June 25, 2011

Songbirds Protect Nests By Eavesdropping On Chipmunks

Some birds use information gathered from eavesdropping on their enemies to find safer areas to build their nests, scientists have found.

Birds that nest on the ground, such as ovenbirds and veeries, fall prey to predators as many eggs and fledglings struggle to survive. Neighboring chipmunks are a major threat.

These birds use a range of cues to determine the location of their nests, but Quinn Emmering and Dr. Kenneth Schmidt from Texas Tech University suggest that the "chips," "chucks" and "trills" made by chipmunks were being eavesdropped on by the birds.

"Chipmunks are vociferous, calling often during the day and sometimes joining in large choruses," says Emmering. "We thought this might be a conspicuous cue that nesting birds could exploit."

Veeries are described as "forest thrushes with warm, rusty-colored backs and cream-colored, faintly spotted chests." They build open, cupped-shaped nests that are directly on the ground or up to 1 meter high in shrubs.

Ovenbirds are known to be larger, squat-shaped warblers with blotchy, dark streaks on their underside, olive above with a bold white eye-ring and an orange-ish crown bordered by two dark stripes. These birds always build their dome-shaped nests made of leaves, pine needles and thatch with a side entrance on the ground. Ovenbirds received their name because their nests resemble a Dutch oven where their eggs are "cooked."

Researchers tested their theory of eavesdropping birds 85 miles north of New York City, in the rolling, forested hills of the Hudson Valley.

Emmering and Schmidt set up speakers at 28 plots that played either chipmunk or grey tree frog calls. In addition, at 16 "silent" control sites there were no recordings played at all.

When compared with the controls, veeries and ovenbirds placed their nests much further away from plots where chipmunk calls were played.

Ovenbirds responded twice as much with nests built at 20 meters further away from the calls than veeries, which nested about 10 meters further away, the research says.

"I was surprise by veeries weakly responding, as their nests are built no higher than a meter making them easily susceptible to chipmunks who regularly scramble up shrubs and trees," Emmering says.

But he suggests that this may be due to the fact that veeries have habitat preferences and alternative predators that could limit their nest-site choices.

"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 says.

The study can be found in this week's publication of the British Ecological Society's Journal of Animal Ecology.


Image 1: Ovenbirds (Seiurus aurocapilla) build dome-shaped nests directly on the ground making them susceptible to ground-foraging rodents. Credit: Quinn Emmering

Image 2: Eastern chipmunks (Tamias striatus) are common predators of songbird nests eating the eggs, nestlings and even fledgling young. Credit: Holly Owen


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