Blue tits show resilience to conifer admixture in European beech forests: Comparing Douglas-fir and Silver fir effects on breeding performance

Climate-driven forest conversion in Central Europe is accelerating shifts in forest management toward mixed stands, often combining European beech with drought-tolerant conifers such as Douglas-fir and silver fir. However, consequences for woodland birds, especially insectivores such as the blue tit ( Cyanistes caeruleus ), remain poorly understood. In particular, it is unclear whether conifer admixture alters habitat quality for cavity-nesting species that depend on deciduous trees for foraging and reproduction. We assessed the impact of conifer admixture on blue tit breeding performance by monitoring standardized nest box occupancy and reproductive success in 63 forest plots located in northern Bavaria, Germany. We compared pure beech stands (control plots) with Douglas-fir–beech, and silver fir–beech mixture stands. At the tree-level, blue tits laid eggs earlier in nest boxes mounted on beech compared to those on coniferous trees, revealing specific effects of tree species on breeding phenology. No significant effects on brood mass or hatching success were detected, suggesting that nest tree identity does not translate into detectable differences in provisioning outcomes. At the stand-level, occupancy, clutch size, brood mass, and hatching success showed no significant differences among forest types. These results highlight the ecological flexibility of blue tits in mixed-stand environments and suggest that mixed beech-conifer stands do not negatively affect their breeding performance when a deciduous matrix persists. Our findings support the compatibility of moderate conifer enrichment with biodiversity conservation in beech-dominated landscapes.