Which bird species respond most to forest structural variation? Implications for biodiversity indicators in Mediterranean forests

The Mediterranean Basin is a biodiversity hotspot facing increasing degradation due to climate change and unsustainable forest management. Although recent land-use changes have led to forest expansion, these forests are often structurally simple, limiting biodiversity and increasing wildfire risk. Enhancing forest structural complexity has become a key management goal, yet practical indicators for assessing biodiversity across large, forested regions remain limited. This study investigates how forest structural features affect bird abundance across a Mediterranean landscape in northeastern Iberia, aiming to identify species that can serve as indicators of Mediterranean forests quality. We modeled the abundance of 43 forest bird species as a function of remotely sensed forest structural attributes, composition, and regional environmental variables. Many generalist species showed low sensitivity to forest structure or yielded poorly fitting models, leaving only 16 species with models showing good fit to the data. For these species, Canonical Correspondence Analysis (CCA) and coefficient comparisons revealed that forest composition and elevation were the main drivers of bird abundance, while structural attributes were especially relevant for canopy and cavity specialists. Our findings highlight the hierarchical nature of species–habitat relationships, where broad environmental gradients shape species distributions, and local structural attributes modulate abundance. These results have direct implications for refining biodiversity indicators and designing regionally tailored forest management strategies. By identifying species most responsive to structural variation, this study contributes to more targeted and ecologically informed approaches to forest restoration in Mediterranean landscapes. • Forest structure significantly influences bird abundance at the regional scale. • Most common forest birds may be poor indicators of structural forest change. • Mature forest bird indicators may also reflect high tree compositional diversity. • A multi-management approach should be considered in Mediterranean forests.