Why does noise reduce response to alarm calls? Experimental assessment of masking, distraction and greater vigilance in wild birds

Abstract

Environmental noise from anthropogenic and other sources affects many aspects of animal ecology and behaviour, including acoustic communication. Acoustic masking is often assumed in field studies to be the cause of compromised communication in noise, but other mechanisms could have similar effects. We tested experimentally how background noise disrupted the response to conspecific alarm calls in wild superb fairy‐wrens, Malurus cyaneus, assessing the effects of acoustic masking, distraction and changes in vigilance. We first examined the birds' response to alarm‐call playbacks accompanied by different amplitudes of background noise that overlapped the calls in acoustic frequency. We then scored and videoed their response to alarm calls in two types of background noise, that did or did not overlap call frequency, but were broadcast at a constant amplitude. Birds were less likely to flee to alarm calls in higher amplitudes of overlapping noise, demonstrating that noise itself compromised communication independently of environmental correlates. Background noise affected the response only if it overlapped in frequency with the alarm calls, implying that the effect was not due to distraction. Further, birds were equally vigilant during background noise of overlapping or non‐overlapping frequency, indicating that the lack of response to alarm calls in overlapping noise was not due to enhanced vigilance and awareness that there was no predator. We conclude that alarm‐call reception was compromised by masking, a mechanism that is often assumed but rarely tested in an ecological context. Masking compromised reception of high‐frequency “aerial” alarm calls and so could reduce survival in background noise of similar frequency. While anthropogenic noise, which is often of lower frequency, is unlikely to affect communication with these calls, it could affect reception of acoustic cues of danger, or other conspecific or heterospecific alarm calls. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13333/suppinfo is available for this article.

Traffic noise and responses to a simulated approaching avian predator in mixed‐species flocks of chickadees, titmice, and nuthatches

“…Carolina chickadees, tufted titmice, and white‐breasted nuthatches generally produced more D notes in their chick‐a‐dee calls (chickadees and titmice) or quank notes (nuthatches) as the amplitude of predator call stimuli increased. However, at sites with greater background traffic noise compared to sites with relatively little traffic noise, titmice, and nuthatches decreased calling behavior in response to owl call playbacks, supporting the masking hypothesis for traffic noise for these two species (see also masking of conspecifics' alarm calls in superb fairy‐wrens, Malurus cyaneus : Zhou, Radford, & Magrath, ). The fact that both titmice and nuthatches decreased calling in anti‐predator contexts as traffic noise levels increased suggests that increased traffic noise levels lessen the abilities for these birds to detect screech owls calling near them.…”

Black-capped chickadees (Poecile atricapillus) alter alarm call duration and peak frequency in response to traffic noise

“…To our knowledge, we are the first to use a taxidermic mount of a predator to elicit alarm calls, which helped ensure that calls were given in predator defense contexts, which strengthens and contextualizes our findings. We also utilized a heterogeneous noise track, which may have better simulated the traffic noise that birds would have encountered in an urban area [ 15 ], but at the same time offered a less controlled measure of tonal frequencies in noise trials [ 12 , 17 , 57 ]. Our study also assessed short-term response of wild birds to noise in otherwise quiet areas, a measure of behavioral plasticity, similar to [ 12 , 17 , 58 ], whereas studies that compare birds already adapted to rural and urban environments [ 22 , 24 , 34 ], or along urban-rural gradients [ 9 , 59 ], may be better positioned to explain evolutionary changes [ 8 , 60 ].…”

Individual differences show that only some bats can cope with noise-induced masking and distraction

“…Similarly, only a few studies to date have investigated the mechanisms of noise disturbance. Some have used bands of noise that are both overlapping or non-overlapping with signals or cues of interest to demonstrate evidence of masking (e.g., Gomes et al, 2016 ; Zhou, Radford & Magrath, 2019 ). Others have shown that noise can disrupt behaviors across sensory modalities (where masking cannot work) via distraction (e.g., Chan et al, 2010 ; Morris-Drake, Kern & Radford, 2016 ).…”