Avian Vocal Production in Noise

Abstract

Birds use acoustic signals to mediate a number of crucial social interactions such as territorial defence, mate attraction and predator avoidance. Thus, differences in signalling efficiency are likely to have major fitness consequences. Acoustic signal transmission is considerably constrained by noise, e.g. sounds in the environment that interfere with the detection, discrimination or recognition of a signal. In this chapter, we discuss noise sources encountered by birds, and the diverse ways birds use to make their signals heard in this noisy world. One concept of signal evolution suggests that bird vocalisations undergo microevolutionary adaptations over time that tailor their sounds to the specific noise profiles of their species-typical habitats. On the individual level, birds across many different taxa also possess the vocal plasticity to make short-term adjustments to their signals to reduce masking in response to changing environmental noise conditions. Such adjustments can take different forms in different species. However, the widespread problem of acoustic communication in noise has also led to the evolution of one shared solution in birds: the Lombard effect, i.e. a noise-dependent regulation of vocal amplitude. In addition, birds may also change the frequency, the duration, the timing, and/or the redundancy of their vocal signals in noise, although in many cases it is not yet clear whether these additional changes are achieved through ontogenetic plasticity or through short-term regulation. In recent years, there has been a flurry of new studies reporting correlations between increased levels of anthropogenic noise and a variety of changes in the vocal behaviour of birds. While many of these studies have focused on increases in song or call frequency in birds exposed to high levels of traffic noise, it is not yet known whether these differences in vocal pitch are actually adaptive. We encourage future research studies to take a more rigorous and integrative approach to the study of vocal signalling in noise. Finally, we note the need for more research on the impact of noise on the evolution and usage of multi-component signals that combine vocal and visual signals.