Sphyrapicus Nuchalis Research Articles

Geographic variability of hybridization between red-breasted and red-naped sapsuckers.

Hybrid zones reveal the strength of reproductive isolation between populations undergoing speciation and are a key tool in evolutionary biology research. Multiple replicate transects across the same hybrid zone offer insight into the dynamics of hybridization in different environments, clarifying the role of extrinsic forces on the speciation process. Red-breasted and red-naped sapsuckers (Sphyrapicus ruber and Sphyrapicus nuchalis) have a long zone of contact over approximately 1,600 km from central British Columbia, Canada to central California, USA. We used Genotyping-by-Sequencing data from three independent sapsucker hybrid zone transects to compare hybridization dynamics between these species under variable geoclimatic conditions. We generated geographic clines of the genomic data to compare hybrid zone widths and used random forests models and linear regression to assess the relationship between climate and sapsucker ancestry along each transect. Our results show variation in the directionality of backcrossing, often indicative of moving hybrid zones. We note variable cline widths among transects, indicating differences in selection maintaining hybrid zone dynamics. Furthermore, random forests models identified different variables in close association with sapsucker ancestry across each transect. These results indicate a lack of repeatability across replicate transects and a strong influence of the local environment on hybrid zone dynamics.

CEDAR WAXWINGS (BOMBYCILLA CEDRORUM) FEEDING ON BIRCH SAP AT RED-NAPED SAPSUCKER (SPHYRAPICUS NUCHALIS) SAP WELLS

In late June 2020 in western Montana we observed up to 10 Cedar Waxwings (Bombycilla cedrorum) feeding on tree sap at Red-naped Sapsucker (Sphyrapicus nuchalis) sap wells excavated on 2 limbs of a Water Birch (Betula occidentalis). These observations constitute (a) the 1st report of waxwings feeding at sap wells created by sapsuckers of any species; (b) the 1st report of waxwings feeding on tree sap in early summer; and (c) the 1st report of the consumption of birch sap by this waxwing species. The Cedar Waxwings may have sought tree sap because of the limited availability of early-summer sugary fruits at the time of our observations in combination with the presence of new clusters of sap wells created by at least 1 pair of sapsuckers near where the waxwings were beginning to breed. The prevalent sugars in birch sap (glucose, fructose) are also those most efficiently assimilated by Cedar Waxwings and may have contributed in attracting the waxwings to the sapsucker wells.

Introgression between Sphyrapicus nuchalis and S. varius sapsuckers in a hybrid zone in west‐central Alberta

Studying species interactions at hybrid zones allows biologists to understand the forces that promote speciation. Hybridization among Sphyrapicus nuchalis, S. varius and S. ruber has long been acknowledged, and hybrid zones between S. nuchalis/S. ruber and S. varius/S. ruber have been characterized with both genetic and genomic data. Using a combination of next‐generation restriction site‐associated DNA sequencing (RAD‐Seq) and traditional genetic methods, we examined patterns of introgression in the poorly characterized S. nuchalis/S. varius contact zone; the two most similar species in the complex, though they are not each other's closest relatives. We found high introgression rates, with several early and many advanced generation hybrids along a 275 km stretch of Rocky Mountain foothill, pointing to a well‐established hybrid zone with hybrid individuals backcrossing with individuals from the parental species and each other. Plumage colouration in the hybrid zone was a relatively poor indicator of parental or hybrid status, which could be attributed to the possible involvement of few large effect genes.

