Forest Bird Populations Research Articles

Over the river and through the woods: Multi-scale habitat associations of two at-risk bird species in riparian forests of the Central Appalachians

Several species-specific habitat management guidelines have been created over the past decade to help recover declining populations of forest birds in eastern North America. Conserving eastern forest birds through forest management requires understanding the factors driving species occurrence patterns and how the needs of various species complement or contrast with each other. Our study assessed and compared occupancy patterns of two at-risk species associated with riparian forests: the Wood Thrush (Hylocichla mustelina) and the Canada Warbler (Cardellina canadensis). We conducted point-count surveys and vegetation sampling at 304 unique locations along 33 stream sections in central and southwestern Pennsylvania from May – June 2018 and 2019. We used single-season occupancy models to examine the influence of vegetation composition (i.e., microhabitat conditions), forest structure (i.e., from Light Detection and Ranging data, “LiDAR”), and other factors (i.e., stream order and elevation) on focal species occurrence patterns. Wood Thrush occupancy (mean ᴪˆ: 0.57) was best explained by the presence of mountain laurel (Kalmia latifolia; negative effect), elevation (negative effect), canopy height (positive effect), and canopy structural heterogeneity (positive effect). Canada Warbler occupancy (mean ᴪˆ: 0.55) was best explained by the presence of mountain laurel (positive effect), stream order (positive effect), elevation (positive effect), density of understory vegetation (positive effect), and canopy structural heterogeneity (negative effect). While both bird species occurred relatively frequently in riparian forests, our results indicate that habitat factors at multiple spatial scales play a role in regulating their occurrence patterns. Moreover, the two species exhibited contrasting habitat associations. Specifically, Canada Warblers preferred sites supporting dense mountain laurel, typically at elevations >600 m, and with a largely intact canopy. These patterns were reversed for Wood Thrushes. These contrasting associations will limit opportunities to manage riparian forests in a manner that conveys co-benefits for our two focal species. As a general rule, forest managers in our study region can target habitat management for Canada Warblers in riparian areas above 600 m and in areas below 600 m for Wood Thrushes. Given our focal species’ use of other (non-riparian) forest types as breeding habitat and broad geographic distributions, it is important to recognize that our findings may not apply outside of riparian forests or beyond the Central Appalachian Mountains.

A modified co-production framework for improved cross-border collaboration in sustainable forest management and conservation of forest bird populations

The border between Canada and the United States poses jurisdictional challenges when it comes to consistently implementing science-based conservation of forests and their biological communities. Through a partnership with the Sustainable Forestry Initiative, Boreal Avian Modelling Project, and American Bird Conservancy, we developed a co-production framework to conduct research that will inform forest management practices for bird conservation in the cross-border region of Bird Conservation Region 12. Our framework first responds to the needs of resource managers and other perceived stakeholders, while investing in relationship-building for long term trust as a foundation for future partnerships with Indigenous rights holders and landowners. Our central question was: How can sustainably managed forests create and/or maintain high quality breeding habitat to support forest bird populations that are resilient to climate change? Engaging with experts in Canada and the United States, we found that the main driver for addressing our central question was our limitation in connecting bird population responses to specific forest management practices. We describe how experts are contributing avian count and forest inventory data that researchers will use to produce a requested decision-support tool. We continue to engage with land managers to link forest resource inventory data to specific forest management practices and refine the components of products by including more diverse perspectives.

Forest Bird Populations at the Big Island National Wildlife Refuge Complex, Hawai‘i

