Warming Temperatures Give Snakes a Taste For Birds

By Emma Bryce

Reporter, Audubon Magazine

Published July 15, 2013

An Acadian Flycatcher - one of the Missourian birds threatened by snakes - at the nest (Photo by Kelly Colgan Azar / CC BY-ND 2.0)

As temperatures change, many bird populations shift their ranges, or appear to decrease. It’s a fact that’s gaining widespread recognition, and serious investigation into the various triggers. But in one case, researchers believe they have honed in on the cause of one such heat-related avian dip: snakes.

A biologist at the University of Missouri found, over 20 years of fieldwork, that when temperatures rose in Missouri’s Ozark Forest, some bird populations took a hit. But temperature works in strange ways for snakes too: heat awakens cold-blooded reptiles and intensifies their need for food, and bird nests become a convenient target. The logic goes that just as snakes’ appetites get stirred up by the heat, a population dip in some birds results.

“A warmer climate may be causing snakes to become more active and seek more baby birds for food,” said the lead researcher, John Faaborg, in a university press release. “Although our study used 20 years of data from Missouri, similar threats to bird populations may occur around the world.”

But there’s another intriguing angle on this tale: some threatened birds included in the study, like the Acadian Flycatcher, are also responsible for eating mosquitoes that might carry pathogens such as the West Nile virus, which since 1999 has killed 1,200 people in the United States. Without such vital birds being present and populous enough to eliminate some portion of the infected mosquito population, there’s reason to believe that viruses like the West Nile could develop, unchecked.

The research team studied the impact of heat on birds by surveying forested areas. Deep in Missouri’s Ozark Forest, animals can usually escape the heat. But during especially hot years, even the cooler depths grow warm, and reptilian activity goes up. By monitoring nests in forested parts, Faaborg showed in the paper, published in Global Change Biology, that during those hotter years, nest productivity fell in populations of Acadian Flycatchers and Indigo Buntings.

Since video footage in former studies has shown that snakes prey on nestlings, Faaborg traced the link between the forest’s birds and snakes. But the events in the Ozark Forest can’t be isolated. “Low survival in the Ozark nests harms bird numbers in other areas…Birds hatched in the Ozark forest spread out to colonize the rest of the state and surrounding region,” Faaborg said in the release. “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.”

Reduced bird populations might also have consequences for agriculture, since birds prey on other insects that are recognized as crop ‘pests’. “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,” Faaborg added.

As for the West Nile virus: with over 200 fatal cases last year alone, the understanding that birds might play a central role in controlling its spread could spur on further efforts to protect their delicate nests—both from the immediate threat of predators, but perhaps also from the more abstract threat of climate change.