Abstract
Birds communicate through acoustic variation in their songs for territorial defense and mate attraction. Noisy urban conditions often induce vocal changes that can alleviate masking problems, but that may also affect signal value. We investigated this potential for a functional compromise in a neotropical songbird: the bananaquit (Coereba flaveola). This species occurs in urban environments with variable traffic noise levels and was previously found to reduce song elaboration in concert with a noise-dependent reduction in song frequency bandwidth. Singing higher and in a narrower bandwidth may make their songs more audible in noisy conditions of low-frequency traffic. However, it was unknown whether the associated decrease in syllable diversity affected their communication. Here we show that bananaquits responded differently to experimental playback of elaborate vs. simple songs. The variation in syllable diversity did not affect general response strength, but the tested birds gave acoustically distinct song replies. Songs had fewer syllables and were lower in frequency and of wider bandwidth when individuals responded to elaborate songs compared to simple songs. This result suggests that noise-dependent vocal restrictions may change the signal value of songs and compromise their communicative function. It remains to be investigated whether there are consequences for individual fitness and how such effects may alter the diversity and density of the avian community in noisy cities.
Highlights
In the last decades, the noise levels in human-altered and natural habitats have substantially increased and affected the way birds sing (Rabin and Greene, 2002; Mennitt et al, 2015; Buxton et al, 2017)
Individuals responded in acoustically distinct ways to each playback type. Their songs had fewer syllables and were lower in frequency and wider in frequency bandwidth when they responded to the elaborate song stimuli compared to when they responded to the simple song stimuli (Figure 4)
We found the following answers to our questions: (1) playback of simpler songs did not trigger stronger behavioral responses than playback of more elaborate songs; (2) individuals did not match song elaboration to the stimulus categories, and even decreased syllable numbers in their song in response to more elaborate songs; (3) songs triggered by elaborate song playback had a lower minimum frequency and wider frequency range compared to songs sung before the playback
Summary
The noise levels in human-altered and natural habitats have substantially increased and affected the way birds sing (Rabin and Greene, 2002; Mennitt et al, 2015; Buxton et al, 2017). Noise-Related Song Variation Affects Communication been reported in city birds (Brumm, 2004; Potvin and Mulder, 2013; Gil et al, 2014), which typically yield an increase in song detectability and improved efficiency of communication (Brumm and Slabbekoorn, 2005; Pohl et al, 2012). Vocal changes may affect signal detectability and signal value (Slabbekoorn and Ripmeester, 2008; Gross et al, 2010) and noise-dependent song variation may thereby involve a functional compromise (Slabbekoorn, 2013; Luther and Magnotti, 2014; Luther et al, 2016; Phillips and Derryberry, 2018). Birds are reported to sing at higher amplitudes if noise levels rise and they can sing shorter or in alternating time periods when noise levels are fluctuating (Brumm, 2004; Gil et al, 2014; Gentry et al, 2017; Derryberry et al, 2017)
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