Aquila Fasciata Research Articles

A comprehensive analysis of spatial and temporal patterns of anthropogenic adult mortality of Bonelli's eagles in eastern Spain

AbstractIn eastern Spain, Bonelli's eagle (Aquila fasciata) abundance has declined almost 50% in the last 25 years and, consequently, the species is listed as regionally endangered. Therefore, the aim of this study is to report the mortality causes of territorial breeding Bonelli's eagles, including the spatial and temporal patterns, and to assess the effects on population dynamics. From 2015–2023, we tracked 60 Bonelli's eagles via global positioning system‐global system for mobile communications (GPS‐GSM) telemetry; 33 of the eagles died (median survival time = 1,519 days). Survival did not differ in relation to sex, age at capture (subadult vs. adult birds), breeding site (coastland vs. inland territories), or transmitter's model. One‐year probability of survival (95% CI) was 0.716 (0.607–0.845); 2‐year survival was 0.640 (0.524–0.781); and 3‐year survival was 0.581 (0.464–0.729). Anthropogenic mortality (66.7% of cases) prevailed over natural (27.3%) and unknown causes (6.0%). Causes of mortality were intra‐ and interspecific predation (24.2% of cases), electrocution (18.2%), poisoning (15.1%), collision with power lines (9.1%), shooting (9.1%), drowning (9.1%), collision with other man‐made infrastructure (6.1%), and disease (3.0%). Deaths were most frequent during the early breeding season. Only 2 (6.1%) of 33 deaths recorded occurred within a protected area. Natural causes of mortality were more frequent away from the Mediterranean coast. In contrast, anthropogenic deaths were more frequent near the Mediterranean coast, where human population density is high. We estimated that probability of extirpation of the species in our study area is 17.8% in the next 50 years and 99.2% in 100 years. Nonetheless, a small reduction in adult mortality by ≥15% could prevent extirpation in the next 50 years. Therefore, actions to reduce adult mortality are urgently needed to preserve the Bonelli's eagle in eastern Spain.

Widespread use of anticoagulant rodenticides in agricultural and urban environments. A menace to the viability of the endangered Bonelli's eagle (Aquila fasciata) populations

Anticoagulant rodenticides (ARs) are one of the most toxic groups of compounds currently used worldwide for rodent pest control. Toxic baits are often, directly or indirectly, ingested by non-target animals, resulting in secondary poisoning and frequently affecting apex predators. Their presence in many species of raptors is quite common, particularly scavenger species, with some of these acting as sentinels for the presence of these substances in the environment. However, there is less data on the presence of ARs in Bonelli's eagle, one of the most endangered eagle species in Spain and which is experiencing a negative population trend in Europe. This medium-sized eagle feeds predominantly on live species, and rarely consumes carrion.In this study, 17 carcasses of Bonelli's eagles from the Eastern Spain were necropsied. Both first and second generation ARs in their livers were analyzed by HPLC-MS-TOF revealing that all the eagles studied had been exposed to at least 5 ARs, out of a total of 10 ARs analyzed, with 7 being the highest number of ARs detected in a sample. Second generation ARs were the most prevalent, particularly bromadiolone and brodifacoum, with the highest concentrations in 94% of the cases. More than a third of the eagles presented a liver concentration of greater than 200 ng/g ARs, suggesting AR poisoning.The elevated presence of these compounds in Bonelli's eagles could be a new cause of mortality for this species or could explain other causes of death, such as the increased mortality in power lines, and should be taken into account for their conservation. At the same time, the presence of these compounds in the environment also represents a risk to public health, as the most frequent species in the diet of Bonelli's eagle (rabbits and partridges) are also hunted and consumed by hunters and their families.

Both movements and breeding performance are affected by individual experience in the Bonelli's eagle Aquila fasciata.

