Estimating Wind Turbine-Caused Bird Mortality

ABSTRACT Mortality estimates are needed of birds and bats killed by wind turbines because wind power generation is rapidly expanding worldwide. A mortality estimate is based on the number of fatalities assumed caused by wind turbines and found during periodic searches, plus the estimated number not found. The 2 most commonly used estimators adjust mortality estimates by rates of searcher detection and scavenger removal of carcasses. However, searcher detection trials can be biased by the species used in the trial, the number volitionally placed for a given fatality search, and the disposition of the carcass on the ground. Scavenger removal trials can be biased by the metric representing removal rate, the number of carcasses placed at once, the duration of the trial, species used, whether carcasses were frozen, whether carcasses included injuries consistent with wind turbine collisions, season, distance from the wind turbines, and general location. I summarized searcher detection rates among reported trials, and I developed models to predict the proportion of carcasses remaining since the last fatality search. The summaries I present can be used to adjust previous and future estimates of mortality to improve comparability. I also identify research directions to better understand these and other adjustments needed to compare mortality estimates among wind farms.     

Response to Huso and Erickson's comments on novel scavenger removal trials

Trials involving volitionally placed carcasses are often used to estimate the portion of the collision-caused fatality population that is undetected by periodic fatality searches at wind turbines. Huso and Erickson criticized our paper reporting on a comparison of carcass persistence rates between what we termed conventional versus novel approaches to these trials. In our novel approach, we measured carcass persistence rates by placing only 1–2 fresh carcasses per week, instead of the typical 10 or more carcasses at a time, often using found carcasses of unknown time since death. Huso and Erickson directed most of their critique to this novel aspect of our approach, although the novelty of our approach also included the use of event-triggered camera traps, which we used to record exact times of removals and to identify vertebrate scavenger species responsible for the removals. In our replies to Huso and Erickson's major criticisms, we acknowledge flaws in our field methods for arriving at fatality rate estimates, but we also point out the larger flaws in the methods used by Huso and Erickson, especially in their use of mean days to removal as a measure of carcass persistence. We conclude by introducing a more appropriate detection trial, which combines searcher detection and scavenger removal trials, and integrates this detection trial into periodic fatality monitoring. © 2013 The Wildlife Society.     

Factors affecting searcher efficiency and scavenger removal of bat carcasses in Neotropical wind facilities

Bat fatalities at wind facilities have been reported worldwide, and environmental impact assessments depend on searches for carcasses around wind turbines to quantify impacts. Some of the carcasses may go undetected by search teams or be removed by scavengers during search intervals, so these biases must be evaluated and taken into account in fatality estimation. We investigated the influence of different factors on searcher efficiency and scavenger removal in a dry forest area in northeastern Brazil, one of the regions with the highest density of wind turbines in the Neotropics. We conducted searcher efficiency and scavenger removal trials around 34 wind turbines from January 2017 to January 2018. Searcher efficiency was influenced by cover type, season, and carcass size, ranging between 12% for small bats in shrub vegetation during the rainy season and 96% for large bats in absent or sparse vegetation during the dry season. Carcass type and season affected scavenger removal; carcass persistence time was shorter for chicks (1.2 days) than for bats and mice (2.1 days), and the probability of a carcass persisting for a whole day was higher in the rainy season, while the probability of carcass persistence for 7, 14, and 28 days was higher in the dry season. The scavenger community was composed of canids, birds of prey, and insects, with systematic removal of carcasses by the crab-eating fox (Cerdocyon thous) throughout the year and by dung beetles in the rainy season. Based on our findings, impact assessments of wind facilities on bats should conduct searcher efficiency trials in all seasons and cover types around wind turbines, using bat carcasses or models of different sizes. Scavenger removal trials should cover all seasons as well, and use mouse carcasses (but not chick carcasses) as surrogates for bats.     

