Raptor Species Research Articles

Sexual Dimorphism in Plumage Coloration of Swainson's Hawks (Buteo swainsoni)

ABSTRACT Plumage pigmentation and ornamentation can indicate male phenotypic quality in many avian species and represent traits that are often selected for by females. Melanins are some of the most widespread classes of pigments in the animal kingdom and form the many shades of black and brown feather colors across raptor taxa. Eumelanin is responsible for the darker shades of black and brown, whereas pheomelanin is responsible for the lighter brown shades and rusty red coloration that are found in many buteonine raptors, including Swainson’s Hawks (Buteo swainsoni). Plumage polymorphism has been examined in various raptor species along with morphometric differences as a basis for sexual dimorphism; however, the potential for sexual dimorphism in the form of pheomelanin-pigmented plumage has rarely been discussed. We examined this phenomenon in Swainson’s Hawks by digitally sampling breast feathers from adult museum specimens, and we utilized an imaging toolbox for multispectral image analysis in the open-source ImageJ software to compare red pixel values between sexes. We found a significant difference in normalized red pixel value means between male and female birds, with higher redness values strongly correlating with male individuals, regardless of morph type. This indicates a relationship between pheomelanin pigmentation and sex, illustrating a basis for sexual dimorphism within the species, as well as the potential for sexual selection of redder males.

Wintering raptor species distribution in a semiarid Mediterranean region: the relevance of lowlands and open habitats as stopover sites

As land-use cover types affect wintering birds’ strategies, studying their habitat suitability for species conservation is relevant. Predictive spatial models are considered excellent tools for conservation planning and improving our understanding of species distribution. Here we build models to predict the spatial distribution of raptor species that overwinter in SE Spain. We modelled the wintering raptor species distribution based on the presence-only data obtained between 2017 and 2019 and analysed their habitat preferences based on elevation and land-use cover variables. Our results show that altitude and distance to the coastline are the most important environmental factors to affect most species’ habitat suitability. Habitat type-related factors are other important predictors, and raptors subject of this study prefer lowlands and areas close to the coastline for overwintering by selecting open habitats, which include wetlands and irrigated and rainfed crops. This study highlights the importance of the habitat heterogeneity generated by wetlands and herbaceous crops within a low-altitude range as the optimal environment for wintering raptors. Such information should be considered for raptor conservation planning in human-altered landscapes.

Quantitative evaluation of the global impacts of human land modification on raptors.

Raptors are threatened by anthropogenic land modifications, but targeted quantitative assessment of these impacts is lacking. We conducted the first global quantitative evaluation of the impacts of human-modified land on raptors. We used eBird data from 2001 to 2020 on 425 raptor species and occupancy models to assess the impacts of human-modified land on raptor distribution. The mean spatiotemporal correlations of human settlement, cropland, and pasture with raptor occupancy probability were -0.048 (SE 0.031), -0.134 (0.032), and -0.145 (0.032), respectively. The mean sensitivity of raptor occupancy probability to settlement, cropland, and pasture was -5.760 (2.266), -3.128 (1.540), and -2.402 (1.551), respectively. The occupancy probability of raptors with a large body mass was more negatively correlated with cropland (phylogenetic generalized least squares regressions: slope = -0.052 [SE 0.022], t=-2.335, df=1,407, p=0.020, λ=0.006) and more positively correlated with pasture (slope=0.047 [0.022], t=2.118, df=1,407, p=0.035, λ=0.013). The occupancy probability of raptors with a more extensive range size was more positively correlated with cropland (slope=0.002 [0.004], t=0.399, df=1,407, p < 0.001, λ=0.000). Raptors that prefer open habitats were more positively correlated with cropland (analysis of variance: F=3.424, df=2, p=0.034, λ=0.000) and pasture (F=6.577, df=2, p=0.002, λ=0.000). In Africa and South America, where raptor species are most abundant, raptor occupancy probability decreased over 20years, most likely due to habitat fragmentation associated with human land modification. Although raptors with different ecological characteristics had different responses to human land modification, the impacts of settlement, cropland, and pasture on mean raptor occupancy probability were negative, regardless of space and time.

