Megafire attributes and pre-fire structural legacies shape short-term avian responses in an Atlantic-Mediterranean ecotone

Background Fire regimes are rapidly shifting due to climate change and increasing vegetation flammability, with these dynamics often intensified in areas undergoing widespread rural abandonment, a trend particularly evident in mountainous landscapes of sub-Mediterranean Europe. We assessed avian community responses, including post-fire beta diversity, during the first breeding season following a megafire—the largest recorded in the region—within a depopulated mountain landscape in northwestern Iberia, located in a poorly studied transitional biogeographic zone. We employed a stratified sampling design across major habitat types to survey bird communities and quantify fire attributes and vegetation structure, combining field-based measurements with satellite-derived spectral indices. Results We recorded 2,928 individuals representing 56 bird species, classified into multiple functional guilds. Fire severity was the main negative driver of community structure and composition, significantly impacting most functional groups. In contrast, spatial heterogeneity in fire severity fostered a broader range of ecological niches, enhancing the coexistence of diverse guilds and buffering the immediate effects of high fire severity. Postfire vegetation structure was a key determinant of community reassembly: snag-rich stands, unburned forest patches, and early post-fire open habitats facilitated both avian persistence and recolonization. These components also provided critical resources for highly specialized guilds, including cavity-nesting and open-habitat species. Bird community composition differed significantly but weakly between burned and unburned areas, and fire heterogeneity had a strong positive effect on post-fire beta diversity only when interacting with post-fire habitat structure. Conclusions Our findings demonstrate that fire attributes alone cannot account for short-term avian responses; rather, their interaction with pre-fire structural legacies is critical to understanding community reassembly. The conservation of snag-rich stands, early-successional open habitats, and unburned forest refugia—alongside the maintenance of fine-scale heterogeneity—should be prioritized to support post-fire bird community recovery in abandoned sub-Mediterranean mountain landscapes.