Novel Scavenger Removal Trials Increase Wind Turbine—Caused Avian Fatality Estimates

ABSTRACT For comparing impacts of bird and bat collisions with wind turbines, investigators estimate fatalities/megawatt (MW) of rated capacity/year, based on periodic carcass searches and trials used to estimate carcasses not found due to scavenger removal and searcher error. However, scavenger trials typically place ≥10 carcasses at once within small areas already supplying scavengers with carcasses deposited by wind turbines, so scavengers may be unable to process and remove all placed carcasses. To avoid scavenger swamping, which might bias fatality estimates low, we placed only 1–5 bird carcasses at a time amongst 52 wind turbines in our 249.7-ha study area, each carcass monitored by a motion-activated camera. Scavengers removed 50 of 63 carcasses, averaging 4.45 days to the first scavenging event. By 15 days, which corresponded with most of our search intervals, scavengers removed 0% and 67% of large-bodied raptors placed in winter and summer, respectively, and 15% and 71% of small birds placed in winter and summer, respectively. By 15 days, scavengers removed 42% of large raptors as compared to 15% removed in conventional trials, and scavengers removed 62% of small birds as compared to 52% removed in conventional trials. Based on our methodology, we estimated mean annual fatalities caused by 21.9 MW of wind turbines in Vasco Caves Regional Preserve (within Altamont Pass Wind Resource Area, California, USA) were 13 red-tailed hawks (Buteo jamaicensis), 12 barn owls (Tyto alba), 18 burrowing owls (Athene cunicularia), 48 total raptors, and 99 total birds. Compared to fatality rates estimated from conventional scavenger trials, our estimates were nearly 3 times higher for red-tailed hawk and barn owl, 68% higher for all raptors, and 67% higher for all birds. We also found that deaths/gigawatt-hour of power generation declined quickly with increasing capacity factor among wind turbines, indicating collision hazard increased with greater intermittency in turbine operations. Fatality monitoring at wind turbines might improve by using scavenger removal trials free of scavenger swamping and by relating fatality rates to power output data in addition to rated capacity (i.e., turbine size). The resulting greater precision in mortality estimates will assist wildlife managers to assess wind farm impacts and to more accurately measure the effects of mitigation measures implemented to lessen those impacts.     

Dogs Detect Larger Wind Energy Effects on Bats and Birds

As wind turbine-caused mortality of birds and bats increases with increasing wind energy capacity, accurate fatality estimates are needed to assess effects, identify collision factors, and formulate mitigation. Finding a larger proportion of collision victims reduces the magnitude of adjustment for the proportion not found, thus reducing opportunities for bias. We tested detection dogs in trials of bat and small-bird carcasses placed randomly in routine fatality monitoring at the Buena Vista and Golden Hills Wind Energy projects, California, USA, 2017. Of trial carcasses placed and confirmed available before next-day fatality searches, dogs detected 96% of bats and 90% of small birds, whereas humans at a neighboring wind project detected 6% of bats and 30% of small birds. At Golden Hills dogs found 71 bat fatalities in 55 searches compared to 1 bat found by humans in 69 searches within the same search plots over the same season. Dog detection rates of trial carcasses remained unchanged with distance from turbine, and dogs found more fatalities than did humans at greater distances from turbines. Patterns of fatalities found by dogs within search plots indicated 20% of birds and 4–14% of bats remained undetected outside search plots at Buena Vista and Golden Hills. Dogs also increased estimates of carcass persistence by finding detection trial carcasses that the trial administrator had erroneously concluded were removed. Compared to human searches, dog searches resulted in fatality estimates up to 6.4 and 2.7 times higher for bats and small birds, respectively, along with higher relative precision and >90% lower cost per fatality detection. © 2020 The Authors. The Journal of Wildlife Management published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.     

Dogs as a tool to improve bird-strike mortality estimates at wind farms

Management of avian populations near anthropogenic infrastructures, specifically wind farms, has been hampered due to biased bird fatalities estimates. Currently, these estimations are based on field surveys performed by humans, which is a method with low efficiency and accuracy. Detection dogs have been used for decades to assist humans, and their use for wildlife surveys is of increasing interest to scientists and wildlife managers. We evaluate the accuracy rates of human and dog-handler teams in real field conditions to address if dogs could be used instead of humans for bird carcass searches. Furthermore, to verify the efficiency of detection dogs (determined by the time spent to detect each bird carcass) searching for bird carcasses, we investigate the influence of several factors that affect the performance of dogs (carcass decomposition condition, distance to the target and weather conditions). Results indicate that dogs are more accurate than humans, independently of vegetation density. Furthermore, carcass decomposition condition, distances to the carcass and weather conditions significantly affect the efficiency of working dogs. The influence of these factors on detection time was minor. Results demonstrate the usefulness of dogs in field surveys to improve bird-strike mortality estimates at wind farms and other anthropogenic structures that cause bird fatalities worldwide.     