Bill shape reflects divergent prey consumption for island compared to mainland American kestrels (Falco sparverius)

Abstract Feeding morphology permits animals to adapt to changing environments and is often under strong selection. We evaluated if bill shape varies according to differences in dietary prey taken across geographical ranges (North America, Central America, South America, and Caribbean islands) in a ubiquitous, New World raptor species, the American kestrel (Falco sparverius). Specifically, we predicted that bills in geographies where kestrels consumed a larger proportion of vertebrates would be shorter and wider, with a larger tomial tooth. We reviewed the literature on kestrel diets across their range and quantified potential differences in bill shape using geometric morphometric methods for 245 museum specimens. The literature review revealed that most prey consumed by kestrels in North, South, and Central America were invertebrates (51.6–69.1%), whereas Caribbean kestrels consumed mostly vertebrates (61.4%), most of which were reptiles (77.3%). Morphometric analyses agreed with these findings; bill shape differed for island versus mainland kestrels but not among mainland regions. Bill shape on islands reflected a more robust bill with a larger tomial tooth, but had a longer hook, which we suggest is adaptive for consumption of lizard prey, more available on islands due to reduced competition with other raptors compared to mainland regions.

First report on daily activity and feeding behavior of Javan hawk-eagle (Nisaetus bartelsi) in the protected forest of Kondang Merak Beach, Malang, Indonesia

Abstract. Ilmi AR, Luthfi M, Elfidasari D, Hidayat YS, Prawiradilaga DM. 2023. First report on daily activity and feeding behavior of Javan hawk-eagle (Nisaetus bartelsi) in the protected forest of Kondang Merak Beach, Malang, Indonesia. Biodiversitas 24: 6770-6779. The Javan Hawk-Eagle (JHE) (Nisaetus bartelsi) is an endemic bird of prey (raptor) on Java Island that is increasingly threatened due to land conversion, habitat destruction, and illegal hunting. These factors have led to its classification as a critically endangered species. There is a need to monitor one of the natural conservation areas in the southern region of East Java, namely the protected forest of Kondang Merak Beach, which serves as a habitat for Javan hawk-eagle to preserve the existence of this species. Therefore, this research aimed to understand and identify the daily activity and feeding behavior of Javan hawk-eagle in the protected forest of Kondang Merak Beach, Malang, East Java. The implication of this research is to provide ecological data for further research and disseminate information to the community to support conservation efforts of Javan hawk-eagle species in the protected forest of Kondang Merak Beach area. The methods used included field surveys, observation of daily activity and feeding behavior, collection of daily temperature data, habitat assessment, disturbance assessment, and interviews. Based on the observation of daily activity, the percentage of perching behavior was 71%, while flying behavior was 29%. The peak flying activity occurred between 9:00 and 12:00. Environmental factors such as temperature, rainfall, and human activities influenced daily activity in the conversation area. The observation of feeding behavior showed that the highest frequency belonged to the category of 23 occurrences, where the peak occurred between 14:00 and 17:00. Furthermore, the activity was affected by nesting seasons and the presence of other raptor species besides Javan hawk-eagle.

Urban Parks Are Related to Functional and Phylogenetic Filtering of Raptor Assemblages in the Austral Pampas, Argentina

Urban parks are hot spots of bird diversity in cities. However, their role as urban filters for raptor species has not been assessed yet. This study aimed to compare the functional and phylogenetic traits of raptor assemblages in urban parks with the regional species pool of raptors in east–central Argentina. Diurnal raptors were surveyed in 51 urban parks in six cities during breeding and nonbreeding seasons. The regional species pool was assessed through raptor surveys and published maps surrounding the cities. The observed functional and phylogenetic relatedness of urban raptors was compared with 999 simulated raptor assemblages from the regional species pool. A total of five species were recorded in urban parks. The Chimango Caracara (Milvago chimango) was the numerically dominant species, comprising 95% of the 172 individuals recorded. The regional species pool was composed of 20 diurnal species. The functional and phylogenetic relatedness of urban raptors was higher than expected by chance, suggesting filtering induced by urban parks. Urban raptors were, in general, generalist species with small body sizes. Moreover, species tended to be part of the Falconidae family. Based on the results obtained here and in other published work, a model of raptor assembling in urban parks is proposed. The design of urban parks needs to be more heterogeneous, promoting the occurrence of specialist raptors.

