Many authors have suggested that the negative effects of roads on animals are largely owing to traffic noise. Although suggestive, most past studies of the effects of road noise on wildlife were conducted in the presence of the other confounding effects of roads, such as visual disturbance, collisions and chemical pollution among others. We present, to our knowledge, the first study to experimentally apply traffic noise to a roadless area at a landscape scale-thus avoiding the other confounding aspects of roads present in past studies. We replicated the sound of a roadway at intervals-alternating 4 days of noise on with 4 days off-during the autumn migratory period using a 0.5 km array of speakers within an established stopover site in southern Idaho. We conducted daily bird surveys along our 'Phantom Road' and in a nearby control site. We document over a one-quarter decline in bird abundance and almost complete avoidance by some species between noise-on and noise-off periods along the phantom road and no such effects at control sites-suggesting that traffic noise is a major driver of effects of roads on populations of animals.
Decades of research demonstrate that roads impact wildlife and suggest traffic noise as a primary cause of population declines near roads. We created a "phantom road" using an array of speakers to apply traffic noise to a roadless landscape, directly testing the effect of noise alone on an entire songbird community during autumn migration. Thirty-one percent of the bird community avoided the phantom road. For individuals that stayed despite the noise, overall body condition decreased by a full SD and some species showed a change in ability to gain body condition when exposed to traffic noise during migratory stopover. We conducted complementary laboratory experiments that implicate foraging-vigilance behavior as one mechanism driving this pattern. Our results suggest that noise degrades habitat that is otherwise suitable, and that the presence of a species does not indicate the absence of an impact.traffic noise pollution | songbird migration | habitat degradation | foraging-vigilance trade-off | perceived predation risk H uman infrastructure shapes animal behaviors, distributions, and communities (1, 2). A meta-analysis of 49 datasets from across the globe found that bird populations decline within 1km of human infrastructure, including roads (2). Observational studies of birds near roads implicate traffic noise as a primary driver of these declines (3). Road ecology research has also shown negative correlations between traffic noise levels and songbird reproduction (4, 5). Birds that produce low frequency songs, likely masked by traffic noise, show the strongest avoidance of roads (6).There is now substantial evidence that anthropogenic noise has detrimental impacts on a variety of species (3, 7-10). For example, work in natural gas extraction fields has demonstrated that compressor station noise alters songbird breeding distribution and species richness (11-13). However, explicit experiments would help to further rule out other characteristics of infrastructure, such as visual disturbance, collisions, chemical pollution, and edge effects, which might be driving these patterns (3). In addition, although these studies implicate noise as a causal factor in population declines, many individuals remain despite noise exposure (3), but at what cost? Proposed causes of decreased fitness for birds in noise include song masking, interference with mate evaluation, nonrandom distribution of territorial individuals, disruption of parentchick communication, reduced foraging opportunities, and/or alterations in the foraging/vigilance trade-off (3, 4).Here we parse the independent role of traffic noise from other aspects of roads experimentally by playing traffic sounds in a roadless area, creating a 'phantom road'. We focus on birds during migratory stopover, because energy budgets are streamlined; foraging, vigilance, and rest dominate activity (14). To meet the amplified physiological needs of sustained nocturnal migratory flights, birds must increase foraging during periods of stopover while maintaining appropriate vigil...
Several past studies have demonstrated the effects of anthropogenic noise on populations of animals. Yet, differing effects of noise by age and subsequent changes in the age structure of populations are poorly understood. We experimentally tested the effects of traffic noise alone on the age structure of a community of migrating birds at a fall stopover site in south-western Idaho using an array of speakerscreating a phantom roadthat replicated the sound of a roadway without other confounding aspects of roads. Both hatch-year and adult birds were negatively affected by noisehaving lower capture rates, lower body condition and lower stopover efficiency along the phantom road when the noise was on compared to control conditions. However, hatch-year birds responded more strongly which lead to a significant shift in the ratio of hatch-year to adult birds under noisy conditions. Our previous work using the phantom road demonstrated that traffic noise can degrade the quality of a stopover site by affecting the ability of migrating birds to gain body condition. Here, we demonstrate differences between age classes such that although noise degrades habitat for both hatch-year and adult migrants, there are still differences in responses to noise between age groups. Despite alternative explanations of our results such as changes in behavior affecting capture likelihood, evidence suggests that younger birds avoided the phantom road more than adult birds perhaps because of different tradeoffs between foraging and predation risk and differing strategies of site selection during migration.
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