Woodpecker nest survival, density, and a pine beetle outbreak

ABSTRACTMountain pine beetle (Dendroctonus ponderosae) outbreaks in western North American coniferous forests are increasing in size and severity. An understanding of wildlife population responses to pine beetle outbreaks is needed to inform habitat conservation strategies. We monitored 355 nests of 5 woodpecker species during 2 sampling periods, before (2003–2006) and after (2009–2014) the peak of a pine beetle outbreak in dry mixed conifer forest of Montana, USA. Three of 5 woodpecker species represented the beetle‐foraging group: American three‐toed (Picoides dorsalis), hairy (Dryobates villosus), and downy (D. pubescens) woodpeckers. The other 2 species studied were northern flicker (Colaptes auratus), a foraging and habitat generalist, and red‐naped sapsucker (Sphyrapicus nuchalis), a sap forager and bark‐gleaning insectivore. We analyzed daily survival rate of nests in relation to pine beetle outbreak (445,000 ha) severity and timing, along with covariates unrelated to the outbreak (temp, nest height, and nest tree diameter). Our results provided stronger evidence for relationships between woodpecker nest survival and the non‐outbreak variables than those associated with outbreaks. Our results indicated limited support for nest survival relationships with beetle severity (annual and cumulative pine tree mortality at 0.81‐ha and 314‐ha scales). Nevertheless, we observed a significant increase in densities of hatched nests for beetle‐foraging woodpeckers following the outbreak. Our results suggest that woodpeckers, particularly beetle foragers, respond numerically to pine beetle outbreaks through increased nesting densities more so than functionally via nest survival. © 2019 The Authors. Journal of Wildlife Management Published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.

Phenotypic and genetic introgression across a moving woodpecker hybrid zone.

In hybrid zones in which two divergent taxa come into secondary contact and interbreed, selection can maintain phenotypic diversity despite widespread genetic introgression. Red-breasted (Sphyrapicus ruber) and red-naped (S.nuchalis) sapsuckers meet and hybridize along a narrow contact zone that stretches from northern California to southern British Columbia. We found strong evidence for changes in the structure of this hybrid zone across time, with significant temporal shifts in allele frequencies and in the proportions of parental phenotypes across the landscape. In addition to these shifts, we found that differences in plumage predict genetic differences (R2 =0.80), suggesting that plumage is a useful proxy for assessing ancestry. We also found a significant bimodal distribution of hybrids across the contact zone, suggesting that premating barriers may be driving reproductive isolation, perhaps as a result of assortative mating based on plumage differences. However, despite evidence of selection and strong patterns of population structure between parental samples, we found only weak patterns of genetic divergence. Using museum specimens and genomic data, this study of sapsuckers provides insight into the ways in which phenotypic and genetic structure have changed over a 40-year period, as well as insight into the mechanisms that may contribute to the maintenance of the hybrid zone over time.

Differences in aggressive responses do not contribute to shifts in a sapsucker hybrid zone

ABSTRACT Interspecific interactions, such as aggressive territorial behaviors, can be important in establishing range boundaries, especially in hybrid zones where 2 divergent taxa meet and interbreed. Further, differences in the aggressive responses among hybridizing taxa can be important in moving hybrid zones, especially when there are strong asymmetries in their aggressive interactions. To determine how between-taxon interactions may be contributing to hybrid zone movement between 2 species of sapsucker, we studied an aspect of territory maintenance: aggression toward a territory intruder between species. Using a series of field experiments involving playback and taxidermic mounts, we tested whether Red-naped (Sphyrapicus nuchalis) and Red-breasted (S. ruber) sapsucker differ in their levels of aggressive behavior, thereby influencing the dynamic nature of the hybrid zone between these species. We found that 2 of the 3 aggressive response variables we measured differed between the 2 species. Red-naped ...

Changing climate mediates sapsucker (Aves: Sphyrapicus) hybrid zone movement.

Hybrid zones, where two divergent taxa meet and interbreed, offer unique opportunities to investigate how climate contributes to reproductive isolation between closely related taxa and how these taxa may respond to climatic changes. Red‐naped (Sphyrapicus nuchalis) and Red‐breasted (Sphyrapicus ruber) sapsuckers (Aves: Picidae) hybridize along a narrow contact zone that stretches from northern California to British Columbia. The hybrid zone between these species has been studied extensively for more than 100 years and represents an excellent system for investigations of the evolution of reproductive isolation. Shifts in the proportions of phenotypes at hybrid localities since 1910 that were inferred using specimens from museum collections were confirmed using species distribution models. We predicted the historical, current, and future distributions of parental and hybrid sapsuckers using Random Forests models to quantify how climate change is affecting hybrid zone movement in the Pacific Northwest. We found observed distribution shifts of parental sapsuckers were likely the result of climate change over the past 100 years, with these shifts predicted to continue for both sapsuckers over the next 80 years. We found Red‐breasted Sapsuckers are predicted to continue to expand, while Red‐naped Sapsuckers are predicted to contract substantially under future climate scenarios. As a result of the predicted changes, the amount of overlap in the distribution of these sapsuckers may decrease. Using hybrid phenotypes, we found the climate niche occupied by the hybrid zone is predicted to disappear under future conditions. The disappearance of this climate niche where the two parental species come into contact and hybridize may lead to a substantial reduction in genetic introgression. Understanding the impacts of global climate change on hybrid zones may help us to better understand how speciation has been shaped by climate in the past, as well as how evolution may respond to climate change in the future.

