Vocal variation in Chiroxiphia boliviana (Aves; Pipridae) along an Andean elevational gradient

Bird vocalizations are likely shaped both by natural and sexual selection. Here we test the sensory-drive hypothesis, which states that communication signals diverge as a direct adaptation to the signaling environment and can evolve to minimize degradation and maximize transmission. We examined the effects of elevation and other habitat variables on variation in vocalizations of Chiroxiphia boliviana (Aves, Pipridae) along an elevational gradient (1300–2500 m) in cloud and humid montane forests in the Andes of Bolivia. We also conducted sound transmission experiments to determine if reverberation and attenuation changed along the gradient. Reverberation increased at higher elevations, and attenuation decreased at higher elevations and increased for higher frequencies. We recorded vocalizations from ~?50 individuals throughout the elevational gradient and examined variation in duration and bandwidth of short calls (used as contact calls between males), 2 display calls (advertisement for females) and 2 types of male–male duets (including interval times between males). Duration of short calls, display 1 and duet 1 increased with elevation. Bandwidth of short calls increased at mid-elevation categories and decreased at high elevations, whereas bandwidth of display 1 and duet 1 decreased with elevation. We also directly related the transmission properties to vocalizations and found that bandwidth of short calls decreased with reverberation and attenuation, bandwidth of display 2 decreased with reverberation, and duration of duet 1 both increased and decreased with attenuation (at 3 and 4 kHz, respectively). This study suggests that vocalizations by C. boliviana may be adapted to the habitat transmission properties along the elevational gradient; and perhaps that increasing song length and concentrating energy within a narrow bandwidth may lead to an increase in amplitude and improvement in transmission. Overall, our results support the sensory-drive hypothesis and suggest that this form of selection is likely common along tropical elevational gradients.