Data from: Innovative microphone transmitter reveals differences in acoustic structure between broadcast and whisper songs of Myadestes obscurus (ʻŌmaʻo)

Low-amplitude ‘whisper songs’ are a taxonomically broad phenomenon in
birds that could play an important role in the suite of behaviors birds
use to communicate. Due to its cryptic nature, there are inherent
difficulties in capturing high-quality whisper song recordings without
interrupting natural behaviors. Thus, whisper song acoustic structure is
poorly understood and its potential function remains the subject of
debate. Here, we present one of the first quantitative assessments of the
acoustic structure of whisper song in birds. Using an innovative
microphone transmitter we collected high quality recordings of broadcast
and whisper songs from the Myadestes obscurus (ʻŌmaʻo), a thrush species
endemic to the Island of Hawai‘i. The transmitter was attached to the
birds and broadcasted radio-signals of all vocalizations produced by the
individual to distances over 100 m away which minimized disruption of the
birds’ normal behavior while recording. We demonstrate that ʻŌmaʻo whisper
songs are a distinct class of vocalization that differ from broadcast
songs in acoustic characteristics beyond amplitude, such as song length,
frequency, and length of silent intervals between notes. These findings,
in conjunction with habitat-associated variation in the rate at which
ʻŌmaʻo emit these vocalization classes, indicate broadcast and whisper
songs likely serve separate functions. This work provides evidence
supporting the ‘acoustic adaptation hypothesis’ which posits that densely
vegetated habitats promote the evolution of songs with specific acoustic
features that maintain signal integrity as the sound propagates through
the environment.