Lesions and viral antigen distribution in bald eagles, red-tailed hawks, and great horned owls naturally infected with H5N1 clade 2.3.4.4b highly pathogenic avian influenza virus.

An epidemic of highly pathogenic avian influenza (HPAI) began in North America in the winter of 2021. The introduced Eurasian H5N1 clade 2.3.4.4b virus subsequently reassorted with North American avian influenza strains. This postmortem study describes the lesions and influenza A virus antigen distribution in 3 species of raptors, including bald eagles (Haliaeetus leucocephalus, n = 6), red-tailed hawks (Buteo jamaicensis, n = 9), and great horned owls (Bubo virginianus, n = 8), naturally infected with this virus strain based on positive reverse transcriptase polymerase chain reaction and sequencing results from oropharyngeal swabs. The birds presented with severe neurologic signs and either died or were euthanized because of the severity of their clinical signs and suspected influenza virus infection. Gross lesions were uncommon and included forebrain hemorrhages in 2 eagles, myocarditis in 1 hawk, and multifocal pancreatic necrosis in 3 owls. Histological lesions were common and included encephalitis, myocarditis, multifocal pancreas necrosis, multifocal adrenal necrosis, histiocytic splenitis, and anterior uveitis in decreasing frequency. Influenza A viral antigen was detected in brain, heart, pancreas, adrenal gland, kidney, spleen, liver, and eye. In conclusion, bald eagles, red-tailed hawks, and great horned owls infected with the HPAI clade 2.3.4.4b virus strain and showing neurological signs of illness may develop severe or fatal disease with histologically detectable lesions in the brain that are frequently positive for viral antigen.

African savanna raptors show evidence of widespread population collapse and a growing dependence on protected areas.

The conversion of natural habitats to farmland is a major cause of biodiversity loss and poses the greatest extinction risk to birds worldwide. Tropical raptors are of particular concern, being relatively slow-breeding apex predators and scavengers, whose disappearance can trigger extensive cascading effects. Many of Africa's raptors are at considerable risk from habitat conversion, prey-base depletion and persecution, driven principally by human population expansion. Here we describe multiregional trends among 42 African raptor species, 88% of which have declined over a ca. 20-40-yr period, with 69% exceeding the International Union for Conservation of Nature criteria classifying species at risk of extinction. Large raptors had experienced significantly steeper declines than smaller species, and this disparity was more pronounced on unprotected land. Declines were greater in West Africa than elsewhere, and more than twice as severe outside of protected areas (PAs) than within. Worryingly, species suffering the steepest declines had become significantly more dependent on PAs, demonstrating the importance of expanding conservation areas to cover 30% of land by 2030-a key target agreed at the UN Convention on Biological Diversity COP15. Our findings also highlight the significance of a recent African-led proposal to strengthen PA management-initiatives considered fundamental to safeguarding global biodiversity, ecosystem functioning and climate resilience.

Nest Construction Costs Bald Eagles Time But not Breeding Success or Productivity

