by John Davis
The Sounds of Nature
Chipmunks produce three types of calls consisting of multiple, twittery notes: a high pitched “chip,” a lower pitched “chuck” and a quieter “trill.” Chips and chucks are often given in a series when a predator is detected, and trills are usually in response to being chased.
Eastern chipmunks (Tamias striatus) produce a variety of calls to communicate with one another. Listen to a recording of the chipmunk sounds. Recording courtesy Quinn C. Emmering.
Ovenbirds are ground-nesting warblers that studies show avoid nest predators by listening to surrounding sounds when selecting nest sites.
According to the study, ground-nesting veeries only weakly responded to chipmunk calls. Photo courtesy Holly Owen.
Study Shows Ground-Nesting Birds Eavesdrop to Find Safe Neighborhoods
Ground-nesting birds face an uphill struggle to successfully rear their young, with many eggs and chicks falling prey to predators.
However, two researchers at Texas Tech have found that some birds eavesdrop on their enemies, using this information to find safer spots to build their nests. The study – one of the first of its kind – was recently published in the “British Ecological Society’s Journal of Animal Ecology.”
Ovenbirds (Seiurus aurocapilla) and veeries (Catharus fuscescens) both build their nests on the ground, running the risk of losing eggs or chicks to neighboring chipmunks.
Nesting birds use a range of cues to decide where to build their nests, but doctoral candidate Quinn Emmering and Kenneth Schmidt, an associate professor in the Department of Biological Sciences, wondered if the birds eavesdrop on the chips, chucks and trills that chipmunks use to communicate with each other.
“Veeries and ovenbirds arrive annually from their tropical wintering grounds to temperate forests,” Emmering said. “They must immediately choose where to nest. A safe neighborhood is paramount, as many nests fail due to predation. Predators are abundant. However, many predators communicate with one another using various calls, scent marks or visual displays that become publicly available for eavesdropping prey to exploit.”
Testing the Theory
Working in the forested hills on the property of the Cary Institute of Ecosystem Studies, 85 miles north of New York City, Emmering and Schmidt tested their theory that ovenbirds and veeries might be eavesdropping on chipmunks’ calls before deciding where to nest. At 28 study plots, a triangular arrangement of three speakers played either chipmunk or grey tree frog calls (a procedural control), and at 16 control sites no recordings were played.
“Chipmunks call often during the day and sometimes join in large choruses,” Emmering said. “We thought this might be a conspicuous cue that nesting birds exploit.”
The researchers found that the two species nested farther away from plots where chipmunk calls were played. The size of the response was twice as high in ovenbirds, which nested 65 feet farther away from chipmunk-playback sites than controls; veeries nested 32 feet farther away from chipmunk-playback sites.
The weaker response by veeries suggests they may not attend to chipmunk calls as ovenbirds do. This difference could ultimately have an effect on how their respective populations are able to respond to dramatic fluctuations in rodent numbers that closely follow the boom-to-bust cycles of masting oak trees.
“We found that by eavesdropping on chipmunk calls, the birds can identify hotspots of chipmunk activity on their breeding grounds, avoid these areas and nest instead in relatively chipmunk-free spots,” Emmering said.
The Feathered Subjects
Veeries (pictured above) are forest thrushes with warm, rusty-colored backs and cream-colored, spotted chests. Ovenbirds are large warblers with dark streaks on their underside, and are olive above with a bold white eye-ring and an orange crown bordered by two dark stripes.
Ovenbirds and veeries primarily forage on the ground and the shrub layer of the forest. Veeries build open, cup-shaped nests directly on the ground or up to 3 feet high in shrubs. Ovenbirds, on the other hand, always nest on the ground, building dome-shaped nests made of leaves, pine needles and thatch with a side entrance. Ovenbirds are so-called because their nests resemble a Dutch oven where they “cook” their eggs.
The British Ecological Society was established in 1913 by academics to promote and foster the study of ecology. The learned society has 4,000 members in the UK and abroad.
Kenneth Schmidt is an associate professor in the Department of Biological Sciences in the College of Arts and Sciences.
His research focus is the ecology of information, how organisms acquire information to manage daily activities and the significance of information for populations, communities and ecosystems. His research projects include eavesdropping and predation risk, settlement and nest-site decisions, and eavesdropping and cognitive abilities of caching in tree squirrels. More
Quinn Emmering is a doctoral candidate in the Department of Biological Sciences in the lab of Kenneth Schmidt. His research primarily focuses on avian ecology and behavior, but particularly, exploring what information birds use to make adaptive decisions.
His current research, conducted at the Cary Institute of Ecosystem Studies, investigates the spatial refugia and cues used by songbirds to assess heterogeneity in predation risk as it applies to nesting decisions. More
Photos courtesy Quinn C. Emmering and Kenneth Schmidt.
John Davis is a Sr. Writer in the Office of Communications & Marketing at Texas Tech University.