Testing the acoustic adaptation hypothesis with native and introduced birds in Hawaiian forests

The acoustic adaptation hypothesis (AAH) states that animals communicating acoustically adapt their vocalizations to the local conditions to optimize signal transmission. We tested select predictions of the AAH by studying the relationships between avian acoustics and forest structural parameters for a community of forest birds, including native and introduced species, on the Big Island of Hawai’i, USA. In areas of dense vegetation, where sound degrades more easily, we expect animal species to reduce the frequency at which they vocalize to reduce sound distortion. Because introduced species may have had limited time to adapt to the local habitat, we also hypothesize that their vocalizations will not change with differences in vegetation. Automated sound recorders were used to obtain information on the birds’ acoustic traits. Vegetation structural characteristics were calculated using a terrestrial light detection and ranging (LiDAR) sensor, which provides highly detailed information on the structure of the vegetation, including woody and leaf density. Of the seven native species studied, only two followed the predictions of the AAH. Interestingly, these two species had the shortest vocalizations, i.e., these vocalizations have the highest chance of information loss. Likewise, for the two introduced species, we did not observe any significant correlation with LiDAR-based vegetation structure metrics. Our study indicates that the predictions of AAH only partially account for the observed acoustic patterns observed in the study system. Other factors affecting acoustic divergence may be more important than the vegetation structure for most of the studied forest birds.