ACOUSTIC PROPERTIES OF TWO URBAN SONG DIALECTS IN THE ORANGE-TUFTED SUNBIRD (NECTARINIA OSEA)

LIKE OTHER FORMS of animal communication, bird song requires transmission from a signaler to a receiver. Over distance, however, biologically important sounds may attenuate (i.e. lose amplitude as a result of geometrical spreading and general absorption of sound energy), degrade (i.e. change acoustic

structure through reverberation, frequency-dependent attenuation, and irregular amplitude fluctuations), and be masked by background noise (Morton 1975, Wiley and Richards 1978, Endler 1992, Forrest 1994). The predominant selection pressures that may modify the structure of long-distance acoustic signals, so as to maximize their transmission, are (1) sound transmission properties associated with local microclimate and vegetation structure (Wiley and Richards 1982, Brown and Handford 2000, Slabbekoorn et al. 2002) and (2) signal interference by local ambient noise patterns (Brenowitz 1982a, Ryan and Brenowitz 1985, Nelson and Marier 1990, Slabbekoorn and Smith 2002, Slabbekoorn and Feet 2003).