Movement is a key behaviour to better understand how individuals respond to their environment. Movement behaviours are affected by both extrinsic factors that individuals face, such as weather conditions, and intrinsic factors, such as sex and experience. Because of the energy costs it entails, movement behaviours can have direct consequences on an individual's demographic parameters-and ultimately on population dynamics. However, the relationship between extrinsic factors, intrinsic factors, daily movement behaviour and demographic parameters such as breeding performance is poorly known, in particular for central place forager territorial species. We investigated here the link between movement behaviours and breeding performance of the French population of Bonelli's eagle (Aquila fasciata), a territorial and sedentary long-lived raptor, and how this link may depend on extrinsic and intrinsic factors. By using data from annual monitoring of breeding performance for the population and GPS tracking of 48 individuals (26 males and 22 females), we found that the breeding performance of this population was mainly driven by whether a new individual was recruited into the territory, and only slightly by weather conditions. Movement behaviours (proportion of time in flight, range of movement and straightness of trajectories) showed large between-individual variation. Those behaviours were related with weather conditions (wind and rainfall) at a daily scale, as well as with individual's experience. We found only one significant correlation between movements and breeding performance: male Bonelli's eagles spending more time flying during chick-rearing phase had lower productivity. Movement behaviours and breeding performance were also indirectly linked through individual's experience, with more experienced birds having better breeding success and a shorter range of movement and spent less time in flight. This suggests that experienced individuals progressively acquire knowledge of their breeding territory, are more efficient in finding prey, and adapt their foraging strategies to weather conditions to minimise energy costs, allowing them higher breeding performance.

Wildlife following people: A multidisciplinary assessment of the ancient colonization of the Mediterranean Basin by a long‐lived raptor

Abstract Modern humans widely shaped present ecosystems through intentional and unintentional geographical redistribution of wildlife, both in historical and pre‐historical times. However, the patterns of ancient human‐mediated indirect changes in wildlife range are largely unknown, and the mechanisms behind them remain obscure. We used a multidisciplinary approach to (a) reconstruct the process of colonization of the Mediterranean Basin by a long‐lived bird of prey, the Bonelli's eagle (Aquila fasciata), and (b) test the hypothesis that this colonization was unintentionally favoured by anatomically modern humans through a release of competition by dominant species, primarily golden eagles (A. chrysaetos). The fossil record of Bonelli's eagles in the Mediterranean Basin was restricted to the last c. 50 ky. This timing matches the period of modern human presence in Europe. Distribution modelling showed that Bonelli's eagles find more suitable conditions in interglacial periods, while glacial maxima are largely unfavourable unless in coastal refugia. In agreement with this, all Bonelli's eagle's fossils were found in coastal areas, and demographic inference from genetic data revealed a drop in the effective population size by around the last glacial maximum. In today's communities, we found a strongly asymmetric competitive relationship between (subordinate) Bonelli's and (dominant) golden eagles, with the former occupying far more humanized areas than the latter both at the landscape scale and the local (i.e. nesting cliff) scale. Moreover, the nesting habitat overlap analysis indicated that, in the absence of the other species, a notably higher population of Bonelli's eagle, but not of golden eagle, could be expected. Our findings are consistent with the human‐mediated competitor release hypothesis, by which anatomically modern humans could have unintentionally favoured the large‐scale colonization by Bonelli's eagles of a previously competitively hostile Mediterranean Basin. Reconstructing the role of ancient humans in shaping present ecosystems may help to understand the historical, current and future population trajectories of competing species of conservation concern under the ongoing scenario of global environmental change. It also illustrates how human‐mediated apparent competition may promote large‐scale redistribution and colonization of wildlife, including long‐lived species. Read the free Plain Language Summary for this article on the Journal blog.

A new view of territoriality in large eagles: the territory pre‐exists regardless of its occupants

AbstractKnowledge of vertebrate territoriality and spatial use is a topic of great interest in the study of animal behaviour and conservation biology. Investigating the plasticity of territory boundaries, the shape of territories and how territories can be modified depending on their owners is important to deepen our knowledge of the behaviour of territorial species. We analysed the variation and tested the similarity of the distribution area of individuals from the same territory between 2015 and 2021, using data from 51 Bonelli's eagles (Aquila fasciata) from 22 different territories, tagged with GPS/GSM transmitters in eastern Spain. We calculated the percentage of overlap between the territories of the same individual in different years using the 95% kernel density estimator. We also analysed the changes in territory size and shape following the replacement of territory owners either by a single individual or by the whole pair. Our results show that territories retain the same shape and extent regardless of the occupying members, and that their boundaries change little over time. Identifying and maintaining large eagle territories, regardless of their owners, is therefore key to ensuring the long‐term recovery of these threatened species.

