Ground-nesting birds face an uphill（向上的） struggle to successfully rear their young, many eggs and fledglings（刚会飞的幼鸟） falling prey to predators. Now, scientists from the USA have found that some birds eavesdrop（窃听） on their enemies, using this information to find safer spots to build their nests. The study – one of the first of its kind – is published this week in the British Ecological Society's Journal of Animal Ecology. Ovenbirds and veeries both build their nests on the ground, running the risk of losing their eggs or chicks to neighbouring chipmunks. Nesting birds use a range of cues to decide where to build their nests, but Quinn Emmering and Dr Kenneth Schmidt from Texas Tech University wondered whether the 'chips', 'chucks' and 'trills' chipmunks use to communicate with each other were being eavesdropped on by the birds.

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

Working in the rolling, forested hills of the Hudson Valley 85 miles north of New York City, Quinn Emmering and Dr Schmidt tested their theory that ovenbirds and veeries might be eavesdropping on chipmunks' calls before deciding where to nest by setting up a playback experiment. At 28 plots a triangular arrangement of three speakers played either chipmunk or grey tree frog calls, while at 16 'silent' control sites no recordings were played.

They found that compared with the controls, veeries and ovenbirds nested much further away from plots where chipmunk calls were played.

Interestingly the size of the response was twice as high in ovenbirds, which nested 20m further away from chipmunk-playback sites than controls, while veeries nested only 10m further away.

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（波动，起伏） in rodent numbers that closely follow the boom-and-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," says Emmering.

Veeries (Catharus fuscescens) are forest thrushes（画眉） with warm, rusty-coloured backs and cream-coloured, faintly spotted chests. Ovenbirds (Seiurus aurocapilla) are larger, squat-shaped warblers. They have blotchy（斑点的） , dark streaks on their underside, olive above with a bold white eye-ring and an orangish crown bordered by two dark stripes.

Both ovenbirds and veeries primarily forage on the ground and low down in the shrub layer of the forest. Veeries build open, cupped-shaped nests directly on the ground or up to 1m high in shrubs. Ovenbirds, on the other hand, always nest on the ground, building dome-shaped nests made of leaves, pine needles and thatch（茅草，杂草） with a side entrance. Ovenbirds are so-called because their unique nests resemble a Dutch oven where they 'cook' their eggs.

Chipmunks produce three types of calls: a high pitched 'chip', a lower pitched 'chuck' and a quieter 'trill' consisting of multiple, twittery notes. Chips and chucks are often given in a series when a predator is detected and trills are usually in response to being chased by a predator or another chipmunk.