Abstract Endemic Hawaiian forest birds have experienced dramatic population declines. The Big Island National Wildlife Refuge Complex (Refuge Complex) was established for the conservation of endangered forest birds and their habitats. Surveys have been conducted at two units of the Refuge Complex to monitor forest bird populations and their response to management actions. We analyzed survey data from 1987 to 2019 at the Hakalau Forest Unit (HFU) and from 1995 to 2019 at the Kona Forest Unit (KFU). We analyzed three strata at HFU: open-forest, closed-forest, and afforested-pasture, and two strata at KFU: upper (>1,524 m elevation) and lower (<1,524 m). In all years, ‘i‘iwi Vestiaria coccinea, ‘apapane Himatione sanguinea, and Hawai‘i ‘amakihi Chlorodrepanis virens virens were the most abundant species at HFU. Three endangered forest bird species, Hawai‘i ‘ākepa Loxops coccineus, ‘alawī Loxops mana (also known as Hawai‘i creeper) and ‘akiapōlā‘au Hemignathus wilsoni, had much lower densities. The most abundant species at KFU was ‘apapane, followed by Hawai‘i ‘amakihi at much lower densities. We found a continuation of several trends observed in previous analyses at HFU up to 2012, with most species’ trends upward in afforested-pasture stratum, stable in the open-forest stratum, and downward in the closed-forest stratum. However, more species were showing downward trends in all three strata during the most recent decade. Results were mixed at KFU, with most species’ trends downward in the upper stratum and upward in the lower stratum. Populations of endangered species were either locally extirpated at KFU or in numbers too low to reliably estimate population abundance. The Refuge Complex is important for conservation of forest birds on Hawai‘i Island. Our results show that HFU supports the majority of three endangered forest bird species. Threats to forest birds at the Refuge Complex appear to be having a negative impact. These threats include habitat loss, disease, feral ungulates, and nonnative predators. Continuing and enhancing management actions, such as forest restoration and removal of invasive species, could help mitigate these impacts and allow the Refuge Complex to remain a key site for forest bird conservation in Hawai‘i.

Examining the temporal effects of wildfires on forest birds: Should I stay or should I go?

In flammable ecosystems, forest dynamics are affected by fire suppression, climate change and changes in land use that cause the frequency and intensity of fires to diverge from natural fire regimes. Although afforestation provides new habitats for forest species, fire can reverse its benefits by penalizing forest species. Shorter fire recurrence may also affect the capacity of original habitats to recover with post-fire salvage logging adding to the picture and having both direct and indirect effects on birds. To identify the underlying factors driving temporal variations in forest bird populations after wildfires, we performed counts in 685 transects located in 68 burnt areas in Catalonia (NE Iberian Peninsula), ranging from one to 11 years post-fire. Among the 20 studied forest species, 18 related significantly to time since fire and 14 to water deficit. Post-fire bird responses mainly fitted to non-lineal responses, such as irruptive and inverse-irruptive trends; whereas water deficit matched with the inclined and bell-shaped trends. Individual site fidelity seems behind the decision of forest birds to stay or leave after a fire, leading to increase frequency of occurrence immediately after fire but this effect tends to diminish over time. In terms of the spatial characteristics of burnt areas, the majority of species showed a positive relationship with fire severity heterogeneity and the density of unburnt patches. On the other hand, salvage logging negatively affected forest bird diversity. Bird diversity also decreased with the time since fire and in the driest transects but fire heterogeneity was positive for alpha and beta bird forest diversity. The relationships between forest birds and the mosaic of burnt and unburnt areas created by wildfires plays a critical role in maintaining biodiversity in fire-prone ecosystems. Thus, limiting post-fire disturbance (e.g. salvage logging) and promoting natural regeneration could be an effective strategy that will benefit forest organisms such as birds. Maintaining unburnt patches and creating a mosaic of burn severity and vegetation will promote habitat heterogeneity and benefit a diverse range of bird species.

Clear-cuts and warming summers caused forest bird populations to decline in a southern boreal area

