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, the 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 that minimized disruption of the birds’ normal behavior while recording. We demonstrate that M. obscurus whisper songs are a distinct class of vocalization that differs 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 M. obscurus emit these vocalization classes, indicate broadcast and whisper songs likely serve separate functions. This work provides evidence supporting the acoustic adaptation hypothesis that 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.