Strong reproductive isolation and narrow genomic tracts of differentiation among three woodpecker species in secondary contact.

Hybrid zones allow the measurement of gene flow across the genome, producing insight into the genomic architecture of speciation. Such analysis is particularly powerful when applied to multiple pairs of hybridizing species, as patterns of genomic differentiation can then be related to age of the hybridizing species, providing a view into the build-up of differentiation over time. We examined 33809 single nucleotide polymorphisms (SNPs) in three hybridizing woodpecker species: Red-breasted, Red-naped and Yellow-bellied sapsuckers (Sphyrapicus ruber, Sphyrapicus nuchalis and Sphyrapicus varius), two of which (ruber and nuchalis) are much more closely related than each is to the third (varius). To identify positions of SNPs on chromosomes, we developed a localization method based on comparative genomics. We found narrow clines, bimodal distributions of hybrid indices and genomic regions with decreased rates of introgression. These results suggest moderately strong reproductive isolation among species and selection against specific hybrid genotypes. We found 19 small regions of strong differentiation between species, partly shared among species pairs, but no large regions of differentiation. An association analysis revealed a single strong-effect candidate locus associated with plumage, possibly explaining mismatch among the three species in genomic relatedness and plumage similarity. Our comparative analysis of species pairs of different age and their hybrid zones showed that moderately strong reproductive isolation can occur with little genomic differentiation, but that reproductive isolation is incomplete even with much greater genomic differentiation, implying there are long periods of time when hybridization is possible if diverging populations are in geographic contact.

Occupancy of red-naped sapsuckers in a coniferous forest: using LiDAR to understand effects of vegetation structure and disturbance.

Red‐naped sapsuckers (Sphyrapicus nuchalis) are functionally important because they create sapwells and cavities that other species use for food and nesting. Red‐naped sapsucker ecology within aspen (Populus tremuloides) has been well studied, but relatively little is known about red‐naped sapsuckers in conifer forests. We used light detection and ranging (LiDAR) data to examine occupancy patterns of red‐naped sapsuckers in a conifer‐dominated system. We surveyed for sapsuckers at 162 sites in northern Idaho, USA, during 2009 and 2010. We used occupancy models and an information‐theoretic approach to model sapsucker occupancy as a function of four LiDAR‐based metrics that characterized vegetation structure and tree harvest, and one non‐LiDAR metric that characterized distance to major roads. We evaluated model support across a range of territory sizes using Akaike's information criterion. Top model support was highest at the 4‐ha extent, which suggested that 4 ha was the most relevant scale describing sapsucker occupancy. Sapsuckers were positively associated with variation of canopy height and harvested area, and negatively associated with shrub and large tree density. These results suggest that harvest regimes and structural diversity of vegetation at moderate extents (e.g., 4 ha) largely influence occurrence of red‐naped sapsuckers in conifer forests. Given the current and projected declines of aspen populations, it will be increasingly important to assess habitat relationships, as well as demographic characteristics, of aspen‐associated species such as red‐naped sapsuckers within conifer‐dominated systems to meet future management and conservation goals.

