Birdsong tuned to the environment: green hylia song varies with elevation, tree cover, and noise

Animals that communicate acoustically must compete for acoustic space in order to convey their signals effectively. Tropical rainforest birds live in an extremely diverse acoustic community consisting of other birds, mammals, frogs, and many insects. Insects are notable for often producing continuous bands of sound energy at constant frequencies, which vary between species and across habitats. We examined how green hylia (Hylia prasina) song frequencies correlate to insect-generated spectral profiles of ambient noise. We also examined how the environment influenced song frequency by using remote sensing to quantify environmental variables. Using path analysis, we assessed the relative effects of elevation, tree cover, precipitation, and insect sounds on green hylia song frequency. Environmental variables were found to directly influence green hylia song frequencies. Specifically, green hylia sang at lower frequencies at higher elevations and under reduced canopy cover. The environment also influenced green hylia song indirectly through its effect on insect sounds. Green hylia sang at lower frequencies presumably to avoid masking by lower frequency insect sounds. Habitat-dependent divergence in songs within species potentially plays an important role in ecological speciation through its impact on species recognition and mate choice. Our data show that factors related to climate, vegetation, and vocal community can promote such habitat-dependent song variation.