DOI: 10.5061/dryad.95x69p8tb
title: Tree cavity density is a limiting factor for a secondary cavity nester in second-growth Andean temperate rainforests

description: 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.
description: <strong>Study area</strong> We
 studied a small bodied secondary cavity-nesting bird species in Andean
 temperate rainforests of southern Chile (Thorn-Tailed Rayadito, 11.74 g;
 Altamirano et al. 2020), in the La Araucanía Region (39º16’ S, 71º48’ W)
 (Fig. 1). The South American temperate rainforests are one out of only
 seven of these temperate rainforest ecosystems in the world (Alaback,
 1991). These forests cover more than 40,000 km<sup>2</sup>
 along the south-western edge of the continent, mainly in Chile and a small
 adjacent area in Argentina (CONAF et al., 1999; Donoso, 1993), stretching
 in latitude south between 35° and 55° (Armesto et al., 1998). They are
 characterised by cool summers, with precipitation occurring throughout the
 year. Furthermore, they have been classified as among the world’s 200
 biologically most valuable and critically endangered ecoregions (Olson and
 Dinerstein, 1998) and are considered a Global Biodiversity Hotspot (Myers
 et al., 2000). In Andean areas, native forest has been reduced by 44% over
 the last two decades (Altamirano and Lara, 2010), and in coastal areas,
 67% of native forests have been replaced by other types of land cover
 since 1975 (Echeverria et al., 2006). We quantified
 forest attributes, cavity density, and the abundance of a secondary cavity
 nester (SCN) in 10 forest stands with different degrees of disturbance in
 the Andean Cordillera (Fig. 1). The stands ranged from recently burned
 areas with very low and occasional selective logging, representing
 second-growth forests (&lt; 80 years), to non-burned old-growth
 forests (&gt; 200 years old; Caviedes and Ibarra, 2017). Six of the 10
 stands corresponded to second-growth forests, mainly with a predominance
 of broadleaf species such as <em>Nothofagus obliqua</em>,
 <em>Nothofagus dombeyi</em> and <em>Laurelia
 sempervirens</em>. The other four corresponded to old-growth stands
 of conifer-broadleaf mixed forest, principally with <em>Saxegothaea
 conspicua</em>, <em>Laureliopsis philippiana</em> and
 <em>N. dombeyi</em>. Forest stands were considered as
 old-growth when they had a core area with a minimal edge effect and
 maintained a complex vertical structure and the mostly unmodified species
 composition (Armesto et al., 2009). In both second-growth and old-growth
 forest stands, understory composition was dominated by bamboo species
 (<em>Chusquea spp.</em>), <em>Rhaphithamnus
 spinosus</em>, and a variety of species of
 <em>Azara</em> and tree saplings. As the home range of the
 focal bird species is unknown, we assumed a range of 3-4 ha for the
 Thorn-Tailed Rayadito, given the observed home-range for its close
 relative, <em>Aphrastura masafuerae </em>(Hahn et al., 2010).
 Our study stands were separated by a minimum linear distance of 1.6 km,
 3.3 to 3.8 times the median natal dispersal reported by Botero-Delgadillo
 et al. (2017; 490 m females and 420 m males) to ensure that populations in
 one stand would not affect birds inhabiting adjacent study stands (Wiebe,
 2011). <strong>Forest attributes and cavity
 density </strong> To quantify forest structural
 attributes and cavity density in the stands, we established five
 vegetation plots per stand. These plots had their centre at each point
 where the abundance of rayaditos was surveyed (details below), with a
 survey area of 0.04 ha (radius = 11.2 m). Within these vegetation plots,
 we quantified the following stand attributes: density of live trees,
 density of dead standing trees, and for all live and dead trees with a
 tree diameter at breast height (DBH) ≥ 12.5 cm, we measured DBH and the
 number of small cavities per tree. We used tree DBH to calculate the basal
 area of trees with DBH ≥ 35 cm and the density of larger trees (DBH ≥ 60
 cm). Tree cavities, including round or square entrances, crevices and
 branch holes, among others, were considered if they had a minimum entrance
 diameter of 2.5 cm, and a maximum diameter of 5 cm (Cornelius, 2008;
 Altamirano et. al. 2017; Ibarra et. al. 2020). We counted cavities with a
 minimum depth of 10 cm from the lower lip of the entrance (Altamirano et
 al. unpublished data). Cavity heights between 0.2 m and 30 m on the trees
 were considered, consistent with reports from previous studies (Altamirano
 et al., 2012; Cornelius, 2008; McGehee et al., 2010).
