Nest Web Research Articles

Ecology and conservation of cavity-nesting birds in the Neotropics: Recent advances, future directions, and contributions to ornithology

ABSTRACT About 35% of tree-cavity-nesting bird species inhabit the Neotropics, a region crucial to understanding their breeding ecology, conservation, and roles in social-ecological systems. Sixteen years ago, Cornelius et al. (2008) reviewed published knowledge and identified research priorities for Neotropical cavity-nesting birds. Advances since 2008 have not been synthesized and many remain excluded from dominant ornithology because of barriers that disproportionately affect people and ideas from the Global South. Here, we review recent advances in knowledge about Neotropical cavity-nesting birds, introduce the Special Feature series “Ecology and conservation of cavity nesters in the Neotropics,” and outline possible directions for future research. Research in the Neotropics has advanced knowledge of breeding biology, demonstrated that nest sites are limited and birds compete for cavities (mainly in humid forests), identified non-excavated cavities (formed by wood decay) as the main source of cavities and demonstrated the importance of understanding Indigenous and local community relationships to birds. With field studies across the Neotropics, the Special Feature series shows how environment, people’s common imaginaries, vegetation management, and behavior of avian excavators can interact to influence cavity availability, with ecological consequences for many cavity-using organisms. In the future, researchers should center ethno-knowledge and natural history to create an accurate list of cavity-nesting birds in the Neotropics, and integrate this knowledge into studies of population and community ecology. It is also important to study factors that influence cavity dynamics, especially using a social-ecological systems framework and especially in arid and semi-arid regions. We recommend expanding the concept of nest webs (ecological networks of cavity nesters) to incorporate additional cavity substrates (e.g., termitaria, cliffs), cavity alternatives (e.g., bulky enclosed stick nests of many Furnariidae), and cavity-using taxa beyond birds and mammals (e.g., social insects, snakes), which abound in the Neotropics but were not contemplated in the original nest web formulation. Translated versions of this article are available in Supplementary Material 1 (Spanish) and Supplementary Material 2 (Portuguese).

Persistence, changes and robustness of nest webs along a latitudinal gradient in the Canadian boreal forest

IntroductionIn eastern Canada, the boreal forest is associated with an important latitudinal shift in forest composition and structure, which occurs in the transition between the mixed southern boreal forest and the coniferous northern boreal forest. Along this transition, upland mixedwood stands with large deciduous trees (important for cavity-dependent vertebrate species) are gradually replaced by forests with smaller conifer trees, primarily black spruce (Picea mariana). Concomitantly, the availability of lowland forests flooded by the American beaver (Castor canadensis), which can provide adequate conditions for tree-cavity users, is also decreasing.MethodsWe hypothesized that this latitudinal gradient would bring important changes in the functional diversity and network structure of vertebrate cavity-using communities. Along this latitudinal gradient we used a nest web approach to analyze the structure and robustness of networks of cavity users in upland forests and in lowland forests flooded by beavers.ResultsDespite their low availability in the northern forest region, we found that mixedwood stands persisted throughout the boreal forest in being the main drivers of nest webs network structure of upland forests whereas old black spruce stands contribution was low. In lowland forests, beaver ponds harbored nest webs with a rich and complex structure in both forest regions. Species removal simulations revealed that across our latitudinal gradient upland and lowland forest nest webs responded differently. In upland forests, the removal of trembling aspen and the Pileated Woodpecker (Dryocopus pileatus) caused the highest proportions of secondary extinctions, showing low robustness of nest webs given that these two species were highly connected to the other species. Contrastingly, nest webs in beaver ponds were more robust mainly because excavator species used a higher diversity of tree species despite the removal of the Northern Flicker (Colaptes auratus) which induced numerous secondary extinctions. The Pileated Woodpecker remained the pivotal species across the two forest regions in upland forests whereas the Northern Flicker became the main large cavity provider in beaver ponds across the latitudinal gradient.DiscussionWe discuss how mixedwood forests and beaver ponds, which are key habitat types for the cavity-using vertebrate community across our latitudinal gradient, should be maintained and protected in landscapes under industrial timber harvesting.

