Many birds feeds on mosquitoes that spread the West Nile virus, a disease that killed 286 people in the United States in 2012 according to the Centers for Disease Control. Birds also eat insects that can be agricultural pests.

However, rising temperatures threaten wild birds, including the Missouri-native Acadian flycatcher, by making snakes more active, according to University of Missouri biologist John Faaborg. He noted that farmers, public health officials and wildlife managers should be aware of complex indirect effects of climate change in addition to the more obvious influences of higher temperatures and irregular weather patterns.

"A warmer climate may be causing snakes to become more active and seek more baby birds for food," said Faaborg, professor of biological sciences in MU's College of Arts and Science. "Although our study used 20 years of data from Missouri, similar threats to bird populations may occur around the world. Increased snake predation on birds is an example of an indirect consequence that forecasts of the effects of climate change often do not take into account."

In the heart of Missouri's Ozark forest, cooler temperatures usually make snakes less active than in the edge of the forest or in smaller pockets of woodland. However, during abnormally hot years, even the interior of the forest increases in temperature. Since snakes are cold-blooded, warmer temperatures make the reptiles more active and increase their need for food. Previous studies using video cameras found that snakes are major predators of young birds.

Over the past twenty years, fewer young Acadian flycatchers (Empidonax virescens) survived during hotter years, according to research by Faaborg and his colleagues published in the journal Global Change Biology. Survival of young indigo buntings (Passerina cyanea) also decreased during warmer years.

Faaborg suggested that a likely reason for decreased baby bird survival in hot years was an increase in snake activity. Faaborg, his colleagues and his former students, collected the data used in the study during two decades of fieldwork.

"Low survival in the Ozark nests harms bird numbers in other areas," Faaborg said. "Birds hatched in the Ozark forest spread out to colonize the rest of the state and surrounding region. Small fragments of forests in the rest of the state do not support successful bird reproduction, so bird populations in the entire state depend on the Ozarks."