Long-term dynamics of the characteristics of tree cavities used for nesting by vertebrates

Many cavity-nesting vertebrates rely on tree cavities for nesting, and these cavities are often a limiting resource. The availability of tree cavities depends on rates of formation, persistence, and degradation or change in characteristics (e.g., entrance area, chamber volume, level of decay). Understanding how cavity characteristics change over time is key to understanding the value of cavities for wildlife as they age. We measured physical characteristics of 960 tree cavities for up to 17years (2490 measures of cavity dimensions, involving 23 species of cavity-using vertebrates) to estimate rates of change in cavities in relation to their age, whether they were in living versus dead trees, and their formation agent. We also analyzed the effects of cavity expansion by renovators. Overall, we found that cavity depth increased by 0.35cmyr−1 (1.7%), horizontal depth increased by 0.09cmyr−1 (0.8%), cavity entrance diameter increased by 0.06cmyr−1 (1.2%), and the decay class of the cavity tree advanced by one category every 12.5years. Cavities in living trees increased in depth by 0.84cmyr−1 versus 0.14cmyr−1 in dead trees; both showed relatively small increases in entrance diameter, with a non-significant trend toward greater increase of entrance diameter in dead trees. Cavities excavated by northern flickers [Colaptes auratus] maintained relatively constant dimensions compared with those created by other excavators, although cavity trees selected by northern flickers decayed 3.3 times more rapidly than those selected by strong excavators (hairy woodpeckers [Picoides villosus], American three-toed woodpeckers [Picoides dorsalis], pileated woodpeckers [Dryocopus pileatus], and red-naped sapsuckers [Sphyrapicus nuchalis]). Northern flickers were the most frequent renovators with 13.5% of cavities they used renovated from cavities formed by other excavators or decay. Renovations by northern flickers increased cavity depth by a mean of 16.5±11.8cm. We show that cavities in living trees significantly increase in cavity volume as they age and likely increase in quality over time, while cavities in dead trees maintain relatively constant dimensions. Thus, retention of living trees as “wildlife trees” is at least as important as retention of dead trees (snags) to support a diverse community of cavity-nesting birds and mammals.

Interactions of sapsuckers andCytosporacanker can facilitate decline of riparian willows

Drought has caused large-scale plant mortality in ecosystems around the globe. Most diebacks have affected upland forest species. In the past two decades, a large-scale decline of riparian willows (Salix L.) has occurred in Rocky Mountain National Park, Colorado. We examined whether climatic or biotic factors drive and maintain the willow community decline. We compared annual growth and dieback of willows inside and outside of 14-year-old ungulate exclosures and measured groundwater depth and predawn xylem pressures of stems as indicators of drought stress. We also performed an aerial photo analysis to determine the temporal dynamics of the decline. Aerial photo analysis indicated willow decline occurred between 2001 and 2005 and was best explained by an increase in moose population and a decrease in peak stream flows. A new mechanism for willow stem dieback was identified, initiated by red-naped sapsucker wounding willow bark. Wounds became infected with fungus that girdled the stem. DNA analyses confirmed Valsa sordida (Cytospora chrysosperma) as the lethal fungus. Captured sapsuckers had V. sordida spores on feet and beaks identifying them as one possible vector of spread. Predawn xylem pressure potentials remained high through the growing season on all study willows regardless of depth to ground water. Our results indicate that additional mechanisms may be involved in tall willow decline.

Habitat-Mediated Variation in the Importance of Ecosystem Engineers for Secondary Cavity Nesters in a Nest Web

Through physical state changes in biotic or abiotic materials, ecosystem engineers modulate resource availability to other organisms and are major drivers of evolutionary and ecological dynamics. Understanding whether and how ecosystem engineers are interchangeable for resource users in different habitats is a largely neglected topic in ecosystem engineering research that can improve our understanding of the structure of communities. We addressed this issue in a cavity-nest web (1999–2011). In aspen groves, the presence of mountain bluebird (Sialia currucoides) and tree swallow (Tachycineta bicolour) nests was positively related to the density of cavities supplied by northern flickers (Colaptes auratus), which provided the most abundant cavities (1.61 cavities/ha). Flickers in aspen groves provided numerous nesting cavities to bluebirds (66%) and swallows (46%), despite previous research showing that flicker cavities are avoided by swallows. In continuous mixed forests, however, the presence of nesting swallows was mainly related to cavity density of red-naped sapsuckers (Sphyrapicus nuchalis), which provided the most abundant cavities (0.52 cavities/ha), and to cavity density of hairy woodpeckers (Picoides villosus), which provided few (0.14 cavities/ha) but high-quality cavities. Overall, sapsuckers and hairy woodpeckers provided 86% of nesting cavities to swallows in continuous forests. In contrast, the presence of nesting bluebirds in continuous forests was associated with the density of cavities supplied by all the ecosystem engineers. These results suggest that (i) habitat type may mediate the associations between ecosystem engineers and resource users, and (ii) different ecosystem engineers may be interchangeable for resource users depending on the quantity and quality of resources that each engineer supplies in each habitat type. We, therefore, urge the incorporation of the variation in the quantity and quality of resources provided by ecosystem engineers across habitats into models that assess community dynamics to improve our understanding of the importance of ecosystem engineers in shaping ecological communities.

