Effects of experimental nest-box addition on cavity-dependent birds in holm oak stands of two Mediterranean national parks (central Spain)

Tree cavities are critical resources for many forest organisms, yet their availability is limited in human-managed landscapes. In Iberian holm oak dehesas, centuries of tree management, deadwood removal, and low oak regeneration have reduced cavity suitability, potentially constraining populations of cavity-nesting birds, particularly secondary cavity-nesters. We experimentally tested whether cavity availability limits bird assemblages using a before–after–control–impact (BACI) design across 22 holm oak stands in Cabañeros and Monfragüe National Parks in Spain. Artificial nest-boxes were added to half of the stands, and acoustic detection indices of cavity-nesting bird assemblages were quantified before and after installation. Nest-box supplementation significantly increased acoustic detection indices of secondary cavity-nesters, but did not affect species richness, which remained dominated by Blue and Great Tits. Importantly, activity indices were not related to nest-box occupancy, indicating that they reflect broader patterns of space use rather than local breeding density. In contrast, excavator species (woodpeckers) remained rare and showed no response to nest-box addition, likely because the scarcity of suitable heartwood and dead trees limits excavation substrate availability. The effect of adding nest-boxes was consistent across stands, regardless of baseline cavity density, indicating pervasive nest-site limitation. Our results indicate that cavity availability is an important constraint in the occurrence of some secondary cavity-nesting birds—particularly tit species—in holm oak woodlands. They also suggest that the classical facilitative “cavity web” framework, in which excavators indirectly support secondary cavity-nesters by creating nesting resources, may be of limited importance in these systems, where woodpecker excavation activity is scarce. Artificial nest-box deployment can provide a short-term increase in nesting opportunities for common species in Iberian holm oak woodlands, but it should be viewed as a complement rather than a substitute for conserving veteran trees, deadwood, and natural cavity-forming processes.