The human population is growing and with it so is the noise pollution that we produce. As urban areas expand, people may complain about the increasing sounds that accompany construction and increasing car traffic in their neighborhoods. However, it is important to consider how these sounds are impacting our non-human neighbors as well.
One bird that may be negatively affected by noise pollution are house wrens (Troglodytes aedon), common to urban natural areas in the USA. Male house wrens have complex songs used in order to communicate at short distances and over long distances. Anthropogenic noise can mask the songs produced by birds like the house wren, so that the signal the song is attempting to transmit cannot be accurately conveyed to its intended receiver like a mate or to ward off intruders of its territory.
House Wren (wiki)
The two components of the male house wren’s song may be influenced differently by urban noise since they are structurally distinct from each other. The introduction section of the song is high in frequency which is good for short-distance communication within pairs of birds as it degrades quickly in the natural environment. The terminal section of the song is low in frequency as it travels over greater distances for functions like mate attraction and defense outside its own territory boundary. In a study by Erin Grabarczyk and Sharon Gill they tested whether ambient noise influences the degradation and attenuation of the song sections of male house wrens at varied levels of anthropogenic noise. Overall, they investigated how well the structure of each song section is maintained within territories and beyond territories in different noise environments.
Grabarczyk and Gill hypothesized that ambient noise affects transmission of the whole song and the sections of the song of the male house wren. They predicted that the effects of noise pollution on the transmission of the two sections would differ in their decay with distance, because the song components have different frequency structures.
The study recorded the songs of male house wrens with both introduction and terminal components and then played them over speakers at natural sites surrounded by varied levels of land use from urban to rural. They then re-recorded the songs at varying distances to measure transmission at distances corresponding to positions within the male’s territory (short-distance communication), territory edge, and at multiple distances beyond the territory (long-distance communication).
The results of the experiment do align with Grabarczyk and Gill’s hypothesis. The study found that at high ambient noise levels from surrounding urban areas, whole wren songs decayed much more quickly and overall show more degradation at each distance than at quiet rural sites. In fact, in the presence of high noise, both song sections for short and long distance communication fell below the hearing thresholds that other wrens could pick up the song signal within the boundary of a male’s territory. The song degradation patterns were different between the sections since the introduction section degraded more quickly than the terminal section, and this pattern was even more pronounced at the noisy sites. Overall, with increasing distance eventually all songs degraded but the decay of each section started at closer distances to the source at the noisy sites.
Song to noise ratio (SNR) of introduction and terminal sections over increasing distance from the source. Sites 1 and 2 are rural (quiet) areas and Sites 3 and 4 are urban (noisy) areas. Noise accelerates patterns of degradation of the song sections with distance. Song sections fell more quickly below the detection threshold (dotted horizontal line) at the noisier sites. Introduction sections degraded faster than the terminal sections, and noise significantly increased the amount of degradation of both sections.
The pronounced degradation patterns of male house wren songs under high noise levels affects their communication abilities. Since both song sections could not transmit beyond the boundaries of a male’s territory with urban noise levels, long-distance communication for the birds is not possible from within their territory. The rapid decay of both short and long distance song sections could potentially lead to a decrease in territory size or to male’s spending more time away from their duties of guarding their nest and mate to defend and broadcast their song along territory edges instead. It is also possible that as urban noise levels become more prevalent, selection will favor lower frequency songs of birds due to the inability for introductory sections to be heard at short-distances.
The results of this study are important for understanding the impact that human noise pollution is having on native species in areas where development is increasing. The ability for house wrens to communicate over a range of distance is crucial to the survival of their populations. It is evident that noisy environments disrupt the transmission of house wren songs, so it is important for humans to be aware of how their expansion near natural areas may be destroying the livelihood of their non-human neighbors.
LITERATURE CITED
Grabarczyk, E.E. and Gill, S.A., 2019. Anthropogenic noise masking diminishes house wren (Troglodytes aedon) song transmission in urban natural areas. Bioacoustics, DOI: 10.1080/09524622.2019.1621209.
