Vocal frequency change reflects different responses to anthropogenic noise in two suboscine tyrant flycatchers

Abstract

Anthropogenic noise is prevalent across the globe and can exclude birds from otherwise suitable habitat and negatively influence fitness; however, the mechanisms responsible for species' responses to noise are not always clear. One effect of noise is a reduction in effective acoustic communication through acoustic masking, yet some urban songbirds may compensate for masking by noise through altering their songs. Whether this vocal flexibility accounts for species persistence in noisy areas is unknown. Here, we investigated the influence of noise on habitat use and vocal frequency in two suboscine flycatchers using a natural experiment that isolated effects of noise from confounding stimuli common to urban habitats. With increased noise exposure, grey flycatcher (Empidonax wrightii) occupancy declined, but vocal frequency did not change. By contrast, ash-throated flycatcher (Myiarchus cinerascens) occupancy was uninfluenced by noise, but individuals in areas with greater noise amplitudes vocalized at a higher frequency, although the increase (≈200 kHz) may only marginally improve communication and may represent a secondary effect from increased vocal amplitude. Even so, the different flycatcher behavioural responses suggest that signal change may help some species persist in noisy areas and prompt important questions regarding which species will cope with an increasingly noisy world.

Figure 1.

Spectrograms (left) and power spectra (right) of (a) ash-throated and (b) grey flycatcher songs and (c) of background noise on a treatment site at 200 m from the compressor. Darker shades in spectrograms indicate more acoustic energy located at those frequencies, which is reflected by higher amplitude values in the power spectra. See figure S2 in the electronic supplementary material for an example of background noise at 50 m from the compressor.

Figure 2.

The occupancy rate estimate for grey flycatchers declined significantly with increased noise amplitude at the point count locations (solid black line). Ash-throated flycatcher occupancy was not significantly affected by noise amplitude (bold grey long-dashed line). Small-dashed lines denote 95% confidence intervals for occupancy estimates with respect to noise amplitude. Points are located at the mean noise amplitude on each site-type and represent the proportion of point count locations where grey flycatchers (black square) and ash-throated flycatchers (grey circle) were detected on treatment sites (filled symbols) and control sites (open symbols).

Figure 3.

Relationship between ash-throated flycatcher vocal frequency (kHz) and background noise amplitude (dB(A)) measured at the location of the individual. Peak frequency of the lowest note for songs (black squares and long-dashed line) and calls (grey circles and solid grey line), plus call minimum frequency (open triangles and short-dashed line), all increased with background noise amplitudes.