In early September, thousands of migrating birds were found dead in New Mexico, Colorado, Texas, Arizona and other states. Although there are still questions remaining about what caused the die-off, a researcher at the University of New Mexico (UNM) reported through the American Birding Association that the fundamental issue appears to be malnourishment.

Over Labor Day weekend, a strong cold front swept through the Rocky Mountain region dropping temperatures from the 90s to below freezing. The front also brought strong winds and snowfall in many areas. The rapid change in weather conditions likely played a factor in the die-off as migrating insectivores, that are already facing declining fat reserves, experienced sudden and dramatic unavailability of food. UNM PhD student Jenna McCollough and her colleague Nick Vinciguerra collected 305 individuals at a site near Velarde, NM on September 13. The dead birds were predominantly insectivores including mostly violet-green swallows (85% of the dead birds collected by McCollough), as well as other swallow and warbler species. The researchers noted the normal average mass of violet-green swallows is around 14 grams, however the specimens gathered from the mortality event (with only minor signs of decomposition) had an average mass of 9.5 grams – about two-thirds the normal weight. With the decreased fat reserves, these birds likely died from hypothermia.

However, a number of other factors might have contributed to the mortality events. Dead birds were found at the White Sands Missile Range in New Mexico in late August, before the weather change, and not all of the birds found dead across the states are insectivores. The malnourishment seen in recovered birds might also be attributed to significant decreases in insect and seed availability caused by other reasons besides the severe cold snap, such as the persistent drought in the region. Massive wildfires in the West potentially caused migrating birds to leave earlier in the season before they had developed sufficient fat reserves. In addition, it is possible that smoke inhalation may have impacted their lungs.

The U.S. Fish and Wildlife Service’s wildlife forensics laboratory in Ashland, OR, has received bird carcasses for necropsy but hasn’t yet released its findings.