Effects of forest fragmentation on feather corticosterone levels in an Amazonian avian community, Balbina, Amazonas, Brasil

In the Amazon, the construction of hydroelectric dams is an emergent driver of biodiversity loss, creating a large number of land-bridge islands, most of them unable to sustain an assemblage of bird species comparable to the intact forest. Although we understand the effects of forest fragmentation on species richness and distribution, we still need to uncover the physiological mechanisms underlying the success of organisms living in disturbed habitats. In this study, we used feather corticosterone levels as a measurement of physiological stress, evaluating whether corticosterone levels mirrored the effects of habitat fragmentation on species occurrence. Since data suggests that smaller islands can reduce habitat suitability, increasing stress in birds that live on them, we predicted that birds living in smaller islands would present increased feather corticosterone levels. We captured birds in 13 islands of varying size and two continuous forests and estimated feather corticosterone levels from 265 individuals from eight different species. Overall, our findings did not support the hypothesis that corticosterone varies in relation to island size, with the exception of Hypocnemis cantator, which presented the expected pattern: decreasing feather corticosterone levels with increasing island size. These differences suggest that species respond differently to stressors driven by fragmentation. Further studies are necessary to assess the reliability of corticosterone levels as a measurement of physiological stress and to determine which parameters are useful to understand how insularization caused by human activities may influence the resilience of avian populations to habitat disturbances.