Fire--fragmentation interactions govern cavity-nesting bird diversity in tropical forests

Wildfires are increasing in frequency and intensity in the Atlantic Forest, reshaping forest structure and the availability of critical nesting resources for cavity-nesting birds. We investigated how fire regimes and landscape fragmentation jointly influence the species richness and abundance of two functional guilds: primary cavity excavators and non-excavators. Specifically, we tested the intermediate disturbance hypothesis (IDH) and whether excavators facilitate non-excavators by creating new cavities. We surveyed cavity-nesting birds in 15 forest landscapes within the southeastern Atlantic Forest of Brazil and combined these data with detailed fire history and landscape configuration metrics. Using generalized linear models and Bayesian multivariate structural equation models, we evaluated direct and indirect pathways linking fire regime, landscape fragmentation, and cavity-nesting guild responses. Contrary to the IDH, no unimodal relationships emerged for abundance, while excavator richness showed a U-shaped response to fire extent, indicating that distinct species assemblages benefit at low and high disturbance levels. Excavator abundance increased linearly with fire frequency and was amplified in more fragmented landscapes, suggesting that repeated fires and edge effects promote cavity formation and persistence. Non-excavator abundance and richness showed weak and statistically uncertain responses to both fire and fragmentation, and the indirect facilitation pathway via excavators was unsupported, indicating that abiotic cavity formation (e.g., decay, breakage) likely plays a dominant role. Our results highlight that the direct effects of fire frequency and landscape configuration are stronger than biotic facilitation for shaping cavity-nesting bird communities. Conservation strategies should integrate fire management and landscape planning to maintain large, cavity-bearing trees and promote habitat heterogeneity, ensuring nesting opportunities for both functional guilds in fire-prone tropical forests.