 <strong>Resource addition and
 removal</strong> We increased the density of
 suitable cavities by installing 40 wooden nest-boxes in each of six
 forests (160 boxes in four second-growth forests and 80 in two old-growth
 forests; n = 240) during the 2010 non-breeding season. The remaining four
 forest stands (two second- and two old-growth forests) were used as
 control stands where nest-boxes were not installed. Nest-boxes were hung
 from branches at a height of 1.5 m. The great majority of nest boxes were
 installed in the forest interior. When there was an open area nearby,
 nest-boxes were placed at least 15 m in from the forest edge. The
 direction in which the entrance to the nest-boxes faced was random. The
 nest-box entrance diameter and depth of the box were 3.1 cm and 17.1 cm,
 respectively (Altamirano et al., 2013; 2015). Nest-boxes of this type
 (i.e., entrance diameter and internal dimensions) were used to improve the
 probability of their occupancy (Lambrechts et al., 2010) as they had
 previously been successfully used by Thorn-Tailed Rayaditos (Moreno et
 al., 2005; Vergara, 2007). In the winter of 2012, we blocked all
 nest-boxes, reducing cavity density back to its original level in both
 second-growth and old-growth forests.
 <strong>Secondary cavity-nesting bird
 abundance</strong> We used an experimental design
 that considered monitoring densities of Thorn-Tailed Rayaditos before and
 after the cavity addition and removal (Aitken and Martin, 2012; Robles et
 al., 2012). We monitored the Thorn-Tailed Rayaditos populations in
 experimental and control stands over a five-year period: two years of
 pre-treatment (2008-2010); two years of treatment (nest-box installation,
 2010-2012), one year of post-treatment (nest-box blocked, 2013).
 Five-point count stations per forest stand were established, with a
 diameter of 50 m and a minimum distance of 125 m between stations. All
 counts were conducted between October and February, during the four hours
 after sunrise (06.30-10.30) and had a duration of seven minutes during
 which we recorded all the bird species that were detected (heard and/or
 seen) within the radius. Point count stations were selected instead of
 transects as they are more efficient in forest conditions (Ralph et al.,
 1996). <strong>Data
 analysis</strong> We compared the following
 forest structural attributes of second-growth and old-growth forests:
 basal area of trees with DBH ≥ 35 cm (m<sup>2</sup>/ha;
 hereafter, basal area), the number of trees with DBH ≥ 60 cm per ha
 (hereafter, density of larger trees), the number of dead trees per ha
 (hereafter, density of dead trees), and the number of small-sized cavities
 per ha (hereafter, cavity density). We tested statistical differences
 using the <em>t-</em>test for pairs comparison, testing first
 for normality (Shapiro Wilk test; p &gt; 0.05) and homoscedasticity
 (Levene’s test; p &gt; 0.05). A critical tree diameter was
 established: i) DBH of 35 cm for the basal area because 80% of the nests
 of rayaditos were in trees with a higher DBH, and ii) DBH of 60 cm to
 calculate the density of larger trees, given that this is the mean
 diameter of the trees used by this species (Altamirano et al. 2017). We
 used two independent linear regressions to assess how the density of
 natural cavities was related to two different scales: tree DBH and basal
 area. For the number of cavities per tree, we used log10(number of
 cavities +1) transformation to achieve the normality assumption. We
 assessed cavity limitation for the Thorn-Tailed Rayadito, analysing its
 responses to experimental cavity addition and removal. Although we did not
 account for detectability, we think this might not have affected our main
 conclusions, as we compared abundances using consistent methods across
 forest stands and seasons. To determine the response to experimental
 increasing and reduction of cavity densities, we used Linear Mixed-Effect
 Models with a Poisson distribution, and the Akaike’s Information Criterion
 (AIC) approach to select the best fit models. Forest type, nest-box
 treatment (pre-treatment, during treatment, and post-treatment) and their
 interaction were included as fixed effects. Season and forest stand were
 included as random effects, allowing us to i) control for any inherent
 capacity of each season to have lower or higher abundances of rayaditos
 (Altamirano et al. 2020) and ii) include repeated measurements in the same
 forest stands (pre-treatment, during treatment, and post-treatment).
 Models with DAIC &lt; 2.0 were considered the best-supported models
 (Burnham & Anderson 2002).
description: # Tree cavity density is a limiting factor for a secondary cavity nester
 in second-growth Andean temperate rainforests   ## Altamirano etal 2024
 OrnApp BirdData Unit number: forest stands id Forest type: OG: Old-growth
 Forest SG: Second-growth Forest STN ID: Point count number Season:
 breeding season NB Treatment 0: before 1: during 2: after APHSPI_TOT:
 Number of individuals of Thorn-tailed Rayaditos ## Altamirano etal 2024
 OrnApp VegData Forest Type: Old- or second-growth forest Dens DBH &gt; 60:
 Number of trees greater than 60 cm DBH per hectare Dens dead trees: Number
 of standing dead trees per hectare Peq_dens: cavity density per hectare Ʃ
 Basal area dens: meters^2 per hectare DBH (cm): Diameter at breast height
 LOG10 (PEQ + 1): Log10 of the Number of small cavities of each tree + 1.