Laying date depends on territorial breeders’ age, climatic conditions and previous breeding success: a long-term study (2004–2021) with Bonelli’s eagle in Spain

Adjustment of reproductive timing within the annual cycle is a crucial issue for both offspring and parents’ survival, and breeding success. Early laying date is closely related to successful breeding outcome and better survival. Obtaining long datasets on threatened long-lived species’ breeding performance can contribute to their conservation. Using field observations and telemetry information, here we analyse how laying date of an endangered long-lived raptor varies with nest characteristics, individual factors and climatic variables during an 18-year (2004–2021) study period in eastern Spain. To this end, we estimated how environmental and individual variables affect laying date. Our results showed that average laying date in our study area took place on February 17th. Laying date showed no trend during the study period. Nests placed in higher elevations showed delayed laying dates. Territories occupied by adults with previous successful experience and high brood size showed earlier laying dates. Current year laying date was highly correlated with previous year laying date. Colder territories delayed laying date and the minimum temperature of the previous December were negatively correlated to previous laying date. Our results highlight the importance of experienced birds able to adjust timing of reproduction to maximise biological fitness. High adult mortality allows sub-adults to settle as breeders and jeopardises species’ persistence. Therefore, the reduction of mortality of territorial birds is of the outmost importance to improve the conservation status of declining long-lived species such as the Bonelli’s eagle (Aquila fasciata).

Assumptions about survival estimates and dispersal processes can have severe impacts on population viability assessments

Population viability analysis (PVA) is a central tool for conservation decision-making. To ensure the reliability of PVA projections, it is important to identify factors that can introduce biases. This study addresses two general but overlooked related issues in PVAs that can significantly affect the reliability of their projections. First, we examined the effects of using apparent versus true survival estimates on PVA outcomes. Second, we incorporated emigration and immigration into the models to assess their influence on the accuracy of projections based on each type of survival estimate. To evaluate these concerns, we implemented PVAs structured by age, sex, and breeding status using as a study system a threatened raptor population (Bonelli's eagle, Aquila fasciata) from which apparent and true survival were available (2008–2020). The performances of PVA projections based on each survival type and dispersal process were assessed by evaluating their fit to census data. Our findings revealed that using apparent survival underestimated census data, while true survival showed a considerably better fit. However, models including dispersal processes showed that each survival type may only deliver precise projections at very specific levels of emigration and immigration. Given the potentially large differences found when using true and apparent survival, we suggest that the relevance of this issue should be elevated to that of other widely reported PVA limitations. Recently developed, accessible analytical methods may permit an easier estimation of true survival and dispersal processes. Otherwise, calibrating projections against observed data may be fundamental to test the adequacy of survival estimates.

Conditioning Fledgling Bonelli's Eagles (Aquila fasciata) to Avoid Power Line Pylons

Conditioning Fledgling Bonelli's Eagles (Aquila fasciata) to Avoid Power Line Pylons

Breeders’ age, nest-site characteristics and climatic conditions but not density-dependent effects determine Bonelli’s Eagle breeding performance: A long-term study (2002–2021)

Abstract The conservation of endangered species is a crucial topic in conservation biology due to the important ecological roles these species play in their respective ecosystems. Top predators are particularly vulnerable to the variation in environmental conditions. The Bonelli’s Eagle (Aquila fasciata) is a long-lived bird of prey that fulfills its role as a top predator in Mediterranean ecosystems, but its overall population has declined in the Iberian Peninsula in recent decades. As with other long-lived species, the Bonelli’s Eagle reaches its maximum fitness at maturity, highlighting the importance of assessing how intrinsic and extrinsic factors affect population dynamics over time. In this study, we conducted annual surveys of a Bonelli’s Eagle subpopulation in the eastern Iberian Peninsula from 2002 to 2021. Our results indicate that the large decline observed in the number of occupied territories, fledged chicks, and successful pairs is not due to negative density-dependent effects but rather influenced by various environmental and demographic factors. These factors include breeders’ age, nest-site characteristics, climatic conditions, and previous breeding costs. We found that productivity was higher in territories with high winter precipitation and temperatures, as well as in coastal areas, leading to earlier laying dates. Additionally, the presence of subadult individuals in the breeding pair decreased breeding performance. Interestingly, the variability in the number of fledglings in each territory was driven by previous breeding costs. Prioritizing efforts to mitigate adult mortality and conserve suitable habitats becomes imperative to halt the ongoing decline of this population and also to enhance productivity, ultimately facilitating the restoration of the species to previous population levels.

