Like human code-breakers intercepting secret messages, red-breasted nuthatches have evolved ways to extract hidden information about threats from predators from the seemingly indistinguishable alarm calls of other species. It is the first time such subtle eavesdropping has been observed in nature.
The nuthatches, common in Canada and the northern US, tune into alarm calls made by black-capped chickadees when the latter are threatened by owls.
Chickadee signals evidently carry a great deal of meaning within the species (see Chirpy chickadees signal deadliness of predators), but to the human ear all chickadee alerts sound very similar.
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Nuthatches, however, have developed ways of telling whether the chickadees are threatened by pygmy owls, which pose a serious threat to small birds, or by much larger great horned owls, which attack small birds less often.
Christopher Templeton and Erick Greene at the University of Washington at Seattle, US, made the discovery after placing speakers at the base of trees in areas frequented by nuthatches and playing recordings of chickadee responses to both types of owls. Listen to the chickadee call warning of a pygmy owl (0.3MB .wav format) and the call warning of a great horned owl (0.3MB).
Surprise attack
Nuthatches became significantly more agitated when they heard recordings of the pygmy owls, approaching the tree where the speaker was placed to 鈥渕ob鈥� the predator. The mobbing behaviour may appear suicidal but it helps to drive the predator away so it cannot launch a surprise attack.
鈥淔or the chickadee calls that signalled a pygmy owl, nuthatches mobbed the speaker more intensely by approaching closer, vocalising more and spending more time mobbing than during playback of chickadee calls signalling a great horned owl,鈥� says Templeton.
Templeton says the nuthatches probably discriminate fine-scaled features in the 鈥渃hick-a-dee-dee-dee鈥� call such as the number of harmonics in the 鈥渄ee鈥� notes, or the length and timing of different syllables. 鈥淭he most important thing our study shows is just how much goes on in nature that we aren鈥檛 aware of,鈥� he says.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0605183104)
