Foraging strategies determine the effect of traffic noise on bats

Anthropogenic noise is a concern in many ecological systems. One important source of noise pollution is traffic noise as it can dominate the soundscape in urban and peri-urban environments. Taxa that rely on acoustics for behavioural strategies are likely to be especially susceptible to noise, as noise can inhibit the perception of informational sounds. Bats use echolocation to hunt prey while foraging and are therefore prime candidates for adverse effects. Captive studies have shown that foraging efficiency can be significantly reduced in noisy environments for some bat species, and that these species actively avoid noisy areas. However, it remains unclear how this selective sensitivity manifests in urban environments. Given that mode of flying and use of echolocation is entwined with foraging strategies, we hypothesised that different foraging guilds (i.e. fast flyers versus slow flyers) may show different levels of sensitivity to noisy roads. We used transects running perpendicular to a major traffic route in Sydney, Australia, to record bat activity and traffic noise levels. Noise amplitude levels across each frequency band dropped by over 50% in the first 50 m, with high frequency components ( > 10 kHz) being especially soft at this distance. Furthermore, all traffic noise above 5 kHz was lost within the first 150 m from the road. Fast flying bats flew close to the road, despite the traffic noise. In contrast, slow flying species appeared to fly more often away from the road. However, few calls of slow flyers were recorded, probably reflecting their difficulty in detecting them using acoustic surveys as well as their earlier decline in these peri-urban environments.