Bird Mortality in the Altamont Pass Wind Resource Area,California

Abstract The 165-km² Altamont Pass Wind Resource Area (APWRA) in west-central California includes 5,400 wind turbines, each rated to generate between 40 kW and 400 kW of electric power, or 580 MW total. Many birds residing or passing through the area are killed by collisions with these wind turbines. We searched for bird carcasses within 50 m of 4,074 wind turbines for periods ranging from 6 months to 4.5 years. Using mortality estimates adjusted for searcher detection and scavenger removal rates, we estimated the annual wind turbine–caused bird fatalities to number 67 (80% CI = 25–109) golden eagles (Aquila chrysaetos), 188 (80% CI = 116–259) red-tailed hawks (Buteo jamaicensis), 348 (80% CI = −49 to 749) American kestrels (Falco sparverius), 440 (80% CI = −133 to 1,013) burrowing owls (Athene cunicularia hypugaea), 1,127 (80% CI = −23 to 2,277) raptors, and 2,710 (80% CI = −6,100 to 11,520) birds. Adjusted mortality estimates were most sensitive to scavenger removal rate, which relates to the amount of time between fatality searches. New on-site studies of scavenger removal rates might warrant revising mortality estimates for some small-bodied bird species, although we cannot predict how the mortality estimates would change. Given the magnitude of our mortality estimates, regulatory agencies and the public should decide whether to enforce laws intended to protect species killed by APWRA wind turbines, and given the imprecision of our estimates, directed research is needed of sources of error and bias for use in studies of bird collisions wherever wind farms are developed. Precision of mortality estimates could be improved by deploying technology to remotely detect collisions and by making wind turbine power output data available to researchers so that the number of fatalities can be related directly to the actual power output of the wind turbine since the last fatality search.     

Utility-scale solar impacts to volant wildlife

To reduce carbon emissions from fossil fuel combustion, United States government agencies, including those in California, initiated aggressive programs to hasten development of utility-scale solar energy. Much of California's early development of solar energy occurred in deserts and annual grasslands, much of it on public land. Measurement of solar energy's impacts to wildlife has been limited to mortality caused by features of solar facilities, and has yet to include impacts from habitat loss and energy transmission. To estimate species-specific bird and bat fatality rates and statewide mortality, I reviewed reports of fatality monitoring from 1982 to 2018 at 14 projects, which varied in duration, level of sampling, search interval, search method, and carcass detection trials. Because most monitors performed carcass detection trials using species of birds whose members were larger than birds and bats found as fatalities, I bridged the monitors' onsite trial results to offsite trial results based on the same methods but which also measured detection probabilities across the full range of body sizes of species represented by fatalities. This bridge preserved the project site's effects on detection probabilities while more fully adjusting for the effects of body size. My fatality estimates consistently exceeded those reported. Projected to California's installed capacity of 1,948.8 MW of solar thermal and 12,220 MW of photovoltaic (PV) panels in 2020 (14,168.8 MW total), reported estimates would support an annual statewide fatality estimate of 37,546 birds and 207 bats, whereas I estimated fatalities of 267,732 birds and 11,418 bats. Fatalities/MW/year averaged 11.61 birds and 0.06 bats at PV projects and 64.61 birds and 5.49 bats at solar thermal projects. Fatalities/km/year averaged 113.16 birds and zero bats at generation tie-ins, and 14.44 birds and 2.56 bats along perimeter fences. Bird fatality rates averaged 3 times higher at PV projects searched by foot rather than car. They were usually biased low by insufficient monitoring duration and by the 22% of fatalities that monitors could not identify to species. I estimated that construction grading for solar projects removed habitat that otherwise would have supported nearly 300,000 birds/year. I recommend that utility-scale solar energy development be slowed to improve project decision-making, impacts assessment, fatality monitoring, mitigation efficacy, and oversight.     

