Acoustic and Morphological Variation in Birds Along a Tropical Elevation Gradient

The Acoustic Adaptation Hypothesis (AAH) predicts that birds adjust their songs to maximize sound transmission under different environmental conditions. However, variation in song spectral properties may also arise indirectly through morphological adaptations to environmental gradients, a mechanism described by the By‐product Hypothesis (BPH). Montane forests, where ecological conditions change rapidly with elevation, provide an ideal setting to test predictions derived from both hypotheses. Here, we investigate the direct (AAH) and indirect (BPH) effects of elevation on the song properties of three bird species: Atlapetes rufinucha , Myiothlypis luteoviridis , and Silvicultrix frontalis , in the tropical Andes of Bolivia. We first experimentally quantified how sound transmissibility varies across elevations. We then compared song characteristics (maximum, minimum, and peak frequency, and bandwidth) and morphometric traits (bill morphology and tarsus length) of individuals captured along an elevational gradient. Sound reverberation was related to signal frequency but not to environmental variables. Song spectral properties varied with elevation, with maximum and peak frequencies and bandwidth showing negative correlations with temperature and positive relationships with habitat complexity. Additionally, all three species exhibited shorter and flatter bills at higher elevations, a morphological shift that may influence sound production. Our results indicate that elevation exerts both direct and indirect effects on avian song spectral properties in montane environments. These findings support the combined roles of local acoustic adaptation and elevation‐associated morphological variation in shaping bird song in mountain ecosystems.