Tree cavities as attractors: an experimental test of the effects of woodpecker legacy on local bird activity and species richness

Tree cavities, whether excavated by birds or formed through wood decay, are key structural features of forest ecosystems. They are critical resources for secondary cavity users and influence breeding bird assemblages by affecting the occurrence and abundance of certain species. However, the effect of an individual cavity on fine-scale bird activity, independently of the surrounding habitat context, remains poorly understood. Here, we experimentally tested the effect of cavity presence on bird activity in a coniferous forest in southern Poland. We installed pairs of standardized, debarked snags, each consisting of one snag with an artificial entrance resembling a woodpecker-excavated cavity and one control snag without a cavity entrance. Bird presence and activity were monitored using camera traps and vegetation structure was quantified around each snag. The presence of a cavity entrance had a positive effect on visitation rates and species richness. Snags with cavity entrances received 114% more bird visits than control snags (4.5 [95% CI: 3.1-6.5] vs 2.1 [1.4-3.2]) and were visited by 48% more species (2.0 [1.5-2.6] vs 1.3 [0.9-1.9]). In addition, greater canopy closure around snags was associated with higher visitation rates and species richness. Our study demonstrates that a single cavity can act as a fine-scale attractor for forest birds. These findings highlight the role of cavities and cavity-bearing trees in shaping spatially heterogeneous patterns of fine-scale habitat use by birds.