Experimental food subsidies keep eagles inside protected areas: implications for conservation and resource management

The conservation and management of mobile species, populations and dynamic habitats, presents significant challenges since such species face diverse threats during various stages of their life cycle. Protected areas (PAs) are essential tools in conservation efforts, aiming to preserve native species and their habitats. However, larger mobile animals, such as the Bonelli's eagle (Aquila fasciata), may require much larger areas outside PAs for essential life moments, including foraging, resting, and wintering. As a result, the efficacy of PAs in conserving such species may be questionable. We designed an experiment in an eastern Spain PA that is home to nesting Bonelli's eagles to determine if increasing food availability through experimental feeding within the PA could result in an effective reduction of space use outside the protected area by the eagles and lead to reduce death risk. Over the period of 2016 to 2021, we tracked 10 Bonelli's eagles from six different territories using GPS-GSM transmitters. We evaluated their space use before, during, and after the feeding experiment. Our results indicate that the availability of food within PAs restricted the eagles' movement, resulting in a higher concentration of locations inside the PAs. The eagles spent less time outside the PAs when they were fed and less time in highly human-dominated habitats. Consequently, this reduced mortality risks that were higher outside the PAs. Our findings suggest that management decisions aimed at increasing food availability within PAs, could be critical in conserving endangered species populations such as the Bonelli's eagle in Mediterranean landscapes.

Relationship between blood lead levels and clinical and physiological factors in Bonelli's eagle

AbstractLead is a toxic metal that produces effects on the overall health of animals, resulting in mortality at high exposures, adverse physiological and behavioral effects at lower concentrations, and population declines in many species. Scavengers and predators are more susceptible to lead toxicity because of behavioral and physiologic factors, and long lifespans increase the risk of exposure cumulatively. Bonelli's eagle (Aquila fasciata) is a long‐living predator that may have reduced breeding success because of lead exposure. We analyzed the blood lead levels of 54 wild and captive Bonelli's eagles admitted to a recovery center in Spain from May 2014 to July 2015 and the lead relationship among physiological, clinical, and environmental variables. We detected blood lead using anodic stripping voltammetry. Lead levels were below the limit of quantification (3.3 μg/dL) in 100% of captive animals (n = 22); in contrast, 21.9% of wild birds (n = 32) had higher lead levels than captive birds (P = 0.020). Among wild animals, the 2 adults tested had detectable lead, the only eagle sampled in autumn had the highest lead levels of the study (14.7 μg/dL), and 16.7% of nestlings had detectable lead levels. There is no evidence that the lead levels differ between sexes. There were no symptoms associated with lead intoxication or any correlation between lead levels and hematological or biochemical values; however, subclinical lead exposure at early ages can be harmful and could influence neurological and body development. Researchers should determine if lead could be causing damage to wildlife from early ages to determine if lead exposure affects the conservation of the threatened population of Bonelli's eagles in Spain and elsewhere.

Similar post-fledging behavior observed in wild and reintroduced juvenile Bonelli’s Eagle in two Mediterranean islands