Land use and climate change are the main drivers of biodiversity change. However, their relative roles in the decline of biodiversity are still insufficiently known. Climate change is expected to be globally most pronounced in the boreal biome, where another key driver affecting boreal species is forestry. In NW Europe almost all forest land outside protected areas is subject to systematic forest management practices including clear-cutting, thinning and replanting. In addition, a human-caused increase in herbivores, such as ungulates, may affect the forest vegetation structure and thereby also bird species. We studied population changes of a bird community in a southern boreal area in Finland over a period of 30 years and related changes in the bird community concerning both land use and climate variables. During the study the total bird density declined by 26%, i.e., 1%/year. Long-distance migrants and residents declined the most, both by 44%, and short-distance migrants by 29%, while partial migrants increased by 29%. Clear-cuts were the most important variable explaining the decline in the total bird density and the density change of long-distance and short-distance migrants and residents. In addition, the higher summer mean temperature was related to the decline of the total bird density and short-distance migrants, which were also moderately negatively affected by the ungulate density. The increase in partial migrants was mostly due to an increase in the numbers of two bird species, the great tit Parus major and the blue tit Cyanistes caeruleus, which have benefitted from human actions in many ways. As a whole, the bird community did not recover to the level that prevailed before the clear-cuts in the study period. This indicates that recolonisation and the recovery rate by birds in maturing young stands cannot fully compensate for the loss of birds resulting from the clear-cuts. Forest management practices should adopt more biodiversity-friendly methods, such as continuous cover forestry, to better maintain populations of forest birds.

Diversified farms bolster forest-bird populations despite ongoing declines in tropical forests

While some agricultural landscapes can support wildlife in the short term, it is uncertain how well they can truly sustain wildlife populations. To compare population trends in different production systems, we sampled birds along 48 transects in mature forests, diversified farms, and intensive farms across Costa Rica from 2000 to 2017. To assess how land use influenced population trends in the 349 resident and 80 migratory species with sufficient data, we developed population models. We found, first, that 23% of species were stable in all three land use types, with the rest almost evenly split between increasing and decreasing populations. Second, in forest habitats, a slightly higher fraction was declining: 62% of the 164 species undergoing long-term population changes; nearly half of these declines occurred in forest-affiliated invertivores. Third, in diversified farms, 49% of the 230 species with population changes were declining, with 60% of these declines occurring in agriculture-affiliated species. In contrast, 51% of the species with population changes on diversified farms showed increases, primarily in forest-affiliated invertivores and frugivores. In intensive farms, 153 species showed population changes, also with similar proportions of species increasing (50%) and decreasing (50%). Declines were concentrated in agriculture-affiliated invertivores and forest-affiliated frugivores; increases occurred in many large, omnivorous species. Our findings paint a complex picture but clearly indicate that diversified farming helps sustain populations of diverse, forest-affiliated species. Despite not fully offsetting losses in forest habitats, diversified farming practices help sustain wildlife in a critical time, before possible transformation to nature-positive policies and practices.

Landscape effects on the conservation of rear-edge populations of forest passerines (Passeriformes): current patterns and prospects

Summary Landscape changes affect species abundance and drive biodiversity loss. Here we explored if habitat amount and patch aggregation shape the abundance of forest passerines within the south-western Palaearctic (Morocco). As forests in this region are affected by increasing drought and temperature, we also forecasted their trends according to current predictions of climate change and explored how landscape changes could affect bird distribution. We recorded geo-referenced occurrences of seven forest passerines that were modelled with a set of environmental variables with Maxent to predict their distribution. The occurrence probabilities provided by the models were used as surrogates for the current distribution of habitat amount and patch aggregation within the country. In addition, 190 500-m line transects scattered within the country were used to estimate local bird abundance. Results showed that bird abundance recorded in line transects was positively correlated with habitat amount and patch aggregation of landscape around transects. This supports the idea that changes in these landscape metrics affect the abundance of the study species. Climate-change projections suggest that habitat amount and patch aggregation will decline in southern sectors but will be maintained or will increase at higher elevations. Given their relationship to abundance, these landscape changes suggest that forest birds will have to shift to the northernmost and elevated sectors. These results showed that landscape management can play an important role in the conservation of rear-edge populations of forest birds and suggest that any increase in forest amount and connectivity will improve bird resilience under a global change scenario.

A re-survey in 2019-2021 of winter bird communities in the Oregon Coast Range, USA, initially surveyed in 1968-1970.