ABSTRACT Constructing nests costs birds time and energy and may influence breeding success and productivity. The precise nature and extent of these costs is understudied, particularly among raptor species. We used linear and generalized linear mixed models to examine the time and reproductive costs of nest construction to Bald Eagles (Haliaeetus leucocephalus) in New Jersey, USA, from 2009 to 2021. To differentiate nest construction costs from the influence of experience, we separately analyzed the breeding performance and phenology of new pairs constructing nests, established pairs constructing nests, and established pairs reusing nests. We also considered the influence of weather, year, and ecoregion. New territorial pairs building nests were generally less successful and productive than established pairs building or reusing nests. However, these trends were largely attributable to a greater egg-laying rate among established pairs. Nest building pairs, new and established, were generally delayed in egg laying, hatching, and nestling fledging compared to established pairs reusing nests; new pairs were notably more delayed than established pairs building nests. Territorial pairs in the Atlantic Coastal Pine Barrens were less successful and productive than pairs in other ecoregions. Higher precipitation around the egg laying period was predictive of lower nest success and productivity rates but did not appear to affect phenology. Higher temperatures around this period also lowered these rates, with the further effect of advancing phenology. In New Jersey, climate change appears to be advancing breeding phenology and could constrain productivity by century’s end, though we anticipate that density dependence will regulate and lower Bald Eagle breeding performance prior to this period. Breeding success and productivity rates, as well as the effects of year in our models, indicate that New Jersey’s breeding Bald Eagle population is still healthy and growing. Overall, our results indicate that nest construction imposes minor temporal costs on Bald Eagles but does not meaningfully affect their breeding success or productivity, which may help inform management of Bald Eagle nests and populations.

Scaling up Vulture Safe Zone to conserve a greater number of Endangered raptor species: A case for creating a Raptor Safe Zone in Nepal

Nepal has successfully piloted the Vulture Safe Zone, a broad-scale conservation strategy for Endangered vulture species. An obvious question is: what more can we do to build on this accomplishment? Through this paper, I advocate for expanding the Vulture Safe Zones to include other threatened raptors to create Raptor Safe Zones (RSZs). RSZs are a logical next step for Nepal because this country is a global hotspot for raptors and because the RSZ will help conserve 23 threatened raptor species, both within and beyond existing protected areas. I discuss opportunities, challenges, and the way forward for creating RSZ.

Raptors and linear infrastructure in Chhattisgarh, India: species composition and conservation concern

Abstract We investigated the species diversity of diurnal raptors along the selected linear infrastructure projects in northern Chhattisgarh, India, between December 2020 and September 2022. The study identified a total of 14 raptor species, consisting of 11 species in Accipitridae, two in Falconidae, and one in Pandionidae families. Two species were under threatened category of the IUCN red list, the Vulnerable Indian Spotted Eagle (Clanga hastata) and the Near Threatened Pallid Harrier (Circus macrourus). Linear infrastructure development, such as roads, railways, pipelines, canals, and power lines, is expanding rapidly, causing the degradation and fragmentation of habitats, and leading to the loss of biodiversity. Unfortunately, the impacts of linear infrastructure on bird populations in India have not been adequately studied, resulting in limited understanding and few measures to mitigate these impacts. This study specifically focuses on the status of raptors along selected linear infrastructure intrusions and provides baseline information that can help in understanding their conservation needs. The findings of this study underline the necessity of implementing appropriate measures to mitigate the negative effects of linear infrastructure development in India.

Intra- and inter-specific territorial response of diurnal raptors in the tropical dry forest of western Mexico

Little is known of intra- and inter-specific territoriality in Neotropical raptors that may defend territories throughout the year, and face greater inter-specific competition for resources. We evaluated intra- and inter-specific territorial responses of four species of Neotropical raptors in the tropical dry forest of the Chamela-Cuixmala Biosphere Reserve, Mexico. We conducted broadcast calls in territories of the Collared Forest-Falcon (Micrastur semitorquatus), Crane Hawk (Geranospiza caerulescens), Gray Hawk (Buteo plagiatus), and Roadside Hawk (Rupornis magnirostris), and measured the latency, duration, and type of territorial response to broadcast calls of conspecifics and sympatric raptor species. We applied repeated measures analysis to determine differences in intra- and inter-specific territorial responses of raptors, and GLMMs to evaluate whether traits of body size, nest type, and hunting technique were associated with territorial response by focal raptors. Large-bodied focal raptor species showed high intra-specific territoriality, giving faster, prolonged responses of various types, including duets and combined responses, to broadcast calls of conspecifics compared to sympatric species. All focal raptors showed some inter-specific territorial response, but this was mainly as vocalizations or displacements towards sympatric species broadcast calls. Species traits of body size, nest type, and hunting technique were associated with latency and duration of territorial response by the larger raptor species that nest in the forest canopy. These traits may be more strongly associated with territorial response of forest raptors that defend limited resources in a structurally complex environment.