Abstract Reintroduction projects are gaining popularity; however, their relatively elevated cost and chance of failure make them debatable, hence assessing their effectiveness is essential. Within the early stages of raptors, the post-fledging dependency period (PFDP) is the one in which individuals must face dangers without having completely developed their skills. Thereby, comparing PFDP patterns concerning reintroduced and wild individuals is of major interest as it would help to plan and improve future conservation actions. We analyzed the behavior of 38 juvenile Bonelli’s Eagles (Aquila fasciata) (15 reintroduced and 23 wild) tracked through GPS telemetry, tagged as nestlings in two insular environments. The study period encompassed 9-yr movement data from reintroduced chicks in Mallorca (Spain) and wild chicks from Sicily (Italy). Movement parameters (i.e., age of first flight, age of dispersal, length of the PFDP, revisits to the natal or release area, and residence time in them) were analyzed together with their behavior for reintroduced and wild individuals. Similar movement patterns were obtained for both origins, although wild individuals revisited the natal site more often and dispersed earlier. Behavior was also similar, though it varied throughout the PFDP, observing a more abrupt progress in wild individuals and an earlier development of travelling and hunting behaviors. Observed differences are probably related to food availability, which can improve body condition and thus delay onset of dispersal, together with parental presence, which can prompt an earlier ending of the PFDP by encouraging juvenile independence. In conclusion, our study provides valuable insights into the behavior of reintroduced Bonelli’s Eagles during their PFDP and how it compares to their wild counterparts. Our results suggest that reintroduced individuals can successfully adapt to their environment and behave similarly to wild individuals. These findings can aid in the planning and improvement of future reintroduction projects and conservation actions for this endangered species.

Non-Invasive Sex Determination of Nestlings and Adult Bonelli’s Eagles Using Morphometrics

Biometric analysis allows the sexing of most vertebrates, particularly birds. Birds of prey, and, especially, the Bonelli's eagle (Aquila fasciata), show reverse sexual dimorphism (i.e., females are usually larger than males). In contrast to blood sampling, the use of morphometrics allows sex determination using a non-invasive method, and, therefore, it facilitates fieldwork. By means of a linear discriminant analysis of biometric variables, we obtained different equations that allow the sexing of nestlings and adult Bonelli's eagles. We sampled 137 Bonelli's eagles, 82 nestlings and 55 adults in eastern Spain during the period 2015-2022. The sexes obtained after linear discriminant analysis were compared with their molecular sexing. The validation procedure of the linear discriminant equations facilitated the reduction of the number of variables used and, consequently, optimised working time and sexing accuracy. After validation, some equations showed a 100% sexing efficiency for Bonelli's eagles, particularly for adults. Our results showed that the variables with smaller overlap between the sexes were the lateral tarsus length and dorso-ventral tarsus length, particularly in nestlings. The rest of the variables showed some overlap between the sexes in both age classes. The results we obtained enable the sexing of juvenile and adult Bonelli's eagles in the field using just these two measurements. Hence, this is an easy, accurate, quick and non-invasive method with multiple applications, including in studies on population dynamics, survival analysis or extinction risk assessments, which, ultimately, could contribute to the improvement of the conservation status of this endangered species.

Pulling the Plug on Eagle Electrocution in Israel: High-resolution Modelling of Bonelli’s Eagle Electrocution Risk

The Bonelli’s Eagle (Aquila fasciata) is a critically endangered species in Israel, with electrocution on power lines posing a serious threat on its population. Since insulation of electricity pylons is a slow and costly process, it is important to prioritize the insulation of the pylons in the network for quick and efficient mitigation of eagle mortality. To determine which pylons need to be retrofitted, we applied a three-stage Maximum Entropy Modeling process for identifying the risk factors among different environmental variables. The results show that the environmental feature with the highest correlation to electrocution events is the distance to water reservoirs, which are foraging hotspots of the eagles in Israel’s arid environment. Unfortunately, the only tall perch available for eagles in the vicinity of many of the reservoirs are the electricity pylons that power the reservoirs’ pumping facilities. This combination of anthropogenic alterations has apparently created a detrimental ecological trap. The strong attractiveness of water reservoirs for the eagles may explain the high level of selectivity that was calculated by the model, suggesting that retrofitting only 3.6% of the pylons in the network would achieve 77% reduction in eagles’ electrocution probability. Moreover, insulating pylons according to the model is expected to achieve a significant remedy for other avian species, among them the Eastern Imperial Eagles (Aquila heliaca) and White-Tailed Eagles (Haliaeetus albicilla). Synthesis and applications: The modeling process presented here yielded two electrocution risk maps, one of which is expected to facilitate properly prioritized mitigation of eagle electrocution in the existing power network of Israel, while the second map is designed to support an informed planning of new infrastructures. Applying this approach is expected to substantially aid the conservation of the population and allow it to recover to its initial size, which was three times its current size, about 70 years ago. The work presented here aims to prioritize the mitigation of raptor electrocution in arid and semi-arid areas, geographic zones that are largely under-studied in relation to this mortality factor. Electrocution in arid areas is of a particular concern in many developing countries, where networks of distribution lines are rapidly growing and raptor electrocution rate is high. The modeling approach presented in this study can be applied in these arid developing countries to mitigate raptor electrocution, aiding their conservation.

