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
Thorn-Tailed Rayaditos (Aphrastura spinicauda), a secondary cavity-nesting
bird species, we conducted an experiment in which nest-boxes were added
and removed in old-growth and second-growth forests during a five-year
period (2008-2013). 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), a
three times higher density of larger trees (88.2 vs. 36.4 trees/ha), and a
1.5 times higher number of small cavities (25.9 vs. 10.3 cavities/ha). The
density of cavities also strongly increased with tree diameter and basal
area. In second-growth forests, Thorn-Tailed Rayaditos showed a strong
response to the addition and removal of nest-boxes, with population
abundance increasing by 13% and then decreasing by 50%, respectively. In
contrast, we found no impact in 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 Thorn-Tailed Rayaditos, and likely many other
secondary cavity nesters, increasing their abundances in a Globally
significant Biodiversity Hotspot in southern South America.