Diet Specialization in an Insular Population of Coastal Peregrine Falcons

Individual diet specialization is known to occur in populations of generalist predators, where specific individuals develop specialist feeding strategies. Diet specialization has been reported in many raptor species and it may be an important driver of intraspecific population structure. Here, we quantify the diet of Peregrine Falcons Falco peregrinus from five breeding territories on an island determined from prey remains collected over four breeding seasons. Three prey species accounted for 69.8% of total prey frequency, with Manx Shearwater Puffinus puffinus being the primary prey accounting for 47.3% by frequency and 40.8% by biomass. European Herring Gull Larus argentatus was the second most important prey species by frequency (13.8%) and biomass (29.8%) followed by Feral Pigeon Columba livia (frequency: 8.7%, biomass: 7.0%). Prey frequency on specific prey groups varied substantially between breeding pairs and months. Two pairs specialized on Manx Shearwater, one pair specialized on Herring Gull and Manx Shearwater, with the remaining two pairs having a relatively generalist diet of Manx Shearwaters, Feral Pigeon and small passerines. Prey frequency on Manx Shearwaters increased throughout the breeding season with a peak in total diet frequency of 63.8% in July, with a concurrent decrease in Herring Gull prey frequency. Higher percentage of Manx Shearwater in the diet explained 87% of the variation in a narrower dietary breadth for the Peregrine pairs. Our results suggest individual diet specialization may be one of multiple factors important for understanding population density in insular raptor populations.

Disclosing the hidden nucleotide sequences: a journey into DNA barcoding of raptor species in public repositories.

In nucleotide public repositories, studies discovered data errors which resulted in incorrect species identification of several accipitrid raptors considered for conservation. Mislabeling, particularly in cases of cryptic species complexes and closely related species, which were identified based on morphological characteristics, was discovered. Prioritizing accurate species labeling, morphological taxonomy, and voucher documentation is crucial to rectify spurious data. Our study aimed to identify an effective DNA barcoding tool that accurately reflects the efficiency status of barcodes in raptor species (Accipitridae). Barcode sequences, including 889 sequences from the mitochondrial cytochrome c oxidase I (COI) gene and 1052 sequences from cytochrome b (Cytb), from 150 raptor species within the Accipitridae family were analyzed. The highest percentage of intraspecific nearest neighbors from the nearest neighbor test was 88.05% for COI and 95.00% for Cytb, suggesting that the Cytb gene is a more suitable marker for accurately identifying raptor species and can serve as a standard region for DNA barcoding. In both datasets, a positive barcoding gap representing the difference between inter-and intra-specific sequence divergences was observed. For COI and Cytb, the cut-off score sequence divergences for species identification were 4.00% and 3.00%, respectively. Greater accuracy was demonstrated for the Cytb gene, making it the preferred primary DNA barcoding marker for raptors.

Intraspecific and geographical variation in rodenticide exposure among common kestrels in Tenerife (Canary Islands)

This study assesses the impact of second-generation anticoagulant rodenticides (SGARs) on the common kestrel (Falco tinnunculus canariensis) in Tenerife, Canary Islands. The analysis of 390 liver samples over 19 years using HPLC-MS/MS showed that 93.1 % of kestrels were exposed to SGARs in this island. A notable shift in SGAR profiles was observed, with bromadiolone and flocoumafen decreasing, while brodifacoum levels increased sharply from 2018 onwards. Comparatively, Tenerife kestrels had a higher detection frequency of SGARs (93.1 %) than those in the rest of the islands of the archipelago (68.2 %), with median concentrations nearly double (∑AR = 180.9 vs 102.4 ng/g liver, P < 0.0001). Furthermore, on average, kestrels from Tenerife were found to have a higher number of different rodenticide compounds per individual. A Generalized Linear Model (GLM) analysis revealed that several factors contribute to the likelihood of SGAR exposure: being an adult kestrel, the enactment of legal restrictions on SGAR bait concentrations in 2018, higher livestock density, and greater human population density. These findings suggest that both bioaccumulation over the birds' lifespans and environmental factors related to human and agricultural activity are influencing the levels of SGARs detected. Alarmingly, 44.7 % of kestrels had SGAR levels above the toxicity threshold established for other raptor species (200 ng/g liver), signaling a high poisoning risk. This is despite EU regulations to protect wildlife, with our findings indicating an increase in both exposure rates and SGAR concentrations since these laws were enacted. The data highlight a critical environmental threat to endemic species on islands like Tenerife. The common kestrel, not considered globally endangered, is nonetheless facing regional threats from SGAR contamination. These results emphasize the urgent need for effective regulations to address the persistent and growing impact of SGARs on island biodiversity.

