SONG FREQUENCY SHIFTS IN AN URBAN BIRD SPECIES OPTIMIZE ACOUSTIC TRANSMISSION INSIDE NOISY URBAN AREAS

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In animal acoustic communication is necessary that signals arrive to the receiver with reduced degradation and attenuation for a better transmission of the message. The noise pollution resulting from anthropogenic activities in cities reduces efficiency and efficacy of acoustic communication. Some species respond to high levels of noise increasing the minimum frequency of their vocalizations to avoid noise masking, but this may affect how sounds transmit in the environment because sounds with higher frequencies experience greater levels of attenuation and degradation. Using a transmission experiment, we analyzed how minimum frequency shifts, affect the sound transmission properties of the song of the House Wren (Troglodytes aedon) in urban areas that differ in the level of anthropogenic noise. We broadcasted songs with minimum frequencies between 1.2-1.8 kHz and the same songs with a minimum frequencies increment artificially of one semitone, increasing minimum frequencies to 2.1-2.6 kHz, in high and low noise level territories at four distances. We quantified signal-to-noise ratio, tail-to-signal ratio, blur ratio, and excess attenuation. Our results showed that songs with frequencies low minimum frequencies in low noise territories transmit with higher signal-to-noise ratio, and lower blur ratio and excess attenuation at longer distances. Songs with increased minimum frequencies only showed higher signal-to-noise ratio in noisier territories at longer distances. These results support the hypothesis of producing frequency shift to increase the communication distance in noisier environments. This is the first experimental study that tests the effect of shifting frequency on acoustic communication transmission on bird territories with different noise levels.