Old forest structural development drives complexity of nest webs in a naturally disturbed boreal mixedwood forest landscape

Structural complexity generated by forest development processes and tree species compositional changes provide key habitat features for vertebrate communities that rely upon tree size and decay processes for foraging, denning or nesting. Complexity of forest structure in old stands could not only be key for harboring increased taxonomic species diversity but also greater functional diversity through more complexity in networks of tree cavity dependent species. Using a nest web approach that hierarchically links cavity-bearing trees with cavity formation agents (natural decay processes and avian excavators) and cavity users (non-excavator species), we compared network characteristics of nest webs along a time since fire gradient in a naturally disturbed boreal mixedwood forest landscape in eastern North America. Since 2003, twelve 24 to 40 ha plots ranging from 61 to more than 245 years after fire were surveyed at the Lake Duparquet Research and Teaching Forest in Abitibi, Quebec, Canada to detect active nesting, and denning cavities. We found that network complexity both in terms of number of vertebrate species and number of interactions among species, increased along the age gradient and was significantly higher in the older stands than predicted by chance. Whereas cavity-nesting communities in old forests used a higher diversity of tree species over a wide range of decay stages, trembling aspen remained a key cavity-bearing tree throughout the age gradient. Woodpeckers were the main cavity formation agents whereas less than 1% of cavities originated from natural decay. The structural development of older forests is thus a driver for functional diversity in cavity-using vertebrate communities through higher interaction richness in nest webs, among cavity-bearing trees, excavators and non-excavating users. The pivotal contribution of the entire gradient of old forest cover types to the overall complexity of nest webs in the boreal mixedwood zone is also a key for the resilience of the cavity-using vertebrate community to natural disturbances. We discuss how such resilience may be compromised by even-aged industrial timber harvesting with short rotations that shifts the age structure of boreal landscapes toward regenerating and young pole forests whereas old forest cover types become below their historical range of variability.

Cavities excavated by woodpeckers limit populations of other cavity-nesting birds.

Research Highlight: Trzcinski, M., Cockle, K., Norris, A., Edworthy, A., Wiebe, K., & Martin, K. (2022). Woodpeckers maintain the diversity of cavity-nesting vertebrates in a temperate forest. Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.13626. Whether populations of hole-nesting birds are limited by the availability of cavities is a long-standing, fundamental question in avian community ecology. The structure of cavity-nesting communities, known as nest webs, includes links between tree species that provide natural holes and bird species that nest in those holes (secondary cavity nesters, SCNs), tree species that provide substrates for cavity excavation and bird species such as woodpeckers that create cavities using those substrates (primary cavity nesters, PCNs), and between PCNs and SCNs. Trzcinski et al. (2022) focus on the latter links and provide the most compelling empirical evidence to date that cavities created by PCNs specifically, as opposed to natural holes or holes generally, limit populations of SCNs. Using data from a long-term study, the authors employ three analyses to separate effects of availability of cavities from environmental factors such as food, habitat features and host tree abundance, while controlling for annual variation and autocorrelation within sites, to isolate the relationship between excavated cavities and SCN numbers. They show that nest density of SCNs is positively related to PCN nest density the previous year, an indicator of availability of excavated cavities, and that the effect of PCNs is stronger when other variables are accounted for. North American coniferous forests such as that studied by Trzcinski et al. (2022) are exceptional in that excavated cavities comprise the vast majority of nesting holes. Whether their findings apply to other systems in which PCNs are a major source of cavities, or to particular PCN-SCN relationships in systems in which excavated cavities account for a much lower proportion of nesting holes remains to be investigated.

Use of Nest Web Cameras and Citizen Science to Quantify Osprey Prey Delivery Rate and Nest Success

ABSTRACT Ospreys (Pandion haliaetus) are obligate piscivores and their nesting success depends on sufficient amounts of fish delivered to the nests during the breeding season. Nests are considered successful when pairs raise a minimum of one young to fledging or near-fledging age. Through web cameras and online broadcasts of Osprey nests, citizen scientists quantified daily number of fish deliveries, nest survival, and nest success. We received and analyzed curated data (one to seven seasons, 2014–2020) from citizen scientist groups representing 19 Osprey web cameras from four countries in North America and Europe. We compared the average and the coefficient of variation of the number of fish delivered per day within the early breeding season between the failed and successful nests using a Wilcoxon rank-sum test. We also analyzed the effects of the average and the coefficient of variation of the number of fish delivered per day on the number of days of nest survival and whether a nest was successful or not using generalized linear mixed models. Successful and failed nests had significant differences in the average number of fish delivered per day and the failed nests had a higher variation in the number of fish deliveries. Moreover, the variation and average number of fish delivered per day had strong associations with whether a nest would fail or succeed. The global effort and manner in which these data were collected are novel and can further our understanding of this charismatic species. The combination of citizen science and technology is a powerful modern tool that can provide insights and has the potential to advance raptor research worldwide.

