Not so sexy in the city: urban birds adjust songs to noise but compromise vocal performance

Anthropogenic noise affects the behavior of a variety of animal taxa around the world. In many taxa, minimum frequencies of acoustic signals increase with ambient noise levels, and males and females respond less strongly to these adjusted signals. Lower response may be due to higher minimum frequencies or to the associated decrease in frequency bandwidth. There is a performance challenge to producing notes rapidly at a wide bandwidth, and any reduction in bandwidth necessarily reduces the performance value. This measure of vocal performance is a trait under sexual selection in many taxa. We investigated the relationship between anthropogenic noise amplitude and male white-crowned sparrow vocal performance in San Francisco, CA. Males on louder territories produced songs at higher minimum frequencies but also with reduced bandwidth and lower vocal performance. These results suggest that behavioral adjustments to anthropogenic noise reduce vocal performance of songs. We conducted playback experiments to test if lower responses to urban songs are due to higher minimum frequencies or to narrower bandwidths. Males responded more strongly to normal than to reduced bandwidth songs, but equally to high- and low-pass filtered songs. Our results therefore suggest that reduced bandwidth and performance—not higher minimum frequency—could be the reason that urban birds are less responsive to songs adjusted for anthropogenic noise. Ultimately, the lower vocal performance of birds on territories with high levels of anthropogenic noise could result in fewer mating opportunities and more challenges in defending their territory.