Eavesdropping and cue denial in avian acoustic signals

Although some signals seem adapted to maximize transmission of cues to intended receivers, others appear to have been selected to deny specific types of cues to unwanted receivers. We review three categories of avian vocal signals that have been suggested to show adaptation for cue denial: aerial predator alarm calls, begging calls, and soft songs and calls. Evidence supports the conclusion that aerial alarm calls are adapted to deny localization cues and that begging calls and soft songs are adapted to deny detection. Selection for denial of cues in acoustic signals has also been documented in a variety of other animals. In summary, eavesdropping by unwanted receivers is often as important in shaping the structure of acoustic signals as is selection for transmission to intended receivers.