Effect of noise and temporary road closures on avifauna vocalizations
Noise disturbance from humans affects the behavior and acoustic communication of a wide variety of terrestrial species and it has become a widespread problem for species that rely on acoustic signals for mate attraction, territory defense, and more. This study investigates if noise reduction (road closure) mitigates the effect of vehicle noise on the acoustic communication of Contopus virens, a suboscine passerine, and Cardinalis cardinalis, an oscine passerine, within the greater Washington DC region. Songs were passively recorded at sites where the traffic pattern of the nearest road was either relatively constant, or interrupted on a weekly basis during a 36 hour road closure. The song characteristics dominant frequency, minimum frequency, song length, and bandwidth were measured to understand how birds respond to variation in traffic noise levels. Song measurements were analyzed with mixed-effect general linear models, which incorporated traffic pattern, day type (weekend or weekday), measures of background noise amplitude (Leq), vegetation cover, temperature, and humidity. The song background noise included low-frequency traffic noise and high-frequency biotic noise.
Traffic patterns with routine, temporary relief from traffic noise affected the song behavior of both species. The results suggest that the motivation for song adjustment in both species is due to background noise levels at the time of their song, though the temperature and relative humidity also influence Contopus virens song adjustments. As background noise levels increased, Cardinalis cardinalis decreased song bandwidth and increased minimum frequency, while Contopus virens increased song duration and dominant frequency, all of which are vocal adjustments that increase the contrast of their song with the background noise. Both species modified their songs less during the road closure, suggesting the visual absence of traffic and reduced noise levels may trigger additional changes in song behavior. In addition, a noise threshold at which birds no longer adjusted their songs, presumably because it was too loud, was identified between 56dB and 58dB for frequency related song adjustments and 45dB for song length. This study provides a better understanding of the upper noise thresholds at which birds stop adjusting their songs in response to anthropogenic noise as well as the potential mitigating effect of noise reduction for acoustic communication among terrestrial vertebrates.