Quiet in the nest: The nest environment attenuates song in a grassland songbird

The nest environment may limit the ability of nest-bound birds to hear sounds from the outside world. In vocal learning species, such as humans and songbirds, it is vital for young animals to hear the voices of conspecific animals early in life. In songbirds, nest structure varies considerably across species, and the resulting impact on sound transmission may have consequences for vocal learning in nestlings. In this study, we tested the hypothesis that the nest environment attenuates song of Savannah Sparrows (Passerculus sandwichensis), grassland songbirds that build nests on the ground under cover of dense vegetation. We recorded live Savannah Sparrows singing at variable distances from 21 nests. We recorded songs using one microphone inside the nest (the typical position of a nestling) and another placed 1 m directly above the nest (a typical position of an adult). We found a substantial reduction in signal-to-noise ratio, where songs recorded inside the nest were an average of 11 dB lower than songs recorded directly above the nest. We estimate that the attenuation imposed by the nest reduced the maximum acoustic environment from 117.7 m (for recordings above the nest), to 78.6 m (for recordings within the nest), which is analogous to listening from a position 39.1 m farther away from the singer. Previous research estimated that song transmits up to 100 m in this species, so any adult male within 100 m of a young bird was previously considered a potential vocal tutor. By reducing the nestling acoustic environment from 100 m to 78.6 m, the number of male tutors available to nestlings is reduced by an average of 27%. Given the growing evidence that song learning begins very early in life, future research on vocal development should account for how the structural properties of the natal environment affect the songs that nestlings hear.