The song was first played just outside of the sparrow's territory. Believing the song to come from a hidden aggressor close to their home turf, the provoked bird sang and approached the speaker.
The song was then played from a different speaker placed 20 meters (60 feet) inside his boundary, which simulated movement by the rival into the territory. At the same time, the researchers uncovered the stuffed sparrow just above the speaker, with its beak posed as if it was singing.
It was enough to fool the live birds. Many of the sparrows responded first by matching the intruder's song. When the simulated intruder moved into the territory and persisted in singing, the defending sparrow progressed to higher-level warnings including soft songs and wing waves.
Soft song is not lullaby-like, but is perceived as menacing by enemies. Wing waving, a vigorous vibrating of one wing at a time, likewise looks harmless enough, but it too is an assertive signal. (See wing waving at the 27-second mark of this video: http://www.youtube.com/watch?v=9ynqMTKs_Fc&feature=youtu.be.)
When the stuffed bird did not react to those higher-level warning signals, the live bird attacked (see video: http://www.youtube.com/watch?v=4wgmqNvofW0).
"Birds generally do all this signaling," Akay said, "because it's usually beneficial to avoid getting into a fight if it can be avoided. There are less costly ways to persuade an aggressor to back down."
Most of the sparrows in the study, 31out of 48, eventually attacked. Birds who had song-matched were the mostly likely to attack.
Not all birds showed the same pattern of signaling. A few "bluffers" matched the trespasser's song without following through with a
|Contact: Molly McElroy|
University of Washington