Silvereyes decrease acoustic frequency but increase efficacy of alarm calls in urban noise

• We sampled silvereye alarm calls from urban and rural habitats across Australia.
• Urban alarm calls had lower-frequency profiles than rural calls.
• These findings contrast with current literature on urban bird vocalizations.
• We developed spectrographic noise profiles for urban and rural habitats.
• The observed frequency shifts increased the active space of the calls by 20%.

Many passerines adjust song attributes to avoid potential masking by anthropogenic noise. The costs of masking should be particularly high for vocalizations important for survival (e.g. alarm calls), but few studies have investigated how such calls are affected. We compared urban and rural silvereye, Zosterops lateralis, alarm calls across southeastern Australia, and found that urban calls had lower average, peak and maximum frequencies than rural calls. The average, peak and maximum frequency of alarm calls also decreased linearly with increasing background noise. The direction of this frequency shift runs contrary to expectations and previous findings of higher-pitched avian vocal signals in urban habitats, including higher-pitched song and contact calls in urban silvereyes. However, assuming no change in call amplitude, acoustic modelling indicates that the observed frequency shift would lead to a 20% increase in the predicted active space of alarm calls (i.e. the distance over which the calls can be detected by a conspecific bird) in urban noise, and therefore may be potentially adaptive. Our findings highlight the importance of considering behavioural and ecological contexts in urban acoustic-adaptation studies.