Long-term changes in bird populations during winter are poorly evaluated in many parts of the world. We re-surveyed forest bird communities during winter, 2019-2021, in seven large plots originally surveyed from 1968 through to 1970, near Corvallis, Oregon, USA by Stanley Anderson, a graduate student at Oregon State University in the 1960s. Anderson counted birds and measured forest plant communities within the forests dominated by Douglas-fir (Pseudotsugamenziesii) in the Coast Range Mountain foothills. His thesis included plot locations, summaries of vegetation characteristics and point estimates of bird densities for each plot. To our knowledge, the Anderson data represent the oldest structured survey of Pacific Northwest winter forest bird communities with density estimates. Given the paucity of similar data, we re-surveyed his plots after aligning methods with his and adding modern components (distance and time interval sampling) to facilitate comparisons of changes in abundances. We preserved more extensive metadata than were preserved from Anderson's surveys, including georeferenced point count survey locations to facilitate more precisely repeatable future re-surveys. Original surveys of winter bird populations in the Pacific Northwest, USA, based on georeferenced locations within seven large plots originally surveyed, 1968-1970. In addition to raw count data of all bird species detected, we include information from distance sampling and time-interval sampling methods. To our knowledge, this is one of the only structured surveys of winter forest bird populations in the Coast Range Mountains, USA.

Assessing the effects of Close-to-Nature Forestry on forest birds in the eastern United States: A case study and way forward

Over the last half-century, North American forest birds have experienced staggering declines, while at the same time, many metrics indicate that North American forest health has improved. While the United States has been recognised for global leadership in innovation and research in forestry, the negative impacts of high-severity harvesting methods and forest fragmentation are sometimes overlooked, and a dearth of research studies exist that assess the impacts of lower-severity forestry practices on birds. Globally, some regions are pursuing markedly different forestry practices, and one example is Europe’s Close-To-Nature Forestry (CTNF) that protects forest canopies. Historically, CTNF methods have not been promoted to help avian abundance or diversity; however, forest bird populations in Europe have increased 7% over the last two decades, and we posit whether similar forest management approaches in the eastern USA could benefit North American birds. Therefore, we sought to 1) assess the effects of forest structural characteristics on avian species richness in a CTNF-managed forest in the eastern USA; 2) compare our results to county-level eBird data to identify the challenges of assessing the impacts of forestry practices on birds at landscape scales; 3) highlight the ambiguities in current forest management guidelines for improving avian habitats in the USA and 4) suggest long-term direction for evaluating the impacts of alternative forest management practices on birds. Our case study provides preliminary evidence that CTNF-managed forests may support diverse birds of high conservation value that are normally drawn to different forest management extremes. We also provide recommendations for utilising eBird data to assess alternative forest management strategies at landscape scales and a synthesised look at the forest management guidelines provided by many of the major players in American ornithology. While there may be multiple reasons for forest bird declines noted in the past 50 years in North America, long-standing forest management practices and policies should not be overlooked.

Long-term trends in forest bird populations reflect management changes in Central European forests

In areas where forest is under long-term human influence, we can expect that forest bird populations will be affected by changes in forest management. Consequences of such forest management changes can be studied using long-term population trajectories of habitat-defined groups of bird species because various species are associated to different habitat types. Here we focused on 51 species of common forest birds in Czechia, a central European country, using data from a nation-wide breeding bird monitoring scheme collected from 1982 to 2019. Important changes in forest management took place over this time period in Europe and we asked whether the trends in Czech forest bird populations reflected these changes. For this purpose, we defined nine groups according to the species’ habitat associations in respect of stand size, vegetation layers and tree species composition. We calculated annual population indices using log-linear models with Poisson distribution for every species and composed these annual indices into multispecies indicators (MSIs). MSI of all forest birds, as well as MSIs of three groups of habitat generalists showed significant increases over the focal period. The groups of more specialized species showed more variable population trajectories: birds of the shrub layer decreased, whereas birds of the forest canopy and birds of broad-leaved trees increased, and populations of birds of forest edges, forest interior and coniferous trees were stable. These patterns in forest bird populations are consistent with forest maturation and increasing wood volume that took place in Czech forests over several decades. Such changes in forest environment are likely consequences of the change in purposes of forest management from fuel wood to timber harvesting that took place in 20th century resulting in a longer rotation period. Increases in species associated with broad-leaved trees can be explained by replacement of coniferous stands by stands of broad-leaved trees indicating recovery of forest environment towards more natural conditions as majority of the forested area would be naturally covered by broad-leaved vegetation in Czechia. In addition, the occurrence of some broad-leaved trees within coniferous stands, a common practice in Czech forestry, may also provide benefits to populations of bird species associated with broad-leaved trees. Taken together, these results support suitability of current forest management for forest birds that corresponds to recent findings from several other European regions. However, we should take this conclusion with caution because the increases of generalist birds may be also driven by factors outside the forest environment.