Using Satellite and Airborne LiDAR to Model Woodpecker Habitat Occupancy at the Landscape Scale

Incorporating vertical vegetation structure into models of animal distributions can improve understanding of the patterns and processes governing habitat selection. LiDAR can provide such structural information, but these data are typically collected via aircraft and thus are limited in spatial extent. Our objective was to explore the utility of satellite-based LiDAR data from the Geoscience Laser Altimeter System (GLAS) relative to airborne-based LiDAR to model the north Idaho breeding distribution of a forest-dependent ecosystem engineer, the Red-naped sapsucker (Sphyrapicus nuchalis). GLAS data occurred within ca. 64 m diameter ellipses spaced a minimum of 172 m apart, and all occupancy analyses were confined to this grain scale. Using a hierarchical approach, we modeled Red-naped sapsucker occupancy as a function of LiDAR metrics derived from both platforms. Occupancy models based on satellite data were weak, possibly because the data within the GLAS ellipse did not fully represent habitat characteristics important for this species. The most important structural variables influencing Red-naped Sapsucker breeding site selection based on airborne LiDAR data included foliage height diversity, the distance between major strata in the canopy vertical profile, and the vegetation density near the ground. These characteristics are consistent with the diversity of foraging activities exhibited by this species. To our knowledge, this study represents the first to examine the utility of satellite-based LiDAR to model animal distributions. The large area of each GLAS ellipse and the non-contiguous nature of GLAS data may pose significant challenges for wildlife distribution modeling; nevertheless these data can provide useful information on ecosystem vertical structure, particularly in areas of gentle terrain. Additional work is thus warranted to utilize LiDAR datasets collected from both airborne and past and future satellite platforms (e.g. GLAS, and the planned IceSAT2 mission) with the goal of improving wildlife modeling for more locations across the globe.

Resource Quantity and Quality Determine the Inter-Specific Associations between Ecosystem Engineers and Resource Users in a Cavity-Nest Web

While ecosystem engineering is a widespread structural force of ecological communities, the mechanisms underlying the inter-specific associations between ecosystem engineers and resource users are poorly understood. A proper knowledge of these mechanisms is, however, essential to understand how communities are structured. Previous studies suggest that increasing the quantity of resources provided by ecosystem engineers enhances populations of resource users. In a long-term study (1995-2011), we show that the quality of the resources (i.e. tree cavities) provided by ecosystem engineers is also a key feature that explains the inter-specific associations in a tree cavity-nest web. Red-naped sapsuckers ( Sphyrapicus nuchalis ) provided the most abundant cavities (52% of cavities, 0.49 cavities/ha). These cavities were less likely to be used than other cavity types by mountain bluebirds ( Sialia currucoides ), but provided numerous nest-sites (41% of nesting cavities) to tree swallows ( Tachycineta bicolour ). Swallows experienced low reproductive outputs in northern flicker ( Colaptes auratus ) cavities compared to those in sapsucker cavities (1.1 vs. 2.1 fledglings/nest), but the highly abundant flickers (33% of cavities, 0.25 cavities/ha) provided numerous suitable nest-sites for bluebirds (58%). The relative shortage of cavities supplied by hairy woodpeckers ( Picoides villosus ) and fungal/insect decay (<10% of cavities each, <0.09 cavities/ha) provided fewer breeding opportunities (<15% of nests), but represented high quality nest-sites for both bluebirds and swallows. Because both the quantity and quality of resources supplied by different ecosystem engineers may explain the amount of resources used by each resource user, conservation strategies may require different management actions to be implemented for the key ecosystem engineer of each resource user. We, therefore, urge the incorporation of both resource quantity and quality into models that assess community dynamics to improve conservation actions and our understanding of ecological communities based on ecosystem engineering.

