The adaptation and fitness costs to urban noise in the calls of the tree sparrow (Passer montanus).

Acoustic communication is one of the primary ways birds transmit and acquire important information. Birds living in urban environments are limited in the transmission and recognition of vocalizations due to intense low-frequency anthropogenic noise, but the Acoustic Adaptation Hypothesis (AAH) suggests that birds mitigate the masking effects of noise by modifying the structure and properties of their vocalizations. Although these adaptations can increase the efficiency of communication, they also impose potential fitness costs on the sender and receiver, which can negatively impact survival. The contact and alarm calls of tree sparrows have a frequency range of 1-10 kHz, which makes them likely to be partially masked by urban noise, limiting communication between individuals and interactions within populations. Thus, based on the AAH, we examined the modifications of contact and alarm calls in response to urban noise and confirmed the potential fitness costs of these modifications in tree sparrows. We recorded two contact calls and an alarm call at 80 sites with varying urban noise levels in Gwangju, Naju, and Damyang from March to April 2023. Both types of contact calls tended to have higher peak frequencies and shorter durations as urban noise increased. Alarm calls tended to increase in the number of notes and duration with increasing urban noise. These results indicate that tree sparrows respond to noisy environments through modifications of the spectral and temporal characteristics of their vocalizations. However, these adaptations impose potential fitness costs on both the sender and the receiver.