Varied diversification patterns and distinct demographic trajectories in Ethiopian montane forest bird (Aves: Passeriformes) populations separated by the Great Rift Valley.

Taxon-specific characteristics and extrinsic climatic and geological forces may both shape population differentiation and speciation. In geographically and taxonomically focused investigations, differentiation may occur synchronously as species respond to the same external conditions. Conversely, when evolution is investigated in taxa with largely varying traits, population differentiation and speciation is complex and shaped by interactions of Earth's template and species-specific traits. As such, it is important to characterize evolutionary histories broadly across the tree of life, especially in geographic regions that are exceptionally diverse and under pressures from human activities such as in biodiversity hotspots. Here, using whole-genome sequencing data, we characterize genomic variation in populations of six Ethiopian Highlands forest bird species separated by a lowland biogeographic barrier, the Great Rift Valley (GRV). In all six species, populations on either side of the GRV exhibited significant but varying levels of genetic differentiation. Species' dispersal ability was negatively correlated with levels of population differentiation. Isolation with migration models indicated varied patterns of population differentiation and connectivity among populations of the focal species. We found that demographic histories-estimated for each individual-varied by both species and population but were consistent between individuals of the same species and sampling region. We found that genomic diversity varied by half an order of magnitude across species, and that this variation could largely be explained by the harmonic mean of effective population size over the past 200,000years. Overall, we found that even in highly dispersive species like birds, the GRV acts as a substantial biogeographic barrier.

Ecological and geographical marginality in rear edge populations of Palaearctic forest birds

AbstractAimThe centre–periphery hypothesis predicts that habitat suitability will decrease at the edge of a species’ range, a pattern often questioned by empirical data. Here we explore if habitat suitability decreases southwards and shapes the abundance distribution of rear edge populations of forest birds within the restricted geographical setting of the south‐western Palaearctic. We also test if birds endemic to the area fit more poorly to the latitudinal decrease in habitat suitability due to the putative effect of adaptations to regional conditions.LocationNorth‐western Africa (Morocco).TaxonPasserines (11 species).MethodsBird occurrences were used to model species distribution and line transects were used to estimate bird abundance. Occurrence probabilities provided by species distribution models were used to display the spatial patterning of habitat suitability. Habitat suitability was employed to predict abundance after controlling for the effect of the distance to some regional source areas of forest birds (tree covered large areas). The species were classified as North African endemic according to an updated review of their taxonomic status.ResultsHabitat suitability decreased southwards, supporting the predicted relationship between ecological and geographical marginality in most species. Abundance was positively correlated with habitat suitability and negatively correlated with distance to source areas. The taxonomic status of birds did not affect the patterns.Main conclusionsThe southward decrease in habitat suitability predicted by the centre–periphery hypothesis shapes the distribution of rear edge populations of forest birds within the south‐western Palaearctic. As most of these populations are endemic, the results suggest that they track the gradients in isolation within the geographical setting of north‐western Africa. These results support the vulnerability of these isolated, peripheral populations of forest birds to large‐scale environmental changes in a region under the effect of increasing drought and temperature.

