Surveillance for Survival

Researchers at the Technical University of Munich in Germany tested twigs from 36 different tree and shrub species by exposing them to different temperature and light conditions in climate chambers for six weeks. Photo by Julia Laube/Tum.

For more than a dozen years Ken Schmidt has been studying how animals eavesdrop on one another. The Texas Tech University professor said that one of his study subjects, the veery, changes its singing behavior when owls are hooting to avoid becoming potential prey.

Veeries, which are close relatives of robins, have a beautiful flute-like song they often sing well after sunset. That’s a risky behavior when in the presence of hunting barred owls, which are known to feed on songbirds.

At three sites on the Cary Institute of Ecosystem Studies research campus in Millbrook, New York, where several pairs of barred owls are known to be in residence, Schmidt played three owl songs over the course of 25 minutes shortly after sunset. A microphone recorded all bird vocalizations for 30 minutes after the owl calls. He found that the veeries sang less after the owl sounds were played, and they stopped singing for the evening much earlier than when no owls were heard.

“Singing becomes much more risky in the low light of dusk when owls are around,” Schmidt concluded. “However, by eavesdropping on owls, veeries can adapt their singing behavior to decrease the risk of predation.”

The changes in veery singing behavior make sense to Schmidt, who said that prey must be somewhat flexible in their behavior if that behavior might attract predators: “Prey perceive signals of predators that tell them when it’s riskier to sing, and predators are perceiving cues of prey to find out where they are.”

The findings of this study, which were published in 2013 in the Journal of Behavioral Ecology and Sociobiology, likely apply to singing behavior during the dawn chorus, as well, but Schmidt said this study is especially notable because so little research has been conducted about the dusk chorus. “Further studies of dusk chorus singing may reveal how the risk of being attacked by predators has contributed to the evolution of singing behavior at dusk,” he said.

He also notes that eavesdropping behavior occurs in many species, and not just among birds. Schmidt found, for example, that chipmunks eavesdrop on the alarm calls made by tufted titmice that warn when predators are in the vicinity. He is also curious whether birds may change the structure of their songs in response to predators.

“If you’re a bird that sings at dusk, you want to sing to advertise your territory and to attract a mate, but that exposes you to the threat of getting killed,” Schmidt explained. “Maybe their song has evolved a certain structure that makes it hard to find where the bird is, like a cloaking device. So our next studies will try to measure and analyze the structure of their songs. Do they simply change how frequently they sing, or are they actually changing their voices?”


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