Fragment area and habitat quality underlie temporal decline and community changes in rainforest birds within long-isolated fragments

Abstract Forest fragmentation and associated forest loss reduce bird species diversity in the short term, but long-term consequences of fragmentation on birds are less commonly documented. To address this, we examine changes in avian communities over 19 years across 19 long-isolated (80+ yr) rainforest fragments that differ in habitat quality and area (0.7–4,134 ha) in India’s Southern Western Ghats. We test whether the density, species richness, and compositional similarity of rainforest, range-restricted, and open-country (matrix-associated) species were (1) stable over time, (2) increased with fragment area, and (3) increased with habitat quality, either singly or in combination. In individual-predictor models, between 2000 and 2019, the density and abundance-corrected (rarefied) richness of rainforest and range-restricted species (together, “forest birds”) declined 19–30% and 16–17%, respectively, despite a 15% increase in the overall (Chao) richness of rainforest species over the same period. Avian community structure was positively associated with fragment area and rainforest habitat quality among forest species, while open-country species showed a negative association. Communities of forest, but not open-country species, were more stable in larger fragments. Combined models indicated that temporal declines in bird density and species richness were mediated by changing habitat quality and buffered by fragment area, and not associated with time per se. Results from our study of old fragments add to growing evidence of avian declines worldwide. However, our findings along with threshold indicator taxa analyses indicate that habitat quality and fragment area above fairly low thresholds (18–55 ha) can support stable communities of forest species and mitigate the ingress of open-country species.