Abstract
Anthropogenic noise is an often-overlooked byproduct of urbanization and affects the soundscape in which birds communicate. Previous studies assessing the impact of traffic noise have focused on bird song, with many studies demonstrating the ability of birds to raise song frequency in the presence of low-frequency traffic noise to avoid masking. Less is known about the impact of traffic noise on avian alarm calls, which is surprising given the degree to which predator information within alarm calls may impact fitness. The objective of this study was to assess the impacts of traffic noise on the Black-capped Chickadees (Poecile atricapillus), a small non-migratory songbird with a well-studied and information-rich alarm call. We studied birds at eight locations in Stark County, Ohio, from 15 January to 7 March 2016, and used a taxidermic mount of an Eastern Screech-Owl to elicit alarm calls. In half of the trials, a pre-recorded traffic noise track was also broadcasted at 50 decibels. In noise trials, chickadee calls contained more introductory notes (P < 0.001), more total notes (P < 0.001), were of longer duration (P < 0.001), and had lower introductory and D-note peak frequencies (P = 0.032 and P = 0.041, respectively). No differences were noted in the number of D-notes per call between noise and control trials. Modifying alarm call duration and frequency, without changing the number of D-notes, may be a strategy that chickadees use to convey predator information and to coordinate a threat-appropriate mobbing response when it is not possible to change call type. Our results add to the small, but growing, literature documenting the effects of anthropogenic noise on avian alarm calls, demonstrate the flexibility and complexity of chickadee calls given in response to predators, and may partially explain why chickadees adapt well to urban areas.
Highlights
Anthropogenic noise is an often overlooked byproduct of urbanization [1, 2], and nearly onefifth of the land area of the United States is exposed to traffic noise [3,4,5]
Our results (Table 1) demonstrate a high degree of flexibility in the alarm calling of Blackcapped Chickadees whereby call duration and acoustical structure are altered in the presence of traffic noise
Given the acoustically heterogeneous traffic noise that we presented in noise trials with intermittent high frequency and broadband sounds, lowering the peak frequencies of introductory and D-notes (Table 1) may have helped chickadees better transmit predator information
Summary
Anthropogenic noise is an often overlooked byproduct of urbanization [1, 2], and nearly onefifth of the land area of the United States is exposed to traffic noise [3,4,5]. Traffic noise usually occupies lower frequency ranges [8, 9], and overlap between human and animal noise can lower the threshold at which birds are able to detect and discriminate between vocalizations (i.e., ‘masking’) [2, 3, 10] and appropriately respond [7, 11, 12] To combat this issue, birds can: shift vocalization frequencies (i.e., ‘the acoustic adaptation hypothesis’); reviewed by Roca et al [13], increase the amplitude of vocalizations (i.e., ‘the Lombard Effect’) [7, 14], shift calling to times of the day when anthropogenic noise is less prevalent [15], repeat portions of their vocalizations [16], and alter the duration of vocalizations [17, 18]
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