Duplex auditory distance assessment in a small passerine bird and the relationship with sound attenuation and vocalization production

Abstract

Eastern towhees, Pipilo erythrophthalmus (Emberizidae, Passeriformes, Aves) estimate how loudly conspecific vocalizations are produced using correlated spectral and temporal variables and often misjudge speaker distance in the field when vocalization source sound-pressure level (SPL) is varied experimentally. I demonstrate that towhees often misjudge speaker distance when played SPL altered stimuli produced with sound frequencies below 3.5 kHz but rarely misjudge speaker distance when played SPL altered stimuli produced with sound frequencies above 3.5 kHz. I describe the homogeneous physical-acoustic structure of towhee Florida scrub habitat and demonstrate that sound frequencies below 3.5 kHz attenuate more reliably over distance in comparison with sound frequencies above ∼3.5 kHz (i.e., sound frequencies below 3.5 kHz propagate with less variation in attenuation across sites sampled, several speaker and microphone elevations and between days). Lastly, I describe acoustical measurements obtained from calls produced by 52 caged subjects in the field, which suggest that sound frequencies to each side of 3.5 kHz are produced independently or are otherwise emphasized differently during production. These experiments suggest that auditory distance perception in the towhee might be described as ‘‘duplex’’ and that variation in sound propagation might often mediate a close relationship between vocalization production and auditory perception.