Gobbling across landscapes: Eastern wild turkey distribution and occupancy–habitat associations

Extensive restoration and translocation efforts beginning in the mid‐20th century helped to reestablish eastern wild turkeys (Meleagris gallopavo silvestris) throughout their ancestral range. The adaptability of wild turkeys resulted in further population expansion in regions that were considered unfavorable during initial reintroductions across the northern United States. Identification and understanding of species distributions and contemporary habitat associations are important for guiding effective conservation and management strategies across different ecological landscapes. To investigate differences in wild turkey distribution across two contrasting regions, heavily forested northern Wisconsin, USA, and predominately agricultural southeast Wisconsin, we conducted 3050 gobbling call‐count surveys from March to May of 2014–2018 and used multiseason correlated‐replicate occupancy models to evaluate occupancy–habitat associations and distributions of wild turkeys in each study region. Detection probabilities varied widely and were influenced by sampling period, time of day, and wind speed. Spatial autocorrelation between successive stations was prevalent along survey routes but was stronger in our northern study area. In heavily forested northern Wisconsin, turkeys were more likely to occupy areas characterized by moderate availability of open land cover. Conversely, large agricultural fields decreased the likelihood of turkey occupancy in southeast Wisconsin, but occupancy probability increased as upland hardwood forest cover became more aggregated on the landscape. Turkeys in northern Wisconsin were more likely to occupy landscapes with less snow cover and a higher percentage of row crops planted in corn. However, we were unable to find supporting evidence in either study area that the abandonment of turkeys from survey routes was associated with snow depth or with the percentage of agricultural cover. Spatially, model‐predicted estimates of patch‐specific occupancy indicated turkey distribution was nonuniform across northern and southeast Wisconsin. Our findings demonstrated that the environmental constraints of turkey occupancy varied across the latitudinal gradient of the state with open cover, snow, and row crops being influential in the north, and agricultural areas and hardwood forest cover important in the southeast. These forces contribute to nonstationarity in wild turkey–environment relationships. Key habitat–occupancy associations identified in our results can be used to prioritize and strategically target management efforts and resources in areas that are more likely to harbor sustainable turkey populations.     

A model for the hydrological cycle of a forested catchment and assessment of the changes caused in water balance by cuttings

A physicomathematical model of the hydrological cycle in a forested catchment was constructed. This model describes the interception of liquid and solid precipitation by tree crowns; snow accumulation and melting; vertical transfer of moisture in soil and its evaporation; and surface, subsurface, and channeled runoffs. The model was calibrated and verified using the observation data for the completely forested Taezhnyi catchment within the area of the Valdai Water Balance Station. Then the model was used to assess possible changes in the hydrological cycle after clear cuttings in this catchment. The values of model parameters were compared to the corresponding soil characteristics in the adjacent treeless (field) Usad’evskii catchment. Modeling results demonstrate that the average water reserve in the snow cover before melting can increase by 15% after forest cutting in the Taezhnyi catchment. The losses for snow sublimation are reduced almost two-fold. The snow melting intensity increases by 30% and its duration decreases by 10 days. The annual runoff after cutting increases by 7–10%; however, the seasonal distribution of the runoff and the constituents of the water balance change to a greater degree. During spring flood, the maximal water discharge in the forested catchment is 50% smaller than after forest cutting. The duration of spring flood after cutting is reduced by 5–7 days. The changes in the hydrological cycle depending on the age-related alteration in leaf area index were also studied.     

Automated extraction of bank angles from bird‐borne video footage using open‐source software

The use of miniaturized video cameras to study the at‐sea behavior of flying seabirds has increased in recent years. These cameras allow researchers to record several behaviors that were not previously possible to observe. However, video recorders produce large amounts of data and videos can often be time‐consuming to analyze. We present a new technique using open‐source software to extract bank angles from bird‐borne video footage. Bank angle is a key facet of dynamic soaring, which allows albatrosses and petrels to efficiently search vast areas of ocean for food. Miniaturized video cameras were deployed on 28 Wandering Albatrosses (Diomedea exulans) on Marion Island (one of the two Prince Edward Islands) from 2016 to 2018. The OpenCV library for the Python programming language was used to extract the angle of the horizon relative to the bird’s body (= bank angle) from footage when the birds were flying using a series of steps focused on edge detection. The extracted angles were not significantly different from angles measured manually by three independent observers, thus being a valid method to measure bank angles. Image quality, high wind speeds, and sunlight all influenced the accuracy of angle estimates, but post‐processing eliminated most of these errors. Birds flew most often with cross‐winds (58%) and tailwinds (39%), resulting in skewed distributions of bank angles when birds turned into the wind more often. Higher wind speeds resulted in extreme bank angles (maximum observed was 94°). We present a novel method for measuring postural data from seabirds that can be used to describe the fine‐scale movements of the dynamic‐soaring cycle. Birds appeared to alter their bank angle in response to varying wind conditions to counter wind drift associated with the prevailing westerly winds in the Southern Ocean. These data, in combination with fine‐scale positional data, may lead to new insights into dynamic‐soaring flight.     