Disturbed Amazonian forests support diminished breeding bird communities

Abstract Twenty percent of the Brazilian Amazon has now been deforested, and deforestation rates are increasing. This compels us to evaluate the conservation potential of human-modified landscapes; yet the ecological value of regenerating and fragmented Amazonian forests remains poorly understood. To date, most faunal studies in disturbed forests have examined metrics derived from presence or abundance. Although valuable, these data cannot tell us how old-growth species are using disturbed forests. In this study, we complement presence data with breeding observations to assess more comprehensively the habitat quality of disturbed forests in central Amazonia. We synthesized breeding evidence from standardized passive mist-netting, mixed-species flock-following, and opportunistic nest-searching across a full annual cycle in 30–35-year-old secondary forest, 10-ha fragments, and undisturbed forest. We then used multistate occupancy models to estimate the number of undetected breeding species in each forest type, which illustrated that fewer species bred in secondary forest (–43%) and 10-ha fragments (–17%) than in undisturbed forest. Both these losses are larger than the associated decrease in species richness alone (–17% and –10%, respectively). Notably, we confirmed breeding by only three terrestrial and near-ground insectivores in fragments and secondary forest combined (of the 9 species found in undisturbed forest). Disturbed forests also supported fewer breeding individuals (–35% to –50%) and, in secondary forest, fewer successful breeding attempts (–24%). Encouragingly, however, some forest-dependent birds are breeding and producing fledglings in disturbed forests, including representatives from almost every guild. This was especially apparent for mixed-species flocks and army-ant followers, two guilds that have historically been considered vulnerable to anthropogenic disturbance. Therefore, despite a loss of breeding habitat in disturbed forests, these data suggest that landscapes composed of regenerating forest and small fragments have conservation potential for forest bird populations.

Assessing terrestrial wildlife populations in the Toronto and Region Area of Concern

Beneficial use impairments (BUIs) under the Great Lakes Water Quality Agreement identify environmental issues requiring remedial action within the Great Lakes Areas of Concern (AOCs). We conducted this study to support the assessment of the wildlife component of BUI 3: degradation of fish and wildlife populations. We compared bird and amphibian (frogs and toads) data from the Toronto and Region Conservation Authority’s Terrestrial Long-term Monitoring Program in the Toronto and Region AOC to an adjacent, but otherwise similar, reference watershed, Duffins Creek. Twelve of 13 targets were met within the AOC for forest bird, wetland bird, meadow bird and amphibian populations based on averages of mean annual values at sites within the AOC that were within two standard deviations of averages at sites in the Duffins Creek reference watershed between 2008 and 2017. Even though wildlife populations within the AOC were within the normal range of variability expected from a reference watershed, they were often at lower levels than within the Duffins Creek reference watershed. In addition, forest bird and amphibian populations were negatively affected by urbanization within the AOC and meadow bird indices declined. We conclude that while wildlife populations within the AOC currently meet targets for BUI 3, they continue to be negatively impacted by numerous stressors that are primarily related to past and ongoing urbanization. Thus, continued restoration of wildlife habitat and protection of existing habitat within the AOC is highly recommended.

An analysis of trends, uncertainty and species selection shows contrasting trends of widespread forest and farmland birds in Europe

Abstract 1. Composite, multispecies biodiversity indices are increasingly used to report against international and national environmental commitments and targets, the Wild Bird Index being a prominent example in Europe, but methods to assess trends, error and species selection for such indices are poorly developed. 2. In this study, we compare methods to compute multispecies supranational indices and explore different approaches to trend and error estimation, the presentation of indices, and species selection. We do so using population trend data on forest and farmland birds from 28 European countries, 1980–2015. 3. We find relative stability in common European forest bird populations over this period, but a severe decline in farmland bird populations. Altering the benchmark year affects index characteristics and ease of interpretation. We show that using annual species’ indices and their SEs to calculate confidence intervals delivers greater precision in index estimates than bootstrapping across species. The inclusion of individual species within indices has limited leverage on index characteristics, but subjective selection of species based on specialisation has the potential to generate bias. 4. Multispecies indices are valuable policy-relevant tools for describing biodiversity health. Their calculation and presentation need to be tailored to meet specific policy objectives, and they must be supported by clear interpretative information. We recommend methods for indicator analysis, forms of presentation, and the adoption of an objective species selection protocol to ensure indicators are representative and sensitive to environmental change.