Burrow webs: Clawing the surface of interactions with burrows excavated by American badgers.

Ecosystem engineers are organisms that influence their environment, which includes alterations leading to habitat provisioning for other species. Perhaps the most well‐examined guild of species provisioning habitat for other species is tree cavity excavators or woodpeckers (Picidae). Many studies have examined the suite of secondary cavity users that rely on woodpeckers, and how the ecological network of secondary users, collectively referred to as the nest web, changes across communities. Despite similar habitat provisioning processes, fewer studies have assessed the suite of species associated with burrowers providing access to subterranean habitat. Here, we begin to characterize the burrow web provisioned by American badgers (Taxidea taxus) and evaluate the diversity and frequency of species interactions we detected at abandoned badger burrows in Wyoming, USA. We deployed camera traps at 23 badger burrows and identified interactions with the burrow by birds, mammals, and reptiles. Overall, we discovered 31 other species utilizing badger burrows, consisting of 12 mammals, 18 birds, and 1 reptile. Mammals, other than American badgers themselves and other fossorial species such as ground squirrels (Urocitellus sp.), frequently using burrows included mice (Peromyscus sp.), long‐tailed weasel (Mustela frenata), pygmy rabbit (Brachylagus idahoensis), and desert cottontail (Sylvilagus audubonii). Of the 18 bird species detected, most accounted for <5% of overall detections, besides chipping sparrows (Spizella passerina) at 7.2%–11.5% of detections. The most common category of detection by bird species was foraging, contrary to mammals, which used the burrow frequently and were commonly observed entering and exiting the burrow. This work provides additional context on the ecological role of American badgers within their environment. More broadly, this work scratches the surface of many remaining questions to explore with the aim of advancing our understandings about burrow webs across the diversity of burrowing species and the communities in which they occur.

Overlap in reproductive phenology increases the likelihood of cavity nest usurpation by invasive species in a tropical city

Abstract Multiple invasive cavity-nesting bird species can be present in a nest web, the network linking birds using cavities. We investigated the nest preferences and breeding phenologies of the cavity-nesting guild in the region surrounding Miami, Florida, USA, where invasive starlings, mynas, and parrots potentially usurp cavities from native woodpeckers and secondary cavity-nesters. We asked if the timing of reproduction determines which invasive species will usurp cavities from native birds with similar nest preferences. Nest usurpations between European Starlings (Sturnus vulgaris) and the woodpecker species present in Miami is well documented, but we predicted that a recently arrived sturnid species and introduced psittacids would also usurp nests. European Starlings had the largest breeding population of any species in our nest web, breeding during the peak of nesting season, and usurped the largest number of active nest cavities. We found that a small population of Common Mynas (Acridotheres tristis) usurped nests, sharing the peak-season nesting period with starlings and native woodpeckers. Parrots bred later than we expected, avoiding nest-site overlap with similarly large native birds that use cavities with similar characteristics. Parrots did not usurp any active nest cavities from native birds. Our results demonstrate how to use analysis of cavity characteristics and reproductive timing to evaluate threats to a cavity nest web posed by multiple invasive species. Common Myna currently usurp few nests; if they increase greatly in population, they could pose a problem for native cavity-nesters.