Curtailment and acoustic deterrents reduce bat mortality at wind farms

The impacts of wind energy on bat populations is a growing concern because wind turbine blades can strike and kill bats, and wind turbine development is increasing. We tested the effectiveness of 2 management actions at 2 wind-energy facilities for reducing bat fatalities: curtailing turbine operation when wind speeds were <5.0 m/second and combining curtailment with an acoustic bat deterrent developed by NRG Systems. We measured the effectiveness of the management actions using differences in counts of bat carcasses quantified by daily and twice-per-week standardized carcass searches of cleared plots below turbines, and field trials that estimated searcher efficiency and carcass persistence. We studied turbines located at 2 adjacent wind-energy facilities in northeast Illinois, USA, during fall migration (1 Aug–15 Oct) in 2018. We estimated the effectiveness of each management action using a generalized linear mixed-effects model with several covariates. Curtailment alone reduced overall bat mortality by 42.5% but did not reduce silver-haired bat (Lasionycteris noctivagans) mortality. Overall bat fatality rates were 66.9% lower at curtailed turbines with acoustic deterrents compared to turbines that operated at manufacturer cut-in speed. Curtailment and the deterrent reduced bat mortality to varying degrees between species, ranging from 58.1% for eastern red bats (Lasiurus borealis) to 94.4 for big brown bats (Eptesicus fuscus). Hoary (Lasiurus cinereus) and silver-haired bat mortality was reduced by 71.4% and 71.6%, respectively. Our study lacked a deterrent-only treatment group because of the expense of acoustic deterrents. We estimated the additional reduction in mortality with concurrent deployment of the acoustic deterrent and curtailment under the assumption that curtailment and the acoustic deterrent would have reduced mortality by the same percentage at adjacent wind-energy facilities. Acoustic deterrents resulted in 31.6%, 17.4%, and 66.7% additional reductions of bat mortality compared to curtailment alone for eastern red bat, hoary bat, and silver-haired bat, respectively. The effectiveness of acoustic deterrents for reducing bat mortality at turbines with rotor-swept area diameters >110 m is unknown because high frequency sound attenuates quickly, which reduces coverage of rotor-swept areas. Management actions should consider species differences in the ability of curtailment and deterrents to reduce bat mortality and increase energy production.     

Snow evens fragmentation effects and food determines overwintering success in ground-dwelling voles

Climate instability strongly affects overwintering conditions in organisms living in a strongly seasonal environment and consequently their survival and population dynamics. Food, predation and density effects are also strong during winter, but the effect of fragmentation of ground vegetation on ground-dwelling small mammals is unknown. Here, we report the results of a winter experiment on the effects of habitat fragmentation and food on experimental overwintering populations of bank voles Myodes glareolus. The study was conducted in large outdoor enclosures containing one large, two medium-sized or four small habitat patches or the total enclosure area covered with protective tall-grass habitat. During the stable snow cover of midwinter, only food affected the overwintering success, body condition, trappability and earlier onset of breeding in bank voles. However, after the snow thaw in spring, habitat fragmentation gained importance again, and breeding activities increased the movements of voles in the most fragmented habitat. The use of an open, risky matrix area increased along the habitat fragmentation. Our experiment showed that long-lasting stable snow cover protects overwintering individuals in otherwise exposed and risky ground habitats. We conclude that a stable winter climate and snow cover should even out habitat fragmentation effects on small mammals. However, along prolonged snow-free early winter and in an earlier spring thaw, this means loss of protection by snow cover both in terms of thermoregulation and predation. Thus, habitat cover is important for the survival of small ground-dwelling boreal mammals also during the non-breeding season.     

