Many passerines regularly use flight calls to communicate during migration. However, flight-calling is inconsistent throughout the day, with a markedly lower degree of calling by free-flying individuals during the early evening hours and with peaks in calling typically around dawn. Based on these observations, we hypothesized that individual flight call response would vary by time of day, and the likelihood to respond would be lower at dusk than during the daytime. We tested daytime and dusk flight call responsiveness of captive American Redstarts (Setophaga ruticilla) and Magnolia Warblers (S. magnolia) during spring migration at Braddock Bay Bird Observatory (near Rochester, New York, USA). We played a conspecific flight call sound stimulus for individual birds placed inside a soundproof recording studio and recorded their acoustic responses. In our experiment, American Redstarts and Magnolia Warblers were significantly less likely to give flight call responses at dusk than the individuals tested during the daytime. The almost absent responsiveness to flight calls at dusk suggests that these species either minimally communicate with one another during take-off or do not use flight calls as a source of communication at this time. These results have important implications for detection and quantitative bioacoustics migration monitoring. They also demonstrate the need for additional studies in temporal flight-calling behavior.

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