Avian Responses to Cloud Line Along an Elevational Gradient of a Tropical Island

Abstract

Cloud lines, defined as persistent bands of stratus clouds along elevational gradients, create abrupt shifts in temperature, humidity, and visibility that can profoundly influence biodiversity. We investigated the effects of cloud lines on avian species richness and community structure along a one-kilometer elevational gradient (480–1420 m) located in the sub-montane zone of Sri Lanka—a tropical continental island in the northern Indian Ocean. Standardized avian surveys were conducted over 1 year using visual, acoustic, mist-net, and camera-trap methods across ten permanently marked transects situated along contour lines spanning the gradient. The elevation of the cloud line was recorded using visibility charts, and environmental covariates were measured in each transect. Species richness declined significantly with increasing distance from the cloud line (Pearson r = −0.22, p = 0.05). In negative binomial models controlling for elevation, light intensity, and habitat complexity, the cloud line emerged as a significant predictor of avian richness (ΔAIC > 18). Among the 96 species recorded—including 23 endemics—those with broader elevational ranges exhibited a stronger association with the cloud line (p < 0.001). The distribution of birds was non-random with respect to cloud line elevation (Chi-square: p < 0.001), particularly at lower elevations. These findings underscore critical conservation concerns, as climate-driven shifts in cloud line position may disproportionately affect biodiversity confined to cloud-associated habitats such as tropical cloud forests.