Habitat fragmentation and forestry practices affect forest structure and composition, and hence, their intrinsic value for biodiversity conservation. While higher tree species diversity is commonly proposed to result in habitat of higher quality for forest species, how these tree diversity and tree composition effects interact with forest fragmentation in terms of critical resources and demographic effects on forest birds remains poorly understood. We investigated here possible synergistic effects of forest fragmentation and tree species composition on the breeding performance of two common, insectivorous forest birds in a human-dominated landscape in northern Belgium. We monitored the breeding performance of Great Tits and Blue Tits in 53 plots across independent gradients of tree species composition and forest fragmentation. In addition, data on the biomass of the main food source of these two species (i.e. caterpillars) was collected during the breeding season. Both tree composition and habitat fragmentation impacted the breeding performance of Great and Blue Tits. Effects of tree species composition were mainly driven by tree species identity, rather than by tree species diversity, and the highest breeding performance was obtained in monocultures of Pedunculate Oaks. Fragmentation effects were only observed in resource-poor Beech monocultures with breeding performance declining with reduction in forest area. Structural Equation Modelling revealed diverse and species-specific pathways: for Great Tits tree composition effects on breeding performance were driven by resource availability while for Blue Tits these effects were driven by variation in clutch size. Thus, forestry practices aiming at promoting forest-dependent birds could benefit from including tree species that support high arthropod numbers and by maintaining forest patches of larger sizes.
