Increased vulnerability of alert responses to combined call sequences under anthropogenic noise in bird communication

Noise pollution disrupts animal communication, often via acoustic masking. Beyond masking alone, complex vocal sequences that combine distinct call types may impose additional processing challenges in noisy environments, for both conspecific and heterospecific receivers. We assessed how traffic noise differentially affects behavioral responses to Japanese tits' simple (recruitment) versus complex (combined alert-recruitment) calls in conspecifics and sympatric heterospecific tits using playback experiments over three breeding seasons. Noise reduced the probability of approach more strongly for combined calls than for single calls in both Japanese tits and heterospecific tits, suggesting greater disruption of responses to complex call structures. Noise also increased latency to approach, with marginally stronger effects on heterospecific than conspecific receivers to combined calls. By contrast, noise exposure increased minimum approach distances to Japanese tits' calls consistently across call types and receiver species. Overall, these results suggest that noise generally disrupts approach behaviors, with disproportionately strong effects on responses to complex combined calls and a tendency toward greater disruption in heterospecific receivers, although not consistently across all behavioral responses. Our study highlights that species relying on complex acoustic signals to coordinate behaviors may face uneven survival costs in increasingly noisy environments.