Savanna Nightjars (Caprimulgus affinis stictomus) adjust calling height to gain amplitude advantage in urban environments

Abstract

Anthropogenic noise and buildings are two significant challenges to birds in urban living environments. The Savanna Nightjar Caprimulgus affinis stictomus is a successful urban nocturnal caprimulgiform in Taiwan. It has been found to prefer calling in areas with more buildings to calling in less-developed areas. However, research has yet to be conducted to investigate the relationship between calling height and its possible advantages with regard to acoustic adaptation for the species. We performed field observation and playback experiments to test three hypotheses: the obstacle reduction advantage, the masking reduction advantage, and the amplitude increase advantage. Our results of field observation showed that the mean calling heights were all lower than the tallest building heights within the territories; thus, the obstacle reduction advantage hypothesis was not supported. We also found that calling heights were significantly affected by the building structures but not the ground noise levels; therefore, the masking reduction advantage hypothesis was rejected. To further verify the amplitude increase advantage, we conducted playback experiments at open and walled sites. We found that amplitude measurements were significantly higher for calls broadcast at the walled site than those at the open site. Our findings suggest that Savanna Nightjars adjust their calling heights to reach optimal building wall surroundings to make their calls reverberate and thus increase their call amplitudes, and the benefits of increasing call amplitude may be one of the vital selective forces for Savanna Nightjars to call preferentially in urban areas with buildings.