Can animals detect differences in vocalizations adjusted for anthropogenic noise?

Most animals use vocalizations to attract mates and defend territories. Many species alter these signals in the presence of anthropogenic noise, such as rush-hour traffic. Yet, little is known about how intended receivers (territorial rivals, potential mates) respond to these altered signals. Here we investigated responses of free-living male northern cardinals, Cardinalis cardinalis, to computer-generated songs that mimicked (1) the population's average minimum-frequency song ('average frequency' song) and (2) songs that had been shifted to have a higher minimum frequency ('shifted-frequency' song), as occurs in the presence of loud anthropogenic noise. Males gave stronger responses to songs of average frequency than to songs with a shifted frequency. At low levels of background noise, differences in responses to two song types were greatest, but as the amplitude of anthropogenic noise increased, differences in responses to the two song types diminished, and at the highest amplitudes, males had almost equal responses to the two song types. Since the shifted-frequency songs received weaker responses than the average-frequency songs, the shifted-frequency songs do not seem advantageous in terms of communication efficacy, especially at low levels of background noise. Songs with a higher minimum frequency are not necessarily beneficial for signal efficiency and might not be adaptive despite potential benefits of masking avoidance, which could have important consequences for mate selection and resource defence among populations in urban areas.