The Acoustic Adaptation Hypothesis posits that habitat characteristics influence the structure of animal vocalizations and that animals will vocalize and display behaviours optimized for sound transmission. White-eared ground-sparrows Melozone leucotis live in habitats with dense vegetation where vocal communication is an ideal mode of communication for territory defence and mate attraction. On the basis of the Acoustic Adaptation Hypothesis, if solos and duets of these ground-sparrows are used in long-distance communication, we should expect that these vocalizations will exhibit structures that enhance sound transmission. We conducted a sound transmission experiment where we broadcast and re-recorded solo songs and duets to study their transmission properties. We used two speaker heights and two microphone heights to simulate different perch heights of signallers and receivers and four distances between the speakers and microphones to simulate variable distances of separation. We found that solo and duet songs show similar patterns of degradation and attenuation with distance and proximity to the ground. These results suggest that solo and duet songs facilitate communication with receivers at similar distances. The highest perches, for both signallers and receivers, maximized acoustic transmission. This is the first study that evaluates the transmission properties of songs and duets in birds, despite the fact that many bird species in tropical forests produce both types of vocalizations. To our surprise, we found that solo and duet songs degraded to below-detectable levels in less than a typical territory's diameter, suggesting that this species has not experienced strong selection for long-distance communication.
acoustic adaption, long-distance communication, Melozone leucotis, sound transmission, thicket habitats, white-eared ground-sparrow