Nurturing resilient forest biodiversity: nest webs as complex adaptive systems

Nurturing resilient forest biodiversity: nest webs as complex adaptive systems

Interspecific networks of cavity-nesting vertebrates reveal a critical role of broadleaf trees in endangered Araucaria mixed forests of South America

Cavity-nesting animals and their nest trees are linked in interspecific facilitation networks known as nest webs, which play key roles in forest function but vary across biomes and with human perturbation. We examined the composition, structure and function of nest webs between two endangered old-growth forests representing the last remnants of the ancient coniferous family Araucariaceae in South America: pewen (Araucaria araucana; Endangered) in temperate Chile (2010–2018), and Parana pine (Araucaria angustifolia; Critically Endangered) in subtropical Argentina (2006–2018). Pewen and Parana pine accounted for 30 and 9% of forest basal area, but only 2 and 5% of nesting cavities, respectively. Instead, cavity-nesting birds and mammals nested disproportionately in coexisting broadleaf trees. Species richness, interaction richness, and mean number of links per species were much higher in Parana pine forest than in pewen forest, but the two nest webs had similar levels of evenness and nestedness. Most secondary cavity-nesting species depended on cavities formed by decay in Nothofagus spp. (98% of nest cavities in pewen forest) or Apuleia leiocarpa (26% of nest cavities in Parana pine forest). An exception was the globally endangered Vinaceous Parrot, a Parana pine seed disperser, which made 50% of its nests in decay-formed cavities in Parana pine. To conserve the ecosystem functions of endangered Araucaria forests it is important to protect and recruit not only Araucaria trees but also a mix of broadleaf trees that can confer resilience to nest webs in the face of major disturbances.

Sequential Use of Tree Cavities by Birds and Nest Web in a Riparian Forest in Southwest India

We studied the sequential use of tree cavities by the cavity-nesting bird community in a riparian forest ecosystem of south-western India during 2013–2015. During the study period, a total of 376 nests of 20 cavity-nesting bird species comprising six primary and 14 secondary cavity nesters were located in the riparian forest. The cavity use pattern showed that one-third of the cavities were reused by birds and that the majority of the reused cavities were located in live trees. A lesser proportion (5%) of cavities used by birds was reused by other animal taxa. Competition for nest cavity use was observed among bird species in which Common Myna Acridotheres tristis was found to be the major competitor. A strong correlation was found between the bird body mass and nest cavity dimensions. Cluster analysis based on tree and cavity characteristics divided the avian community into five groups. We constructed a nest web for community structure that showed cavity resource use by various primary and secondary cavity-nesters. Woodpeckers (Greater Golden-backed Woodpecker Chrysocolaptes guttacristatus, White-naped Woodpecker C. festivus, and Browncapped Pygmy Woodpecker Dendrocopos nanus) as well as barbets (Brown-headed Barbet Psilopogon zeylanica, Coppersmith Barbet P. haemacephala and White-cheeked Barbet P. viridis) formed the key excavators in the riparian forest. While many of the smaller secondary cavity-nesting birds depend on the excavated cavities for nesting and reproduction, large-sized birds used decay formed cavities.

Nest webs beyond woodpeckers: the ecological role of other nest builders.

Nest webs beyond woodpeckers: the ecological role of other nest builders.

Correction to: The conservation value of tree decay processes as a key driver structuring tree cavity nest webs in South American temperate rainforests

South American temperate rainforests, a global biodiversity hotspot, have been reduced to nearly 30% of their original extent and most remaining stands are being degraded. Cavity-nesting vertebrate communities are dependent on cavity-bearing trees and hierarchically structured within nest webs. Evaluating the actual degree of cavity dependence (obligate, non-obligate) and the preferred attributes of trees by cavity nesters is critical to design conservation strategies in areas undergoing habitat loss. During three breeding seasons (2010–2013), we studied the cavity-nesting bird community in temperate rainforests of Chile. We found the highest reported proportion of tree cavity nesters (n = 29 species; 57%) compared to non-cavity-using birds for any forest system. Four species were excavators and 25 were secondary cavity nesters (SCNs). Among SCNs, ten species were obligate and 15 were non-obligate cavity nesters. Seventy-five percent of nests of SCNs were located in cavities produced by tree decay processes and the remaining 25% were in cavities excavated mainly by Pygarrhichas albogularis and Campephilus magellanicus. Nest web structure had a low dominance and evenness, with most network interactions occurring between SCNs and large decaying trees. Tree diameter at breast height (DBH) was larger in nest-trees (57.3 cm) than in available trees (26.1 cm). Cavity nesters showed a strong preference for dead trees, both standing and fallen (58% of nests). Our results stress that retaining large decaying and standing dead trees (DBH > 57 cm), and large fallen trees, should be a priority for retention in forest management plans in this globally threatened ecosystem.

