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Resource selection plasticity and behavioral dominance may influence the ability of a species to respond to changes in resource availability, particularly if dominant species have highly specialized resource requirements. We examined the response of several dominant and subordinate cavity-nesting species to a reduction in the availability of an essential resource (nesting cavities) using the novel experimental approach of blocking the entrances to high-quality cavities. We monitored nest abundance on seven treatment and 13 control sites (aspen groves in a grassland matrix) in British Columbia, Canada, for two years pretreatment (2000—2001), two years during treatment (2002—2003), and two years posttreatment (cavities reopened; 2004—2005). At the community level, nest abundance declined by 50% on treatment sites following cavity blocking and returned to pretreatment levels following cavity reopening. Nest abundance of European Starlings (Sturnus vulgaris), a dominant secondary cavity-nester (SCN), declined by 89% and failed to recover posttreatment. Conversely, nest abundance of Mountain Bluebirds (Sialia currucoides; a subordinate SCN) increased following cavity blocking and remained high following reopening. Tree Swallows (Tachycineta bicolor) were unaffected by cavity blocking. We suggest that starlings, while being the dominant SCN, may be limited by availability of suitable nest sites, whereas bluebirds may be limited by starling abundance. We propose that plasticity in nest site preferences of subordinate cavity-nesters may enable them to contend with natural variation in availability of critical resources, such as nest cavities and food, in addition to coping with interspecific competition. This is the first community-level, multiyear study involving manipulation of nest site availability via experimental cavity blocking.

Cavity-nesting birds and mammals exhibit species-specific nest-site selection for tree characteristics and cavity dimensions. Although trees and their cavities change as they age, with trees becoming softer and cavities becoming larger, it is not known how their value as nesting resources varies with age. In the context of wildlife and forest management, we investigated the relative value of generating a supply of fresh cavities, which are thought to be of high quality, versus protecting cavities as they age and expand in interior volume. For 21 years (1995-2016), we monitored the formation and occupancy of tree cavities used by >30 species of birds and mammals in interior British Columbia, Canada. Cavity occupancy by secondary users was highest 1 year post-excavation (53%), then declined to 40% after 2 years, remained at 33 ± 7% (SD) between 3 and 16 years of age, and increased to 50% use from 17–20 years post-excavation. Excavators that reused cavities (woodpeckers [Picidae], nuthatches [Sitta spp.]) strongly selected 1- and 2-year-old cavities, large-bodied non-excavators (ducks, raptors, squirrels) selected mid-aged cavities, and mountain bluebirds (Sialia currucoides) and tree swallows (Tachycineta bicolor) selected most strongly for the oldest cavities. Cavities created in living aspen trees (Populus spp.), especially those excavated by northern flickers (Colaptes auratus), maintained high occupancy by secondary users across cavity age, and provided the bulk of cavities used in this system. Altogether, these results show that a diverse excavator community is needed to generate a supply of fresh cavities in the ecosystem, and retention of the mid-aged and older cavities will help support larger species.

Tree cavities are a critical multi-annual resource that can limit populations and structure communities of cavity-nesting vertebrates. We examined the regional and local factors influencing lifetime productivity (number and richness of occupants) of individual tree cavities across two divergent forest ecosystems: temperate mixed forest in Canada and subtropical Atlantic Forest, Argentina. We predicted that (1) species would accumulate more rapidly within cavities in the species-rich system (Argentina: 76 species) than the poorer system (Canada: 31 species), (2) cavity characteristics associated with nest-site selection in short-term studies would predict lifetime cavity productivity, and (3) species would accumulate more rapidly across highly used cavities than across cavities used only once, and in Argentina than in Canada. We monitored and measured nesting cavities used by birds and mammals over 22 breeding seasons (1995–2016) in Canada and 12 breeding seasons (2006–2017) in Argentina. Cavities were used an average of 3.1 times by 1.7 species in Canada and 2.2 times by 1.4 species in Argentina. Species richness within cavities increased with number of nesting events at similar rates in Canada and Argentina, in both cases much slower than expected if within-cavity species assemblages were random, suggesting that lifetime richness of individual cavities is more strongly influenced by local ecological factors (nest site fidelity, nest niche) than by the regional species pool. The major determinant of lifetime cavity productivity was the cavity’s life span. We found only weak or inconsistent relationships with cavity characteristics selected by individuals in short-term nest-site selection studies. Turnover among (vs. within) cavities was the primary driver of diversity at the landscape scale. In Canada, as predicted, species accumulation was fastest when sampling across high-use cavities. In Argentina, the rates of species accumulation were similar across high-and low-use cavities, and fastest when both high-and low-use cavities were pooled. These findings imply that biodiversity of cavity nesters is maintained by a mix of long-lived (highly productive, legacy trees) and many high-turnover (single-use, fast decaying) tree cavities. Conservation of both long-lasting and single-use cavities should be incorporated into decisions about stand-level forest management, regional land use policies, and reserve networks.

