Alerting and message components of white-crowned sparrow song differ in structure and environmental transmission

Abstract

Signals that function over long distances, such as bird songs, must be detectable and discriminable from other signals by receivers despite being attenuated and degraded during environmental transmission. The acoustic design features that enhance detectability may conflict with those that enhance discriminability of different messages (e.g., the sender’s motivation or identity). The songs of many bird species begin with simple tonal notes, hypothesized to alert receivers to the following song parts. We describe structural differences in the songs of the Puget Sound white-crowned sparrow (Zonotrichia leucophrys pugetensis) and performed a transmission experiment to test if the whistle transmits differently than other song parts. As expected for an alerting component, the whistle phrases across different song types were highly similar, suffered less degradation when transmitted, and were produced at higher amplitude than the other two phrase types. These results suggest that in white-crowned sparrows alerting and message-bearing song phrases transmit differently.