The conservation value of tree decay processes as a key driver structuring tree cavity nest webs in South American temperate rainforests

The conservation value of tree decay processes as a key driver structuring tree cavity nest webs in South American temperate rainforests

木材腐朽プロセスと樹洞を巡る生物間相互作用 : 樹洞営巣網の構築に向けて( 枯死木をめぐる生物間相互作用)

木材腐朽プロセスと樹洞を巡る生物間相互作用 : 樹洞営巣網の構築に向けて( 枯死木をめぐる生物間相互作用)

Exploring nest webs in more detail to improve forest management

The interactions between birds that use tree cavities for breeding, roosting and sheltering have been named ‘nest webs’. We applied for the first time in nest-web studies some tools developed in network theory, in order to develop conservation and management recommendations of forest biodiversity. We recorded 109 interactions between 15 bird and 11 cavity-bearing tree species, in a subtropical piedmont forest (PF) of northwestern Argentina. Bird species in this nest-web included four woodpecker species, whose cavities were scarcely (9%) used by non-excavator birds, such as parrots, owls, and woodcreepers. Based on the Importance and Strength indices the most important tree species were Calycophyllum multiflorum (Rubiaceae) and Anadenanthera colubrina (Fabaceae). The nest web contained three main interaction modules: one composed by woodpeckers interacting with both living and standing dead trees; another by non-excavator birds using decay-formed cavities in living trees; and a third small module that had a few birds using woodpecker-excavated cavities in living trees. Important tree species were different for woodpecker and non-excavator modules. Extinction simulation of the most important tree species tripled the negative impact on cavity-using bird assemblage compared with the random extinction of tree species. In logging operations special consideration should be taken to ensure the maintenance of key tree species for the conservation of all the nest-web components in PF.

Lotic cyprinid communities can be structured as nest webs and predicted by the stress-gradient hypothesis.

Little is known about how positive biotic interactions structure animal communities. Nest association is a common reproductive facilitation in which associate species spawn in nests constructed by host species. Nest-associative behaviour is nearly obligate for some species, but facultative for others; this can complicate interaction network topology. Nest web diagrams can be used to depict interactions in nesting-structured communities and generate predictions about those interactions, but have thus far only been applied to cavity-nesting vertebrate communities. Likewise, the stress-gradient hypothesis (SGH) predicts that prevalent biotic interactions shift from competition to facilitation as abiotic and biotic stress increase; this model has been hardly applied to animal communities. Here, both of these models were applied to nest-associative fish communities and extended in novel ways to broaden their applicability. A nest web was constructed using spawning observations over 3 years in several streams in south-western Virginia, USA. Structural equation modelling (SEM) was then implemented through an information-theoretic framework to identify the most plausible nest web topology in stream fish communities at 45 sites in the New River basin of the central Appalachian Mountains, USA. To test the SGH, the per-nest reproductive success of 'strong' (nearly obligate) nest associates was used to represent interaction importance. Eigenvectors were extracted from a principal coordinate analysis (PCoA) of proportional species abundances to represent community structure. Both of these metrics were regressed on physical stress, a combination of catchment-scale agricultural land use and stream size (representing spatiotemporal habitat variability). Seventy-one per cent of SEM model evidence supported a parsimonious interaction topology in which strong associates rely on a single host (Nocomis), but not other species. PCoA identified a gradient of community structure dominated by Nocomis and associates, to communities dominated by other reproductive groups. Both metrics of interaction importance responded positively to physical stress. This study demonstrates that nest webs can be useful in a variety of systems and that SEM can be a quantitative extension of this framework. Likewise, the SGH can be used to understand positive interactions in animal communities and can be extended to predict proportional representation of facilitating and beneficiary species in communities.

Temporal dynamics of a commensal network of cavity-nesting vertebrates: increased diversity during an insect outbreak.

