Gonzalez, Sarahand Sean Mahoney

Title:
Testing the Adaptive Significance of Song in Willow Flycatcher Subspecies

Abstract:
In birds, song is an important characteristic that allows birds to recognize members of their own species. How differences in song characteristics arise and how they are maintained remain important questions in evolutionary ecology. Song may be shaped by the need to acquire mates, or by factors that affect the survival of the individual (natural selection). The Acoustic Adaptation Hypothesis (AAH) predicts that animals should optimize their signal given their environmental context. Under this hypothesis, higher frequency songs with shorter internote intervals (i.e. trills) should transmit more efficiently in open and more humid habitats because of reduced heat loss and overlapping sound waves. Individuals in a population that can optimize song transmission in their environment should have higher reproductive success. Willow Flycatchers (Empidonax traillii) occur across a broad ecological gradient and four subspecies are currently recognized. We tested the AAH in Willow Flycatchers by relating song characteristics from 134 recordings including frequency and internote intervals to canopy cover and climate variables from the PRISM climate database. We found general support for the AAH: Although vegetation density was non-linearly related to song frequency and trills, we found a strong linear relationship between song characteristics and climate variables. Higher frequency songs with more trills were found in mesic areas and lower frequency songs with fewer trills were found in xeric areas. Our findings suggest that the Willow Flycatcher’s song is shaped by the climatic conditions at the site which may indicate that populations of Willow flycatchers occupying different habitats are diverging evolutionarily.

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