Forest structure and composition drive functional variation in bird assemblages of managed temperate forests

Forest management practices shape bird communities by altering habitat structure and composition, yet their effects on functional diversity remain poorly understood. We investigated how forest composition and structure resulting from continuous-cover retention forestry influence the functional diversity of bird assemblages in temperate forests. Using data collected from 135 one-hectare plots in the southern Black Forest, Germany, we assessed three components of birds’ functional diversity, Rao’s Q entropy, functional evenness, and functional divergence, and analyzed how forest structural and composition features underpin responses of specific morphological traits. Our results showed contrasting effects: tree and shrub diversity increased functional diversity up to a maximum, after which competitive processes may lead to functional redundancy. In contrast, structural features such as standing deadwood volume and stand complexity act as environmental filters, reducing trait diversity by favoring species with particular requirements (e.g., bark foragers). Forest composition, especially the proportion of conifers, was the main factor driving morphological variation, impacting beak depth, body mass, and wing shape more than structural complexity. Diet-related traits responded weakly, indicating high functional redundancy in foraging strategies. Overall, our findings suggest that the current bird communities in the Black Forest, as in many temperate forests of central Europe, are functionally redundant and resilient to habitat modification, with tree diversity being the key driver of functional diversity. These results highlight the importance of maintaining compositional heterogeneity in forest management to support birds’ functional resilience to change, while noting that structural retention mainly benefits specific species rather than the entire bird assemblage.