Acoustic properties of two urban song dialects in the Orange-tufted Sunbird (Nectarinia osea)

摘要

Biologically important acoustic signals must be transmitted from a signaler to a receiver. Over distance, however, sounds may undergo modification through attenuation, degradation, and masking. Recent anthropogenic habitat modification occurring in many places-in urban habitats, in particular-has rapidly changed local topography and atmospheric conditions and generated new patterns of noise that are likely to interfere with communicative signals. As part of a study of microgeographic song dialects in an urban population of Orange-tufted Sunbirds (Nectarinia osea) in Israel, we examined the environmental influences on song transmission and reception in a rapidly developing human-altered environment. We examined the physical properties of the two dialect song types, which exhibit a large difference of 2-3 kHz in the maximum frequency of the trill, using sound transmission measurements to test how both song types propagate through a highly obstructed habitat of buildings and vegetation. Additionally, we examined how ambient noise - in particular, low-frequency noise arising mainly from automobile traffic-affects the transmission of both dialect songs. Finally, using song playback, we investigated the consequences of sound degradation on dialect recognition and discrimination by sunbirds. The dialect containing higher frequencies in the trill was found to undergo severe frequency-dependent attenuation, in which the maximum frequency of the trill notes drops by > 2 kHz over a distance of 70-100 m (less than two territories away). Also, the possibility that the use of higher frequencies in that dialect group's song is intended to overcome masking by urban ambient noise, which is concentrated mainly in lower frequencies, was not supported by our findings. Males singing the high dialect responded differently to playbacks of an intact and an attenuated form of their dialect song. Taken together, our results suggest that the dialect containing higher frequencies in the trill may be unsuitable for effective long-range transmission through this particular sunbird habitat.