Experimental river noise alters bird and bat abundance, activity, and foraging behavior

Dylan G. E. Gomes, Cory Toth, Hunter Cole and Jesse R. Barber, Department of Biological Sciences, Boise State University, Boise, ID, Dylan G. E. Gomes, Cooperative Institute for Marine Resources Studies - Hatfield Marine Science Center, Oregon State University, Newport, OR, Clinton D. Francis, Biological Sciences, California Polytechnic State University, San Luis Obispo, CA

Background/Question/Methods
Natural sensory environments, despite strong potential for structuring systems, have been neglected in ecological theory. Here, we test the hypothesis that intense natural acoustic environments shape animal distributions and behavior by broadcasting whitewater river noise in montane riparian zones for two summers. Additionally, we use spectrally-altered river noise to explicitly test the effects of masking as a mechanism driving patterns. We then survey abundance and activity across 60 locations, over two full breeding seasons. Additionally, we deploy 720 clay caterpillars to assess bird foraging and deploy foraging tests across 144 nights, consisting of robotic insect-wing mimics, and speakers broadcasting bat prey sounds to assess bat foraging. We analyze all these data with generalized linear mixed-effects models to parse out the role that noise characteristics have on the abundance, activity, and foraging behavior of these taxa.
Results/Conclusions
We find that both birds and bats avoid areas with high sound levels, while birds avoid frequencies that overlap with birdsong, and bats avoid higher frequencies more generally. However, overlap with birdsong appears to only drive birds away at relatively lower sound pressure levels, suggesting that masking of birdsong is less important in driving patterns in high intensity acoustic environments. We find that intense sound levels decrease foraging behavior in birds. For bats, we find that bats appear to switch hunting strategies from passive listening to aerial hawking as sound levels increase. Natural acoustic environments are an underappreciated niche axis, a conclusion that serves to escalate the urgency of mitigating human-created noise.