Species Identity Coding by the Song of a Rainforest Warbler: An Adaptation to Long-Range Transmission?

In tropical forest, visual communication is limited by obstacles and birds use mainly the acoustic channel to communicate at long range. However, in this dense vegetation environment, sounds may be greatly altered during transmission overdistance. To be e ff ecti ve,information transfer must rely upon parameters resistant to degra- dation, e.g., aslowly-modulated and low-pitched signal. Nevertheless, acommon bird of the Brazilian Atlantic forest, the White-bro wed Warbler Basileuterus leucoblep harus (Oscines, Parulidae ), presents aterritorial song having the opposite characteristics: awide frequenc yband and high-pitched signal. Thus, the aim of our study wastocharacterize the propag ation-induced modifi cations of this signal, and to identify its species-specifi ccod- ing parameters. According to propag ation experiments performed in the field at di ff erent distances, it appears that the signal is particularly sensiti ve to degradation through the vegetation: the fineacoustic structures (rapid frequenc yand amplitude modulations )are strongly modifi ed, and the highest pitched notes tend to disappear at a relati vely short distance (about 25 m).Playback experiments showthat, for species-specifi crecognition, birds use the only feature that is resistant to degradation, i.e., the overall slowfrequenc ymodulation of the song phrase, and ignore those parameters sensiti ve to propag ation distance. Moreo ver, birds do not need to hear the whole song, apart of it being su ffi cient to elicit abehavioural response. Thus, in spite of its structure, the song of the White- browed Warbler succeeds in conveying, overalong range (more than 100m )and through the dense vegetation, the information required for species- specifi crecognition.