Passive acoustic monitoring and acoustic indices reveal noise-related changes in bird vocalisations

Noise may affect acoustic communication in birds, depending on the signal-to-noise ratio and the spectral and temporal features of their signals. This study evaluated potential vocal adjustments in five bird species (Thamnophilus doliatus, Cyclarhis gujanensis, Eucometis penicillate, Formicarius analis and Leptotila verreauxi) to natural and anthropogenic noise using three acoustic indices (normalised difference soundscape index, acoustic complexity index and bioacoustics index). Data were collected through passive acoustic monitoring at 25 forest sites near the Bacalar Lake, Yucatan Peninsula, Mexico. We focused on each species' most frequent vocalisation, measuring five acoustic parameters (peak frequency, 5% and 95% frequencies (Hz), duration (sec) and number of vocalisations). All species, but T. doliatus, exhibited changes in at least one acoustic parameter. C. gujanensis vocalised at a higher peak frequency with greater natural noise. E. penicillata had lower frequency 5% values with greater anthropogenic noise. F. analis vocalisations had longer durations and higher frequency 95% values with greater anthropogenic and natural noise. L. verreauxi performed a greater number of vocalisations with greater anthropogenic noise. Our study shows that passive acoustic monitoring and indices can reveal noise-related changes in bird vocalisations, indicating that birds may adjust their calls to reduce noise masking.