Agriculture erases climate‐driven β‐diversity in Neotropical bird communities

Earth is experiencing multiple global changes that will, together, determine the fate of many species. Yet, how biological communities respond to concurrent stressors at local‐to‐regional scales remains largely unknown. In particular, understanding how local habitat conversion interacts with regional climate change to shape patterns in β‐diversity—differences among sites in their species compositions—is critical to forecast communities in the Anthropocene. Here, we study patterns in bird β‐diversity across land‐use and precipitation gradients in Costa Rica. We mapped forest cover, modeled regional precipitation, and collected data on bird community composition, vegetation structure, and tree diversity across 120 sites on 20 farms to answer three questions. First, do bird communities respond more strongly to changes in land use or climate in northwest Costa Rica? Second, does habitat conversion eliminate β‐diversity across climate gradients? Third, does regional climate control how communities respond to habitat conversion and, if so, how? After correcting for imperfect detection, we found that local land‐use determined community shifts along the climate gradient. In forests, bird communities were distinct between sites that differed in vegetation structure or precipitation. In agriculture, however, vegetation structure was more uniform, contributing to 7%–11% less bird turnover than in forests. In addition, bird responses to agriculture and climate were linked: agricultural communities across the precipitation gradient shared more species with dry than wet forest communities. These findings suggest that habitat conversion and anticipated climate drying will act together to exacerbate biotic homogenization.     

黑龙江流域积雪覆盖时空变化遥感监测

利用MODIS双星数据对黑龙江流域2003—2012年的积雪覆盖面积进行提取和验证,然后基于合成的数据分析研究区积雪覆盖面积的季节和年际变化.结果表明: 双星合成降低了云的影响,总体精度>91%,可以满足分析和研究需求.研究期间,黑龙江流域积雪覆盖面积存在显著的季节变化,7、8月的积雪最少,几乎为零,1月积雪覆盖面积最大,占流域的80%以上.2003—2004、2009—2010年冬季积雪覆盖面积较高(>180×10⁴ km²),2011年冬季最大积雪覆盖面积较低(150×10⁴ km²).积雪覆盖的年际变化与年平均气温和平均降水量的波动存在一定的对应关系:积雪覆盖面积较低年份对应的年降水量较少、平均气温较高,反之亦然.2003—2012年,研究区5、6月的积雪覆盖面积呈减少趋势,降水量增加和气温的升高与积雪覆盖面积减少紧密相连.     

From shade- to sun-grown perennial crops in Sulawesi, Indonesia: implications for biodiversity conservation and soil fertility

Traditional, complex forest farming systems are increasingly convertedto sun-grown monocultures throughout the tropics. Biophysical, soil andbiodiversity effects associated with sun- vs. shade-grown coffee and cacao wereinvestigated in a case study in Sulawesi, Indonesia. Canopy height, tree,epiphyte, liana and bird species diversity, vegetation structural complexity,percent ground cover by leaf litter, and soil calcium, nitrate nitrogen andorganic matter levels in the O horizons were all significantly greater in shadedthan in sun-grown farms. In contrast, photosynthetic active radiation (PAR), airand soil temperatures, weed diversity and percent ground cover by weeds weresignificantly greater in sun compared to shade farms. At the landscape level,conversion of shade-grown crops to sun conditions isolates protected areas andremnant primary forest fragments. Local cultivators are cognizant of theagronomic and socioeconomic risks associated with sun-grown perennialmonocultures and some are increasing the density and diversity of fruit treecultivation in an effort to provide shade and organic matter, and increase anddiversify crop yields. The maintenance of traditional, complex forest farmingsystems, particularly shade-grown perennial crops, warrants greater attention inagricultural development and biodiversity conservation efforts.     

The dilemma of where to nest: influence of spring snow cover,food proximity and predator abundance on reproductive success of an arctic-breeding migratory herbivore is dependent on nesting habitat choice

Pink-footed geese Anser brachyrhynchus nest in two contrasting but commonly found habitats: steep cliffs and open tundra slopes. In Svalbard, we compared nest densities and nesting success in these two environments over ten breeding seasons to assess the impact of spring snow cover, food availability to nesting adults and arctic fox Vulpes lagopus (main terrestrial predator) abundance. In years with extensive spring snow cover, fewer geese at both colonies attempted to breed, possibly because snow cover limited pre-nesting feeding opportunities, leaving adults in poor breeding condition. Nesting success at the steep cliff colony was lower with extensive spring snow cover; such conditions force birds to commit to repeated and prolonged recess periods at far distant feeding areas, leaving nests open to predation. By contrast, nesting success at the open tundra slope was not affected by spring snow cover; even if birds were apparently in poor condition they could feed immediately adjacent to their nests and defend them from predators. Foxes were the main nest predator in the open tundra slopes but avian predators likely had a larger impact at the steep cliffs colony. Thus, the relative inaccessibility of the cliffs habitat may bring protection from foxes but also deprives geese from readily accessing feeding areas, with the best prospects for successful nesting in low spring snow cover years. Our findings indicate that spring snow cover, predator abundance and food proximity did not uniformly influence nesting success of this herbivore, and their effects were dependent on nesting habitat choice.     

