Silvereyes decrease acoustic frequency but increase efficacy of alarm calls in urban noise
Section snippets
Study Species
Silvereyes are common native Australian passerines that inhabit a variety of environments, including urban areas. Silvereye songs and contact calls in city environments are higher in pitch than those in rural habitats (Potvin et al., 2011). Silvereyes also utter alarm calls, which are broadband in nature and consist of a fundamental frequency and a number of emphasized harmonics (Fig. 1).
Study Sites
Each pair of study sites was located at the following specific geographical areas in southeastern Australia,
Noise Profiles, Masking and Active Space in Rural and Urban Habitats
Mean noise levels at urban sites were 9.0 dB (A) higher than those at rural sites (95% CI: 2.09, 15.94 dB (A)). The energy of this noise was also distributed differently across the frequency spectrum in the two habitats (Fig. 2a). Thus, the background noise in each habitat differed not only in intensity (i.e. calls needed to be louder in urban areas to overcome masking by the background noise) but also according to frequency (Fig. 2b). For instance, a call made at 2500 Hz in the urban habitat
Discussion
Across disparate geographical locations, alarm calls of silvereyes in urban areas had lower average, peak and maximum frequencies than those of rural birds. This pattern contrasts with our earlier finding of higher frequencies in the songs and contact calls of urban silvereyes (Potvin et al., 2011). It also contradicts the prevailing expectation from recent empirical studies that the vocalizations of urban birds should generally display upward frequency shifts to reduce masking from urban
Acknowledgments
We thank J. Kruckel for field assistance, G. Fry, A. Leishman, G. Clancy, A. Tarrant, D. Paton, D. Williams, E. Woehler, R. Fuller and A. Fletcher for assistance in locating/banding birds, and Australian National Botanic Gardens (ACT), Namadgi National Park (ACT), Glenorchy City Council (Tas), Brisbane City Council (Qld), Susan Island Trust (NSW), Munghorn Gap Nature Reserve (NSW), Camden Airport (NSW), Kogarah City Council (NSW), Darebin City Council (Vic) and Darebin Parklands Association
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