Nest characteristics of African crowned eagles and black sparrowhawks in urban mosaic landscapes: Potential constraints in finding nesting sites and implications for exotic tree management

The increase in human population and continuous landscape transformations, especially in urban areas, will result in most of the natural habitats being degraded, significantly reduced in size and lost. Consequently, constraints in acquiring resources such as nesting sites and food may occur. In this study, we investigated the nesting site characteristics of two raptor species in the urban mosaic landscape of Durban, KwaZulu Natal, South Africa, to determine the potential constraints affecting acquiring nesting sites. Between October 2020 and July 2021, we visited a total of 52 nests occupied by the African crowned eagle (Stephanoaetus coronatus, hereafter crowned eagle) and the black sparrowhawk (Accipiter melanoleucus) in our study area and compared the nest characteristics of the two raptors. Our results showed that exotic tree plantations were the most commonly used as nest sites by both species, suggesting that exotic trees may be crucial in providing nesting sites for both species in this highly modified landscape. Furthermore, our results showed that there was no significant difference in nest height, tree cover, nest cover and distance to (i) road, (ii) building and (iii) water of nests occupied by the two species. However, there was a significant difference between tree height and nearest neighbour distance. Tall, mature trees, slightly isolated from other trees, influenced nest site use for our study species. Innovative conservation measures and strategies could be vital in protecting raptor nesting sites (i.e., exotic tree plantations) in urban mosaic landscapes with natural and managed green spaces.

An avian Modified Glasgow Coma Scale as a prognostic indicator in raptors with head trauma.

To develop a Modified Glasgow Coma Scale (MGCS) for use in raptors presenting with head trauma and assess the agreement of the MGCS scores between examiners with varying backgrounds, and to assess the prognostic value of the avian MGCS in raptors with head trauma. 156 native raptorial species. All raptors received an MGCS assessment within 8 hours of their presentation, between January 1, 2018, and December 31, 2019. For the first objective, the assessment was performed by a veterinary student, a wildlife veterinarian, and a board-certified or resident veterinary neurologist. Each animal received a score in 3 categories (motor activity, level of consciousness, and brain stem reflexes) and an overall score. For the second objective, the MGCS scoring was performed by the intaking clinical team member and survival after 48 hours was documented. Agreement between the 3 individual scores was assessed via Cronbach α and intraclass correlation. There was excellent-good agreement in all 3 assessment categories as well as the overall score. Univariate associations between survival and demographic factors were determined using the χ2 test. Overall, raptors with a total MGCS of < 10 were less likely to survive than those with a score > 12. An avian-specific MGCS demonstrated good-excellent agreement among raters of various backgrounds in assessing raptors with head trauma. Additionally, this study showed that an avian-specific MGCS may be correlated with the probability of survival within the first 48 hours after presentation to rehabilitation facilities in raptors with head trauma.