Multi-scale habitat requirements of forest bird species in a highly fragmented landscape

Land use changes in Europe have resulted in forest loss and fragmentation that have been proved to be key factors driving the decline of various forest bird populations. Quantifying the environmental factors which allow the persistence of forest birds in highly modified landscapes is therefore essential to enhance conservation efforts. In the present study, we defined the environmental factors determining the distribution of seven forest birds (Picus viridis, Dendrocopos major, Dryobates minor, Garrulus glandarius, Poecile palustris, Sitta europea, Aegithalos caudatus) in a central portion of the Po Plain, northern Italy. In the study area, less than 12% of the whole territory is covered by broad-leaved forests (concentrated along the main rivers) and tree plantations. To obtain a full picture of the habitat requirements of these seven forest bird species, we first conducted a survey of their populations using the point count method during the 2015 breeding season, following which, using resource selection functions, we investigated the effect of environmental variables on each species, mainly focusing on the role of natural forests and tree plantations. The effect of variables was assessed both at a home-range scale and at four different scales in the surrounding context. Tree plantations, in addition to natural forests, proved to be important for most of the seven species studied, including forest specialists such as Dryobates minor and Poecile palustris. The distribution of forest species was better explained by habitat amount than by habitat configuration, both at the home-range scale and in the surrounding context. As expected, Sitta europaea was the most sensitive species to land use changes, and broad-leaved natural forests were essential for its persistence in the landscape.

Increased nesting success of Hawaii Elepaio in response to the removal of invasive black rats

Abstract In Hawaii and other oceanic islands with few native land mammals, black rats (Rattus rattus) are among the most damaging invasive vertebrate species to native forest bird populations and habitats, due to their arboreal behavior and generalist foraging habits and habitat use. We evaluated the nesting response of Hawaii Elepaio (Chasiempis sandwichensis; Monarchidae), a generalist insectivore, to the removal of black rats using rodenticide in a before-after-control-impact study in high- and low-elevation mesic montane habitat recovering from long-term damage from introduced ungulates and weeds. We monitored nesting success and rat abundance during 2015–2016 before applying rodenticide bait in 2017 to remove rats from two 700 × 700 m treatment plots that were paired with 2 nontreatment plots of the same size. Rat abundance was reduced by 90% during treatment, with combined variables treatment and elevation best explaining the change using GLM methods and AIC model selection. The daily survival rate (DSR) of nests (n = 191) was greater on treated plots after rodenticide application (mean ± SE = 0.980 ± 0.004 treatment; 0.964 ± 0.004 nontreatment), modeled nest success increased from 29% to 50%, and apparent nest success (number of successful nests per total nests) increased from 37% to 52%. The most informative model for predicting DSR included the effect of treatment. Predation by rats was documented at 3 of 16 nests using video surveillance, and we observed additional evidence of rat predation during in-person nest monitoring. Rats targeted adults on the nest and sometimes removed intact eggs, leaving little trace of their activity. Our results demonstrate that reducing rat predation can immediately improve the nesting success of even a common bird species in habitat with a long history of forest restoration. Sustained predator control may be critical to accelerating the recovery of native forest bird communities.

Impacts of the Northwest Forest Plan on forest composition and bird populations

The Northwest Forest Plan (NWFP) initiated one of the most sweeping changes to forest management in the world, affecting 10 million hectares of federal land. The NWFP is a science-based plan incorporating monitoring and adaptive management and provides a unique opportunity to evaluate the influence of policy. We used >25 years of region-wide bird surveys, forest data, and land-ownership maps to test this policy's effect on biodiversity. Clearcutting decreased rapidly, and we expected populations of older-forest-associated birds to stabilize on federal land, but to continue declining on private industrial lands where clearcutting continued. In contrast, we expected declines in early-seral-associated species on federal land because of reduced anthropogenic disturbance since the NWFP. Bayesian hierarchical models revealed that bird species' population trends tracked changes in forest composition. However, against our expectations, declines of birds associated with older forests accelerated. These declines are partly explained by losses of older forests due to fire on federal land and continued clearcutting elsewhere. Indeed, the NWFP anticipated that reversing declines of older forests would take time. Overall, the early-seral ecosystem area was stable, but declined in two ecoregions-the Coast Range and Cascades-along with early-seral bird populations. Although the NWFP halted clearcutting on federal land, this has so far been insufficient to reverse declines in older-forest-associated bird populations. These findings underscore the importance of continuing to prioritize older forests under the NWFP and ensuring that the recently proposed creation of early-seral ecosystems does not impede the conservation and development of older-forest structure.