Dispersal Movements and Hot Spot Areas of Juvenile Bonelli’s Eagles on the Island of Crete, Greece

The Bonelli’s Eagle (Aquila fasciata) is the main avian predator in almost all the islands of the Greek archipelagos. The phase of its natal dispersal has never been studied in Greece, though it constitutes a crucial part of the species ecology and conservation. In an effort to investigate the movement patterns and settlement areas of young eagles, we tagged 30 fledglings on the island of Crete during 2019–2023 with GPS/GSM transmitters. Fledging date was set the first day with full satellite coverage and consecutive GPS fixes for individual birds while the initiation of the dispersal period was established when they performed consecutive long flights, overstaying away from their natal territory (>10 km). Mean fledgling date was calculated at May 27 (range = 20/5–8/6), whereas the dispersal period started on average on September 03 (range = 30/8–21/9), namely ca. 90–100 days after the abandonment of the nest. All juvenile birds wandered around over the entire island residing primarily in two distinct habitat types i.e. coastal cliffs and satellite islets around Crete and agricultural areas in the lowlands covered with olive groves and vineyards. Preliminary surveys show that the juvenile birds avoid active Bonelli’s Eagle territories and select rural areas near landfills, water reservoirs, fowl pens, and uninhabited islets, specializing on prey species that are readily available (i.e. gulls, corvids, rock pigeons, wild rabbits and poultry). This fact makes them particularly susceptible to human-induced mortality such as electrocution, drowning in open water tanks and poaching, which accounted for ca. 60% of the casualties of this age group. Overall, the study confirms that land use changes and human pressure are the major threats for the species on Crete, likewise in the rest of its distribution range in the Mediterranean.

Does the Release of Bonelli’s Eagles from Captive Breeding Nucleus Contribute to Population Rehabilitation?

During the late 1960s, the Bonelli’s Eagle (Aquila fasciata) population in Israel was brought to the brink of extinction, due to a combination of factors. A rehabilitation program set in 2000 included few measures, among them the release of eaglets from a captive breeding nucleus, producing two to seven eaglets yearly. In recent years there is an apparent increase in the number of Bonelli’s Eagle pairs breeding in the wild, including the reoccupation of territories that were abandoned for many years. Yet, there is only little evidence for the recruitment of captive-born eagles into the wild population, and their contribution to its growth is unclear. These subjects are now being examined by a telemetry study, comparing dispersal patterns and survival rates of captive versus nature hatched eagles. The results from the first five years of study show significant differences between these two groups, with captive hatched eagles dispersing much farther away from their natal site compared to the dispersal of wild eagles. The different dispersal patterns may be the reason for the lower survival rates exhibited by the captive hatched eagles, as many of them wander to neighboring countries where they are subjected to shooting and trapping. Moreover, these birds also have a higher tendency to being electrocuted on power lines. Genetic analysis and alteration of the releasing methods are being used in order to check whether the different movement and behavior attributes of the captive-hatched eagles are linked to genetic properties or behaviorally mechanisms during their growth.

Potential negative effects of the installation of video surveillance cameras in raptors’ nests

Video surveillance cameras installed on birds’ nests are a cost-effective tool to study many aspects of ecology and behaviour that would otherwise be practically impossible to obtain. However, although most studies report neutral effects of cameras on birds, very few studies analyse in detail the potential negative effects of their use, particularly on raptors. Here, using a long-term database of a population of Bonelli’s eagle (Aquila fasciata) collected from 2000 to 2022, I show how the inappropriate use of video surveillance cameras could result in negative effects on the reproduction of a threatened species through a before-and-after control-impact study design. Pairs under video surveillance showed lower productivity, lower breeding success and unusual delayed laying dates. The installation of cameras close to the laying date, coinciding with the mating phase of individuals, most of them subadult inexperienced birds; in combination to the reiteration of visits to the nests once the cameras were installed to check the system, particularly during the incubation period and early stages of breeding; and the installation of cameras in a particular area subject to constant human disturbance, might explain these results. Potential management actions to mitigate the effect of the installation of video cameras on birds’ behaviour should include the need to plan the intervention dates, testing the systems beforehand under controlled conditions and adequate post-installation monitoring to avoid unnecessary disturbance to animals. Finally, I urge the scientific community to report the potential negative effects observed in their studies, especially if the target species are threatened with extinction.