Snags are important habitat features for many forest-dwelling species, so reductions in the number of snags can lead to the loss of biodiversity in forest ecosystems. Intentional snag creation is often used in managed forests to mitigate the long-term declines of naturally created snags, yet information regarding the use of snags by wildlife across long timescales (>20 yr) is lacking and prevents a complete understanding of how the value of created snags change through time. We used a long-term experiment to assess how harvest treatment (i.e., small-patch group selection, 2-story, and clearcut) and snag configuration (i.e., scattered and clustered) influenced nesting in and foraging on 25–27-year-old Douglas-fir (Pseudotsuga menziesii) snags by cavity-nesting birds. In addition, we compared our contemporary measures of bird use to estimates obtained from historical surveys conducted on the same group of snags to quantify how bird use changed over time. Despite observing created snags for >750 hours across 2 consecutive breeding seasons, we found limited evidence of nesting activity. Only 11% of created snags were used for breeding, with nesting attempts by 4 bird species (n = 36 nests); however, we detected 12 cavity-nesting species present on our study sites. Furthermore, nearly all nests (94%) belonged to the chestnut-backed chickadee (Poecile rufescens), a weak cavity-excavating species that requires well-decayed wood for creating nest cavities. Our surveys also recorded few observations of birds using created snags as foraging substrates, with only 1 foraging event recorded for every 20 hours of observation. We detected 82% fewer nests and recorded 7% fewer foraging observations during contemporary field work despite spending >7.5 times more effort observing created snags relative to historical surveys. We conclude that 25–27-year-old created Douglas-fir snags provided limited opportunities for nesting and foraging by most cavity-nesting birds, and that the period of greatest use by this group occurred within 5–15 years of creation.

Mountain pine beetle (Dendroctonus ponderosae) (MPB) outbreaks are increasingly prevalent in western North America, causing considerable ecological change in pine (Pinus spp.) forests with important implications for wildlife. We reviewed studies examining wildlife responses to MPB outbreaks and postoutbreak salvage logging to inform forest management and guide future research. Our review included 16 studies describing MPB outbreak relationships with 89 bird species and 6 studies describing relationships with 11 mammalian species, but no studies of reptiles or amphibians. We included studies that compared wildlife response metrics temporally (before versus after the outbreak) and spatially (across sites that varied in severity of outbreak) in relation to beetle outbreaks. Outbreaks ranged in size from 20,600 to ≥107 ha and studies occurred 1–30 years after the peak MPB outbreak, but most studies were conducted over the short-term (i.e., ≤6 years after the peak of MPB-induced tree mortality). Birds were the only taxa studied frequently; however, high variability existed among those studies to allow many inferences, although some patterns were evident. Avian studies concluded that cavity-nesting species responded more favorably to beetle-killed forests than species with open-cup nests, and species nesting in the shrub layer favored outbreak forests compared with ground and open-cup canopy nesters that generally showed mixed relationships. Bark-drilling species as a group clearly demonstrated a positive short-term association with MPB epidemics compared with that of other foraging assemblages. Cavity-nesting birds that do not consume bark beetles (i.e., secondary cavity-nesting species and nonbark-drilling woodpeckers) also exhibited some positive responses to MPB outbreaks, although not as pronounced or consistent as those of bark-drilling woodpeckers. Mammalian responses to MPB outbreaks were mixed. Studies consistently reported negative effects of MPB outbreaks on red squirrels (Tamiasciurus hudsonicus). However, there is evidence that red squirrels can persist after an outbreak under some conditions, e.g., when nonhost tree species are present. For small mammal species associated with forest understories, responses may be most pronounced during the postepidemic period (>6 years after the peak of beetle-induced tree mortality) when snags fall to produce coarse woody debris. Postoutbreak salvage logging studies (n = 6) reported results that lacked consensus. Postoutbreak salvage logging may have an impact on fewer wildlife species than postfire salvage logging, probably because only host-specific tree species are removed after beetle outbreaks.

Logging often reduces taxonomic diversity in forest communities, but little is known about how this biodiversity loss affects the resilience of ecosystem functions. We examined how partial logging and clearcutting of temperate forests influenced functional diversity of birds that nest in tree cavities. We used point-counts in a before-after-control-impact design to examine the effects of logging on the value, range, and density of functional traits in bird communities in Canada (21 species) and Chile (16 species). Clearcutting, but not partial logging, reduced diversity in both systems. The effect was much more pronounced in Chile, where logging operations removed critical nesting resources (large decaying trees), than in Canada, where decaying aspen Populus tremuloides were retained on site. In Chile, logging was accompanied by declines in species richness, functional richness (amount of functional niche occupied by species), community-weighted body mass (average mass, weighted by species densities), and functional divergence (degree of maximization of divergence in occupied functional niche). In Canada, clearcutting did not affect species richness but nevertheless reduced functional richness and community-weighted body mass. Although some cavity-nesting birds can persist under intensive logging operations, their ecosystem functions may be severely compromised unless future nest trees can be retained on logged sites.