Positive association between forest management, environmental change, and forest bird abundance

BackgroundThe global decrease in wildlife populations, especially birds, is mainly due to land use change and increasing intensity of land use (Parmesan and Yohe 2003). However, impacts of management tools to mitigate biodiversity loss at regional and global scales are less apparent in forest regions that have a constant forest area, and which did not suffer from habitat degradation, and where forests are sustainably managed, such as in Central Europe or the northeastern USA. A biodiversity assessment for Germany suggested, for example, that bird populations were constant (Bundesamt für Naturschutz 2015).ResultsThis study shows that changes in the environment and in forest management over the past 45 years have had a significant, positive effect on the abundance of non-migratory forest bird species in Central Europe. Economy (timber prices and GDP), forest management (timber harvest and mixed forest area), and environmental factors (atmospheric CO2 concentration and nitrogen deposition) were investigated together with changes in abundances of migratory and non-migratory forest birds using partial least squares path modeling. Climate change, resulting in longer seasons and milder winters, and forest management, promoting tree diversity, were significantly positively related to the abundance of non-migratory forest birds and explained 92% of the variation in their abundance in Europe. Regionally-migrating forest birds had stable populations with large variation, while birds migrating across continents declined in recent decades, suggesting significant, contrasting changes in bird populations in Europe. In northeastern North America we also found evidence that non-migratory forests have experienced long-term increases in abundance, and this increase was related to management. The increase of populations of non-migratory forest birds in Europe and North America is associated with an increase in structural diversity and disturbances at the landscape level.ConclusionsOur results suggest that reports about bird decline in forests should separate between migratory and non-migratory bird species. Efforts to mitigate the general decline in bird abundance should focus on land-use systems other than forests and support sustainable forest management independent of economic conditions.

No evidence of critical slowing down in two endangered Hawaiian honeycreepers.

There is debate about the current population trends and predicted short-term fates of the endangered forest birds, Hawai`i Creeper (Loxops mana) and Hawai`i `Ākepa (L. coccineus). Using long-term population size estimates, some studies report forest bird populations as stable or increasing, while other studies report signs of population decline or impending extinction associated with introduced Japanese White-eye (Zosterops japonicus) increase. Reliable predictors of impending population collapse, well before the collapse begins, have been reported in simulations and microcosm experiments. In these studies, statistical indicators of critical slowing down, a phenomenon characterized by longer recovery rates after population size perturbation, are reported to be early warning signals of an impending regime shift observable prior to the tipping point. While the conservation applications of these metrics are commonly discussed, early warning signal detection methods are rarely applied to population size data from natural populations, so their efficacy and utility in species management remain unclear. We evaluated two time series of state-space abundance estimates (1987–2012) from Hakalau Forest National Wildlife Refuge, Hawai`i to test for evidence of early warning signals of impending population collapse for the Hawai`i Creeper and Hawai`i `Ākepa. We looked for signals throughout the time series, and prior to 2000, when white-eye abundance began increasing. We found no evidence for either species of increasing variance, autocorrelation, or skewness, which are commonly reported early warning signals. We calculated linear rather than ordinary skewness because the latter is biased, particularly for small sample sizes. Furthermore, we identified break-points in trends over time for both endangered species, indicating shifts in slopes away from strongly increasing trends, but they were only weakly supported by Bayesian change-point analyses (i.e., no step-wise changes in abundance). The break-point and change-point test results, in addition to the early warning signal analyses, support that the two populations do not appear to show signs of critical slowing down or decline.