Tree cavity density is a limiting factor for a secondary cavity nester in second-growth Andean temperate rainforests

Cavity-nesting bird populations are most frequently limited by the number of tree cavities available in second-growth forests. However, this possible limitation of a key resource is less clear in old-growth forests. We compared forest attributes (i.e., basal area, density of larger trees, density of dead trees, and tree cavity density) in second-growth and old-growth stands in Andean temperate rainforests in southern Chile. To examine the role of nest-site availability in limiting the populations of Aphrastura spinicauda (Thorn-Tailed Rayadito), we monitored their populations in both forest types during a 5-year period (2008-2013), while we conducted an experiment in which nest boxes were added and then, after two years, removed by blocking cavity entrances. In old-growth forests, as compared to second-growth forests, we found a more than double basal area (99.6 vs. 43.7 m2 ha−1), a 3 times higher density of larger trees (88.2 vs. 36.4 trees ha−1), and a 1.5 times higher number of small cavities (25.9 vs. 10.3 cavities ha−1). The density of cavities also strongly increased with tree diameter and basal area. In second-growth forests, A. spinicauda showed a strong response to the addition, and later to the removal of nest boxes, with population abundance increasing by 13% and then decreasing by 50%, respectively. In contrast, we found no impact on old-growth stands. Our experiment emphasizes the importance of maintaining large and dead trees in second-growth, disturbed, and managed forests. These trees provide suitable cavities for A. spinicauda, and likely many other secondary cavity nesters, increasing their abundance in a Globally significant Biodiversity Hotspot in southern South America. A Spanish translation of this manuscript is available as Supplementary Material.