Tree cavities are a vital multi-annual resource used by cavity-nesting birds and mammals for nesting and shelter. The abundance of this resource will be influenced by the rates at which cavities are created and destroyed. We applied the demographic concepts of survival and longevity to populations of tree holes to investigate rates of loss for cavities in three tree species, as well as how characteristics of nest trees, habitat type, and species of excavator affected the persistence of tree cavities in trembling aspen, Populus tremuloides (95% of cavities were in aspen trees), in interior British Columbia, Canada. By modeling survival of 1635 nesting cavities in aspen over a time span of 16 years, we found that the decay stage of the nest tree was the most important factor determining cavity longevity. Cavities in trees with advanced decay had a relatively short median longevity of 7 years (95% CI 6-9 years), whereas those in living trees had a median longevity of more than 15 years. We found that cavity longevity was greater in continuous forest than in aspen grove habitat. Interestingly, cavities formed by weak excavators survived as long as those created by Northern Flickers ( Colaptes auratus ), despite occurring in more decayed tree stems. Thus, weak excavators may be selecting for characteristics that make a tree persistent, such as a broken top. Our results indicate that retention of cavities in large, live aspen trees is necessary to conserve persistent cavities, and that cavity longevity will have a large effect on the structure and function of cavity-using vertebrate communities.

Background Bartonella is a genus of Gram-negative facultative intracellular Alphaproteobacteria of public health importance. Although they are known to mainly infect mammalian hosts with some blood-feeding arthropods having been confirmed as vectors, there is some evidence of Bartonella association with non-mammalian hosts including birds. Methods Here we used high-throughput sequencing of 16S rRNA and Sanger sequencing of the citrate synthase ( gltA ) genes to test for the presence of Bartonellaceae in the blood of three migratory cavity nesting bird species, purple martins ( Progne subis ), tree swallows ( Tachycineta bicolor ) and eastern bluebirds ( Sialia sialis ) and their most prevalent and abundant nest ectoparasites, Dermanyssus prognephilus (mite), Ceratophyllus idius (flea) and Protocalliphora sialia (bird blow fly larva). We constructed maximum likelihood phylogenetic trees to verify the placement of the resulting sequences in the Bartonellaceae . Results We found evidence of Bartonella in all three bird species and all three arthropod species tested. We report multiple instances of identical Bartonella sequences in both birds and parasites, leading to the likely hypothesis that these ectoparasites are potential vectors of Bartonella . Our phylogenetic analysis suggests that ‘avian Bartonella ’ may form its own sub-clade within the genus Bartonella . Conclusions To the best of our knowledge, we provide the first confirmation of overlapping Bartonella strains among bird hosts and various species of nest-associated ectoparasites from the same system, suggesting a possible Bartonella host–vector relationship between these arthropods and a non-mammalian host. Our study adds to the growing appreciation of the Bartonellaceae as a phylogenetically diverse group with a wide range of hosts.

Understanding vulnerability of endemic taxa to predation is clearly important for conservation management. In New Zealand, predation by introduced mammals such as rats and mustelids is widely recognized as the primary factor responsible for declines of indigenous fauna. The aim of our study was to evaluate the vulnerability of New Zealand’s surviving endemic forest bird species to impacts of introduced mammalian predators, and identify key life history attributes underlying this vulnerability. We measured range contraction following the introduction of exotic mammalian predators for 23 endemic forest bird species using information on both pre-human and current distributions. We used Bayesian modeling techniques to analyze whether variation in range contraction was associated with life history traits potentially influencing species’ predation vulnerability, while accounting for phylogenetic relatedness. Our results showed that the extent of range contraction varied greatly among species, with some species remaining in available forest habitat throughout most of their pre-human range, and others having disappeared completely from the main islands. Cavity nesting was the key trait associated with more extensive range decline, suggesting that cavity-nesting species are more vulnerable to predation than species that nest in more open sites.

New Zealand’s native forest bird species with high taxonomic levels of endemism (deep endemics) are more vulnerable to decline than species that arrived and speciated more recently. Here we use national-scale local occupancy data to show that three endemism-linked life-history traits account for greater vulnerability of deep-endemic species in the extant forest avifauna, but also that other, more subtle traits and mechanisms favour rather than hinder endemic persistence. We suggest that these traits together provide a basis for predicting changes in local occupancy following suppression of introduced mammalian predators in different landscapes. Our analyses disentangle the limiting effects of forest area, predation, and food availability for different species. They support understanding that predation by introduced mammals is the primary cause of forest bird declines and limitation today, but suggest that large tracts of native forest are essential for future restoration of endemic forest avifauna, even following suppression of introduced mammalian predators. We predict that with fewer predators (1) deep-endemic species would most successfully and rapidly recover in large tracts of warm forest, (2) scarcity of forest or food would limit recovery in larger-bodied, less mobile species and in mobile cavitynesters, and (3) interspecific competition would become influential and produce community reassortment.