Do the male blackcaps (Sylvia atricapilla) increase the temporal features of their song in response to traffic noise?

Abstract

The rapid rise in transportation lines and the corresponding increasing levels of noise have detrimental impacts on wildlife, and especially on species using acoustic signals to communicate. Many studies have shown that traffic-generated noise alters the occupancy patterns and the reproductive success of birds. To counteract the masking effect of their song by noise, several species are able to adjust the characteristics of their songs. Spectral and amplitude adaptations have already been widely documented in this context, mostly in urban environments. However, additional factors can also act in synergy with noise in cities and their combined effects are complex to disentangle. Here, our first objective was to investigate if blackcaps (Sylvia atricapilla) are able to increase the temporal features of their vocalizations, such as signal duration or signaling rate, in response to traffic noise. Research has shown that these modifications would enhance the probability to transfer the acoustic signal successfully. We recorded singing males in May both in roadside forests and in narrow roadside strips of vegetation (i.e. hedges), as well as in corresponding reference sites. Vocal activity is also related to male competition, which increases with population density. Our second objective was therefore to evaluate if the forest (2D structure) males have a higher song output than the hedge (1D structure) males. We found that the males living in forests and in hedges along the highway displayed the highest and the lowest song output, respectively, among all habitat types. This unexpected dual response in noisy locations could be related to a difference in vegetation structure. While the forest birds had the possibility to partly escape from the noise exposure by extending their territory deeper into the forest, the birds living in narrow hedges were constrained to constantly endure the elevated noise levels at the road verge. Over a certain threshold, keeping on increasing vocal activity to reduce song masking would come at a disadvantage for the birds in hedges along the road. Indeed, we suggest that it could result in trade-offs with vigilance or feeding time or with other vocal adjustments (frequency, amplitude). We therefore believe that these behaviors are more beneficial in terms of fitness than singing intensively. Concerning the forest birds along the road, we suggest that their high song output is likely the combined result of the noise pressure and the intra-sexual pressure, which is in line with our predictions. Taken together, these findings indicate that the availability of suitable habitat will shape the response of blackcaps to noise. Finally, it indicates that even the species that display acoustic adaptations to artificial noise may be confronted to an increased challenge for survival