Effect of snow cover on seasonal changes in diet, habitat, and regional distribution of raptors that prey on small mammals in boreal zones of Fennoscandia

Seasonal changes in spatial distribution of search effort of birds that prey on small mammals were studied in two structurally different coniferous forest habitats in the northern boreal zone in SE Norway. During the season with snow cover both the proportion of Microtus relative to that of Clethrionomys in the predators' diet, and their use of a clear-cut relative to that of older forest were lower than during the snow-free season. This was related to a lower relative availability of prey ( Microtus agrestis and M. oeconomus ) in the clear-cut when the ground was snow-covered than when it was snow-free. Based on this local pattern I suggest the following explanation for differences in migratory strategy between raptors that prey on small mammals in Fennoscandian boreal zones: species that migrate to snow-free areas in winter are either adapted to hunt by the energetically expensive method of quartering in open grassland habitats, where prey ( Microtus ) availability is relatively low during periods with snow cover (hen harrier Circus cyaneus , short-eared owl Asio flammeus , and longeared owl A. otus ), or by sit-and-wait in open grassland and forest habitats, the latter with relatively high prey availability during periods with snow cover, but unable to locate concealed prey (kestrel Falco tinnuculus , common buzzard Buteo huteo , and roughlegged buzzard B. lagopus ). In contrast, species that remain in areas with permanent snow cover during winter use the energetically cheap sit-and-wait tactic, and are able to hunt in closed forest habitat and localize concealed prey (the remaining owl species). Interspecific differences in prey availability as determined by hunting habitat and hunting mode is probably more important in shaping the migration patterns of Fennoscandian owls than is nest site availability.     

Effects on bird abundance and species richness of edge restructuring to include shrubs at the interface between conifer plantations and moorland

Capsule?Bird species richness and (for most species) abundance were positively related to the extent of shrub cover at the interface between conifer plantations and moorland, but it appears that responses to shrub development vary between different bird guilds.

Aims?To assess the bird assemblages in both winter and breeding seasons at the interface between managed conifer plantations and open moorland, where that interface had been restructured to include a mosaic of shrubs and open ground.

Methods?Timed point counts were used to sample the birds at restructured plantation – moorland interface areas and also in neighbouring plantations (post- and pre-thicket age classes) and neighbouring moorland. Associations between species richness and abundances with measures of shrub cover and composition were assessed using GLMMs.

Results?A total of 60 bird species were recorded including 29 on lists of conservation concern, most of which were associated with shrub interface habitats. Species richness and, for most species, abundance were positively related to the extent of shrub cover. Positive relationships between shrub cover in interface areas and the abundance of some species in neighbouring plantations and open moorland suggested a resource subsidy to birds in neighbouring habitats. In contrast, some birds tended to be less abundant in plantations next to areas with more shrub cover. These species were more abundant in the shrub itself, suggesting redistribution by species with a preference for early successional shrub habitats.

Conclusions?The long-term management of shrub, especially with regard to successional development, is a challenging aspect of forest and landscape management that deserves further study.     

Impact of wind farms on soaring bird populations at a migratory bottleneck

Collision with turbines at wind farms is expected to have a greater impact on birds at particular sites where high concentrations of individuals occur, such as migration bottleneck areas. The Strait of Gibraltar (southern Spain) has long been recognized as the most important bottleneck in western Europe for soaring bird migration. Moreover, this area is within one of the most important potential areas for wind energy generation in Spain. Here, we examine monthly migratory soaring bird abundance in relation to long-term avian mortality rates at 21 wind farms located near the Strait of Gibraltar using zero-inflated hurdle negative binomial and gamma models. Best fit models included an effect of season in the collision mortality rates and in the proportion of adult individuals within the total deaths. However, monthly bird abundance was not directly related to the number of fatalities over the year. The accumulated fatalities during autumn migration constitute a small percentage (1%) of the total migrating population size. Moreover, mortality peak during autumn migration is largely attributable to juvenile birds. In contrast, the number of fatalities coinciding with the breeding period constitutes a substantial proportion (6%) of the local population, and it involved substantial losses among adult birds. Our results show that wind farms probably have an individually low impact on the migratory population of soaring birds. On the contrary, annual losses among adult local birds are remarkably high considering the small size of the local populations, and they may have population level effects.