoke hurt birds?

This question is frequently being asked by those curious about the bird deaths. Simply? Yes, smoke hurts birds. A study from 2017 (that used numerous studies since 1950) found:

“Consistent evidence for adverse health impacts on birds attributable to exposure to gas-phase and particulate air pollutants, including carbon monoxide (CO), ozone (O3), sulfur dioxide (SO2), smoke, and heavy metals, as well as mixtures of urban and industrial emissions. Avian responses to air pollution include respiratory distress and illness, increased detoxification effort, elevated stress levels, immunosuppression, behavioral changes, and impaired reproductive success. Exposure to air pollution may furthermore reduce population density, species diversity, and species richness in bird communities.”

Smoke damages birds in the short-term AND the long-term.


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