Woodpeckers increase in abundance but maintain fecundity in response to an outbreak of mountain pine bark beetles

Many temperate woodpecker species are thought to be highly conservative in their fecundity with little response to fluctuations in availability of resources. In a 15-year field study in interior British Columbia, we evaluated responses in abundance and fecundity of six species of resident and migrant woodpeckers (downy woodpecker [ Picoides pubescens], hairy woodpecker [ Picoides villosus], American three-toed woodpecker [ Picoides dorsalis], pileated woodpecker [ Drycopus pileatus], northern flicker [ Colaptes auratus], and red-naped sapsucker [ Sphyrapicus nuchalis]) to a large-scale outbreak of mountain pine bark beetles that resulted in a strong positive pulse in food supply. Population densities of woodpecker species increased during outbreak years. Despite the year-round multi-annual increase in food resources, and in contrast to the strong increases in fecundity shown by nuthatches and chickadees, annual fecundity (as indicated by clutch size and number of nestlings that fledged) did not change for any woodpecker species over the study. Similarly, we found no changes in fecundity in response to selective forest harvesting despite numerical increases for woodpeckers at these sites. Our study confirms that these woodpecker species are conservative in their reproductive investment patterns even during strong multi-annual increases in food. Our findings indicate woodpecker populations are regulated numerically through variable survival and/or greater immigration rates, which can result in higher breeding densities temporarily during resource pulses.

Nonideal habitat selection by a North American cavity excavator: pecking up the wrong tree?

Nonideal habitat selection occurs when preferred habitat attributes differ from those associated with increased fitness. These mismatches have been widely studied in open cup-nesting birds, but the relationship between habitat-associated preferences and fitness in cavity-nesting birds has received relatively little attention. We studied patterns of preference and fitness during 2006–2007 in an Idaho, USA, population of Red-naped Sapsuckers ( Sphyrapicus nuchalis S.F. Baird, 1858). Using a suite of nonparametric tests, we examined the associations between habitat attributes and both nesting-area preferences and nest productivity (number of fledglings per pair) across four spatial scales. Nest productivity was associated with tree- and cavity-scale attributes, whereas preference was associated with attributes of home ranges. Live trees and southeasterly oriented cavities had higher nest productivity but were not preferred. Microclimates in nests with these attributes may enhance nestling survival, whereas nonpreference for these attributes may be due to energetic constraints in some individuals. Additional studies comparing patterns of nonideal habitat selection between open-cup nesters and cavity nesters should advance our understanding of how life-history characteristics influence selection patterns.

A Forty-year Comparison of the Breeding Avifauna in Grand Teton National Park, Wyoming

Studies comparing historical data with modern surveys can provide important insights into avian population trends. In 1993–1995 I repeated a breeding bird survey completed by G. W. Salt in 1952 and 1954 in Grand Teton National Park, Wyoming. The composition of breeding birds on all transects were comparable during the four decades, with most species showing no discernable difference in relative abundance (60.7%; n = 54), although 45.6% (n = 41) showed strong annual variation in the 1990s. Long-distance migrants represented the greatest number of species present (52.8%; n = 47) and the largest proportion showing a decline (25.5%; n = 12). Resident species had the largest proportion showing an increase (31.8%; n = 7). One resident species (Clark's nutcracker, Nucifraga columbiana) and one short distance migrant (red-naped sapsucker, Sphyrapicus nuchalis) showed a declining trend. American bitterns (Botaurus lentiginosus) may have disappeared from the sites surveyed in this study. Willow flycatcher (Empidonax traillii) abundance, while highly variable in the 1990s, also showed a significant decline. Close to half of the species occurring in spruce-fir forest declined (42.2%; n = 11), while species occurring in lodgepole pine (Pinus contorta) forest increased (35.3%; n = 12). Large national parks, such as Grand Teton National Park, could act as avian refugium, fundamental in preserving species that are experiencing declines in surrounding areas or on wintering grounds. The value of relatively undisturbed landscapes, such as national parks, to protect and conserve species numbers and diversity is increasingly vital, as the landscape and habitat in the surrounding areas continue to change.