Selection of Nesting Habitat and Insular Niche Separation of Two Sympatric Aquila Species

Aquila chrysaetos and Aquila fasciata are two congeneric eagle species distributed in the Mediterranean region which are supposed to compete for similar breeding and foraging resources. In the present study, bioclimatic, topographic, and human-related habitat parameters were investigated for 64 and 75 nest sites of Golden and Bonelli’s eagles, respectively. The nests were found during fieldwork undertaken from 1995–2020. Overall, the habitat parameters that best discriminated nest site selection were associated with elevation, temperature, and land use with topographic variables being most powerful for niche separation. Univariate analysis, regression, and species distribution modeling identified a strong association of the species with altitude pinpointing the Golden eagle’s mountainous and continental character and the Bonelli’s eagle being a lowland and coastal species. Golden eagle nests were situated away from human settlements on steep cliffs in higher altitude areas with transitional woodland-shrub vegetation. In contrast Bonelli’s eagle nests were located on low-altitude warmer zones, closer to the coast and human settlements with more natural grasslands in their vicinity. The ecological niche separation of the two species was best described by altitude and temperature, though no clear-cut evidence was detected for their competitive exclusion. Inter-specific nearest neighboring distance was found statistically significant only for the Golden eagle which seems to be less tolerant in its co-existence with the Bonelli’s eagle. Conservation measures for both species should target territories under human pressure, though more research should focus on the species range use and habitat heterogeneity within overlapping territories.

Early stages of home range settlement in a newly reintroduced raptor on an island: Bonelli’s eagle (Aquila fasciata) on Mallorca, Spain

Early stages of home range settlement in a newly reintroduced raptor on an island: Bonelli’s eagle (Aquila fasciata) on Mallorca, Spain

Home-Range Size and Space Use of Territorial Bonelli’s Eagles (Aquila fasciata) Tracked by High-Resolution GPS/GSM Telemetry

High-resolution GPS/GSM dataloggers provide spatial information of the highest quality, which outperform previous tracking methods, such as Argos telemetry or conventional VHF ground-tracking. As a result, this has improved our knowledge of home-range behavior and spatial ecology of many species, including large raptors. In this paper, we use high-resolution GPS/GSM dataloggers to assess the home-range size and the role of sex, season (breeding or non-breeding season), and breeding status (reproductive or non-reproductive individuals) on the space use of Bonelli’s eagle (Aquila fasciata). To this end, 51 territorial individuals (25 females and 26 males) were equipped with GPS/GSM transmitters and were tracked over 7 years (2015–2021) in eastern Spain. Overall, we recorded 4,791,080 fixes that were analyzed through kernel density methods (50%, 75%, and 95% fixed kernels). The average individual home-range size according to the 95%, 75%, and 50% kernels was 54.84 ± 20.78 km2, 24.30 ± 10.18 km2, and 11.17 ± 4.90 km2, respectively. Overall, the home-range size of individuals occupying the same territory was similar, mainly due to the cooperative hunting behavior exhibited by the species. We did not find interannual differences in the home-range size (95% fixed kernel) of the majority of individuals, showing a strong territorial fidelity of the breeding pairs. In general, females’ home-range size was slightly smaller than males’ size due to the decrease in activity in the breeding season as a result of laying, incubation, and chick attendance at nests. No seasonal variation in the 95% kernel was found, but it was found in the 75% and 50% kernels. In regard to the breeding status, higher home-range size was recorded in the non-reproductive individuals. Moreover, we found a low neighbor overlap among the territories (4.18% ± 3.06%), which evidences a high level of intraspecific competition in the Bonelli’s eagle. Finally, this study highlights the advantages of the use of accurate telemetry information to improve our understanding of the spatial ecology of the endangered Bonelli’s eagle, which ultimately will serve to better inform management actions for its conservation.