Shift in habitat selection during natal dispersal in a long‐lived raptor species

Animals select their habitat along environmental gradients, but the mechanisms that constrain the ecological requirements of an individual can differ between life stages. Dispersal is a key demographic process that determines gene flow and alters species distributions, yet few empirical studies have examined whether habitat selection in animals is changing during dispersal. In this study, we examined changes in habitat preferences during natal dispersal of red kites Milvus milvus, a European raptor species. By deploying solar‐powered GPS‐GSM transmitters on nestlings, we continuously tracked individuals up to six years (2015–2020), from fledging to settlement. We applied habitat selection functions (HSF) to the tracking data using hierarchical generalized additive models, a flexible method which combines individual‐ and population‐level inference, while allowing for the contrast of the prospecting and settlement phases. During the prospecting phase (n = 204 birds), individuals were less responsive to their environment than during the settlement phase, resulting in a predicted wide distribution in western Europe. During the settlement phase, individuals (n = 78 birds) selected a narrower range of environmental gradients, while avoiding areas of high elevation, steep topographic slopes, high human population density and highly heterogeneous landscapes. During this phase, individuals were also more philopatric, i.e. they were more inclined to choose an environment closer to their natal area, than during the prospecting phase. Suitable habitats predicted during settlement were much more spatially contrasted than during prospecting. Our study provides empirical evidence that habitat selection changes across natal dispersal phases in a long‐lived species, indicating that species conservation strategies should account for different environmental constraints before and after settlement. Furthermore, our findings underscore the importance of long‐term tracking data, with sufficient sample size, to study the link between habitat selection and natal dispersal.

American Kestrels Compete with European Starlings over Nest Boxes in Eastern Pennsylvania

ABSTRACT The American Kestrel (Falco sparverius) has been steadily declining throughout most of its eastern North American range, and the cause of this decline is still relatively unknown. As a cavity nesting species, the American Kestrel often competes with other cavity nesters such as the invasive and abundant European Starling (Sturnus vulgaris) over nest boxes. The relationship between European Starling presence at nesting sites and American Kestrel occupancy and nesting success is understudied. We analyzed data from nest boxes monitored in eastern Pennsylvania, USA, from 1992 to 2021 to identify changes in occupancy of American Kestrels and competitors, and to examine the relationship between competition at nest boxes and American Kestrel nesting parameters. We found that American Kestrel occupancy decreased while European Starling occupancy increased over the study period. All other species occupying nest boxes (small mammals, passerines, owls, and snakes) showed no significant occupancy trends. On average 21% of nest boxes remained unoccupied annually, and 7% of occupied nest boxes were used by both American Kestrels and competitors in the same breeding season. The presence of these competitors had negative associations with American Kestrel occupancy, clutch size, number of fledglings produced, and overall nesting success. Specifically, the rate of nesting success decreased by 26% when European Starlings used the same nest box within the same breeding season. In recent years, nesting productivity of American Kestrels has decreased, with the average number of nestlings, fledglings, and nesting success rate all declining, while the average clutch size remained constant. Our results suggest that American Kestrel nesting parameters in eastern Pennsylvania are negatively associated with competition for nest boxes during the breeding season. The opposing trends in occupancy for the European Starling and the American Kestrel in this study area coupled with the declining productivity of American Kestrel nests raise concerns over the future of this raptor species in eastern Pennsylvania.

Hunting Site Behaviour of Sympatric Common Buzzard Buteo buteo and Rough-Legged Buzzard Buteo lagopus on Their Wintering Grounds.

Birds wintering in the northern Palearctic compensate for substantial energy losses and prepare for a food deficit in winter by adjusting their foraging behavior. Apart from weather conditions, interspecific competition also drives hunting strategies. To describe this phenomenon, we observed the behavior of two sympatrically wintering raptor species: the Common Buzzard and the Rough-legged Buzzard. The study was carried out in east-central Poland during four seasons on a study plot where the densities of both species were high. Interspecific differences were detected in the use of available hunting sites. Rough-legged Buzzards conspicuously avoided using fence posts for scanning the surroundings and spent the most time standing on the ground. Common Buzzards more often used trees for this purpose when the snow cover was thick. Thicker snow cover resulted in fewer attempted attacks on prey in both species and caused Common Buzzards to change their hunting sites less frequently. The study also showed that the more often a bird changed its hunting site, the greater the number of attempted attacks. The outcome is that the ultimate effectiveness of hunting is mediated by the overview of the foraging area from different heights and perspectives, not by the type of hunting site. Snow cover was the most important factor in modifying foraging behavior and possibly intensifying interspecific competition.