The importance of excavators in hole-nesting communities: availability and use of natural tree holes in old mixed forests of western Canada

Nest-holes created by woodpeckers or natural decay processes are an essential commodity for secondary hole-nesting species. Hole-making agents may strongly influence the richness and abundance of species in hole-nester communities. However, few studies have examined the characteristics and relative importance of naturally occurring holes for hole-nesters. Between 1995 and 2006, we examined 1371 excavated and non-excavated holes used by 29 bird and mammal species in central British Columbia, Canada. Excavated holes were much more abundant (85%) than non-excavated holes (15%). Red-naped sapsucker (Sphyrapicus nuchalis) and northern flicker (Colaptes auratus) excavated 52% of the holes monitored. At the community level, non-excavated holes were used for nesting less than expected based on their availability (6% of total nests), particularly among woodpeckers (2% of nests). However, secondary hole-nesters used non-excavated holes roughly in proportion to their availability (10% of nests), and some excavators used non-excavated holes for nesting (flicker and red-breasted nuthatch, Sitta canadensis, 4% of nests each; black-capped chickadee, Poecile atricapillus, 13% of nests). Although nests in non-excavated holes tended to be lower on the tree, larger internally and with larger entrances, only European starling (Sturnus vulgaris) appeared to select non-excavated holes with characteristics most similar to their preferred excavated holes. Non-excavated holes may be an alternate nesting resource for secondary hole-nesters that cannot acquire an excavated hole and, occasionally, for excavators. The use of non-excavated holes may provide an advantage for secondary hole-nesters by releasing them from the constraints of excavator nest-site preferences.

Relationship Between Aspen Heartwood Rot and the Location of Cavity Excavation by a Primary Cavity-Nester, the Red-Naped Sapsucker

AbstractWe investigated nest-hole excavation by the Red-naped Sapsucker (Syphrapicus nuchalis) in aspen (Populus tremuloides) woodlands in western Colorado. Sapsuckers excavate nest cavities primarily in aspens infected with a heartwood rot fungus (Phellinus tremulae), which softens the heartwood of infected trees. We assessed the interior condition of fungus-infected aspen trunks by extracting wood samples with an increment corer to determine whether sapsuckers chose nest-hole locations based on the extent of healthy sapwood remaining. Comparing fungus-infected trees with and without cavities, cavity-bearing trees had thinner healthy sapwood. The depth of healthy sapwood also varied with compass direction, being thinnest on the south sides of fungus-infected aspens. Cavity entrance orientations were significantly biased to the south-southeast, corresponding with the directional bias in heartwood rot. These results suggest that the depth of healthy sapwood, and hence excavation effort, may be important in determining nest hole location for the Red-naped Sapsucker.

A Variance-decomposition Approach to Investigating Multiscale Habitat Associations

Abstract The recognition of the importance of spatial scale in ecology has led many researchers to take multiscale approaches to studying habitat associations. However, few of the studies that investigate habitat associations at multiple spatial scales have considered the potential effects of cross-scale correlations in measured habitat variables. When cross-scale correlations in such studies are strong, conclusions drawn about the relative strength of habitat associations at different spatial scales may be inaccurate. Here we adapt and demonstrate an analytical technique based on variance decomposition for quantifying the influence of cross-scale correlations on multiscale habitat associations. We used the technique to quantify the variation in nest-site locations of Red-naped Sapsuckers (Sphyrapicus nuchalis) and Northern Flickers (Colaptes auratus) associated with habitat descriptors at three spatial scales. We demonstrate how the method can be used to identify components of variation that are associated only with factors at a single spatial scale as well as shared components of variation that represent cross-scale correlations. Despite the fact that no explanatory variables in our models were highly correlated (r &lt; 0.60), we found that shared components of variation reflecting cross-scale correlations accounted for roughly half of the deviance explained by the models. These results highlight the importance of both conducting habitat analyses at multiple spatial scales and of quantifying the effects of cross-scale correlations in such analyses. Given the limits of conventional analytical techniques, we recommend alternative methods, such as the variance-decomposition technique demonstrated here, for analyzing habitat associations at multiple spatial scales.