Network analysis offers insight into the structure and function of ecological communities, but little is known about how empirical networks change over time during perturbations. "Nest webs" are commensal networks that link secondary cavity-nesting vertebrates (e.g., bluebirds, ducks, and squirrels, which depend on tree cavities for nesting) with the excavators (e.g., woodpeckers) that produce cavities. In central British Columbia, Canada, Northern Flicker (Colaptes auratus) is considered a keystone excavator, providing most cavities for secondary cavity-nesters. However, roles of species in the network, and overall network architecture, are expected to vary with population fluctuations. Many excavator species increased in abundance in association with a pulse of food (adult and larval beetles) during an outbreak of mountain pine beetle (Dendroctonus ponderosae), which peaked in 2003-2004. We studied nest-web dynamics from 1998 to 2011 to determine how network architecture changed during this resource pulse. Cavity availability increased at the onset of the beetle outbreak and peaked in 2005. During and after the outbreak, secondary cavity-nesters increased their use of cavities made by five species of beetle-eating excavators, and decreased their use of flicker cavities. We found low link turnover, with 74% of links conserved from year to year. Nevertheless, the network increased in evenness and diversity of interactions, and declined slightly in nestedness and niche overlap. These patterns remained evident seven years after the beetle outbreak, suggesting a legacy effect. In contrast to previous snapshot studies of nest webs, our dynamic approach reveals how the role of each cavity producer, and thus quantitative network architecture, can vary over time. The increase in interaction diversity with the beetle outbreak adds to growing evidence that insect outbreaks can increase components of biodiversity in forest ecosystems at various temporal scales. The observed changes in (quantitative) network architecture contrast with the relatively stable (qualitative) architecture of empirical mutualistic networks that have been studied to date. However, they are consistent with recent theory on the importance of population fluctuations in driving network architecture. Our results support the view that models should allow for the possibility of rewiring (species switching partners) to avoid overestimation of secondary extinction risk.

How important is dead wood for woodpeckers foraging in eastern North American boreal forests?

Dead and decaying trees may be a limited resource for woodpeckers in managed forests, especially for species that rely on dead wood for nesting and foraging. Whereas recent nest web studies greatly increased our understanding of nest tree use by woodpeckers, knowledge on woodpeckers foraging requirements is much less developed. We quantified and compared tree selection patterns and foraging behavior of six bark-foraging woodpeckers – downy woodpecker (Picoides pubescens), hairy woodpecker (Picoides villosus), American three-toed woodpecker (Picoides dorsalis), black-backed woodpecker (Picoides arcticus), yellow-bellied sapsucker (Sphyropicus varius) and pileated woodpecker (Dryocopus pileatus) – that co-occur in eastern boreal forests of North America. A total of 271 observation bouts and more than 600 foraging trees were recorded at three study sites characterized as mixedwood, conifer, and burn. Our results show that dead wood represents an important foraging substrate for most bark-foraging woodpeckers in Canadian eastern boreal forests. However, significant differences in individual species were found with regard to substrate use patterns, foraging behavior and associated prey. Woodpeckers were categorized according to their selection for specific stages of tree degradation, with the yellow-bellied sapsucker and the pileated woodpecker representing opposite ends of this gradient. The black-backed woodpecker showed the highest use of dead wood and was very specific in its tree selection by using mostly recently dead trees. We emphasize that providing foraging substrates for most woodpecker species not only requires maintaining dead wood but also paying heed to the underlying dynamics of dead wood (e.g. recruitment and degradation) in managed boreal forest landscapes.

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.

Woodpecker excavations suitability for and occupancy by cavity users in the boreal mixedwood forest of eastern Canada

In the boreal forest, where tree cavities are mainly excavated by woodpeckers, many holes are incomplete excavations that are not suitable cavities for most other cavity users that form nest webs. We assessed cavity suitability for and use by a community of primary excavators and secondary users in managed and unmanaged landscapes in the boreal mixedwood forest of eastern Canada. We compared ground surveys of tree holes with direct inspections of the inside of potential cavities in remnant habitats surrounded by cutover areas and in large tracts of unharvested forest. We found that ground surveys overestimated suitable cavity abundance: only 38% of the potential cavities detected by ground surveys were suitable for nesting in both managed and unmanaged landscapes. Ground surveys of active nests correctly detected a greater proportion of primary (93%) than secondary cavity nesters (48%). In nest webs such as those of the boreal forest, where cavities are mainly created by woodpeckers, our results indicate that a large proportion of holes detected from the ground are not suitable for cavity nesters, thus overestimating the actual availability of nest sites. Furthermore, when nest cavities are active, ground surveys are satisfactory for detecting primary cavity nesters, but they are inadequate for detecting secondary cavity nesters.