The productivity of raptors is affected by different factors such as habitat, food supply, disturbance, weather conditions, and competition. Of the studied breeding parameters, success rate and ...breeding success significantly decreased over the years, while fledgling success and breeding density showed no trend. This study shed light on the main factors influencing the breeding success of a medium-sized raptor expanding in Europe from Asia. Modeling the factors determining the breeding success, we found that grassland availability, mean temperature during fledgling period and rainfall during nestling period influenced Long-legged buzzard reproduction (ΔAIC = 0.00,
wi
= 0.42,
p
= 0.03). The second significant model was described by weather factors excluding presence of grasslands (ΔAIC = 1.03,
wi
= 0.25,
p
= 0.05). Examining the explanatory factors, only presence of grassland in the territories demonstrated some effect on buzzard productivity. The assumption that other factors such as human disturbance, Souslik density, habitat diversity, and presence of competitors shaped buzzard’s productivity did not find support in this study. This study supports the prediction of the future of Long-legged buzzard as it explored the effects of habitat and weather on its reproduction in the southern part of its distribution range. The conditions of a large-scale habitat alteration on one hand and, on the other hand, global climate change, manifested in increased rainfall during spring in Europe, draw an uncertain future for this species. Therefore, restoration of grasslands and proper habitat management, ensuring sufficient and diverse prey, is crucial for conservation of the Long-legged buzzard.
The Optimal Foraging Theory predicts that, to maximize fitness, animals adapt their foraging strategy that provides the most benefit for the lowest cost, maximizing the net energy gained. While the ...diet of many breeding raptor populations is well known, studies on the foraging patterns of non-territorial birds of prey (floaters) are scarce. In this study, we examined the foraging pattern of non-territorial Eastern Imperial Eagle, scrutinizing different aspects of its feeding ecology and behavior. We built a simple model of the optimal foraging strategy of floater eagles including the success of foraging as a currency as well as environmental factors such as seasons, type of prey, habitat, foraging techniques, and eagle age as a limitation affecting the foraging efficiency of birds. We found that floaters focused their diet exclusively on European Souslik, accounting for almost half (44.2%) of the eagle’s prey. Diet differences between floaters and breeders were due to higher Souslik and carrion consumption and lower Hedgehog predation by floater eagles. The diet diversity of breeding eagles (H = 3.297) was much higher than that of floaters (H = 1.748). Our model suggested that the foraging mode, habitat type, and season best explained the feeding success of non-territorial eagles (ΔAIC = 0.00, w = 0.42). Of all explanatory factors, “Kleptoparasitism” (β2 = −4.35), “Rodents” (β2 = −4.52), “Pasture” (β2 = 2.96), “Wheat” (β2 = 4.41), “In the air” (β2 = 4.16), and “Other habitats” (β2 = 4.17) had a pronounced effect. The factors “Spring–summer season” (β2 = −0.67) and “European Souslik” (β2 = −2.76) had a marginal effect in our models. Generally, the mean success rate of attack modes used by non-territorial eagles was 0.54 ± 0.50. Floaters successfully obtained food through: kleptoparasitism (43.10%), carrion feeding (24.14%), and high soar with vertical stoop (14.66%). Several important issues for the conservation of non-territorial Eastern Imperial Eagles arose from our research. The strong relation of floaters with the European Souslik calls for specific conservation measures aimed at the conservation of this type of prey and the restoration and appropriate management of its grassland habitats. The importance of the scavenging behavior of juvenile birds requires increased control of the use of poison baits and subsequent prosecution by state institutions. Protecting the most important temporary areas, improving institutional control against the use of poison baits, and intensifying awareness-raising campaigns among pigeon-fanciers and hunters are also of crucial importance for effective species conservation.
Predator–prey interactions may be linked to different temporal or spatial patterns, including dynamics in prey populations. Therefore, understanding the adaptive capacity and how top predators ...respond to shifts in prey abundance and availability is crucial for their conservation. In this study, we investigated the diet pattern of the endangered Eastern Imperial Eagle facing long-term and large-scale changes. We studied the abundance variation of its profitable prey, sousliks, and how it reflected on eagle population trajectories in a regional and temporal context. We found a significant diet alteration expressed in large decrease of brown hare (β2 = −0.83), poultry (β2 = −0.81), gulls (β2 = −0.71), and water birds (β2 = −0.57), and an obvious increase of northern white-breasted hedgehog (β2 = 0.61) and doves (β2 = 0.60). Raptors and owls raised their participation (β2 = 0.44), but white stork and different reptiles supplied more biomass. Abundance of European souslik decreased through the studied periods (adjusted R2 = 0.25, p < 0.001) which accounted for the lower proportion of this prey in the eagle’s diet. Nevertheless, the eagle population successfully adapted and significantly increased (β2 = 0.97) in most of the distribution area. The trophic strategy used by this top predator related to opportunistic foraging represents an ecological advantage that allows the species to adapt to different habitats and guarantees its future. The observed prolonged diet alteration could result in a significant negative attitude among different groups such as hunters, pigeon fanciers, and poultry keepers towards eagles. Therefore, enhanced communication with key stakeholders is needed. Conservation efforts should be focused also on the preservation of the species’ main foraging habitats and the restoration of damaged ones so as to maintain the good conditions of both primary food source and subsequent prey.
Habitat transformation is identified as major threat to biodiversity loss globally, affecting threatened raptors. Changes in land use can alter the abundance and distribution of birds of prey by ...affecting habitat availability and quality. In this study, we used multivariate analyses to investigate the effect of habitat alteration on the demography of a declining Eastern Imperial Eagle (Aquila heliaca), making assumptions for future population trajectories. We used the Generalized Linear Mixed Models with Poisson distribution and Log link function, searching a relationship between the demographic parameters and the landscape structure and possible effects of fragmentation. In our study area, habitat change affected dramatically permanent grasslands, shrinking their availability. As we expected, the share of grasslands in eagles’ territories significantly affected occupancy rate, but not productivity and breeding frequency. We found that occupancy rate decreased significantly, while productivity and breeding frequency showed no trend. Modeling the effect of habitat alteration on Eastern Imperial Eagle demography, we found out that territory quality was a more powerful factor driving the response of a top predator to the alteration of favorite foraging habitats. Only the habitat quality in source territories had a significant positive effect on eagle productivity. We found that simple rules to predict adverse agricultural impact on territory abandonment or breeding productivity of a top predator were not reliable. Our results could be used for planning conservation of other large territorial raptors, facing the same type of threat.
The Eastern Imperial Eagle (EIE) is a top predator exploiting different prey in different parts of its distribution. In this study, we summarise data collected over a long period of time (for 25 ...consecutive years), identifying key prey species in the different regions, as well as clarifying seasonal preferences in the eagle's diet. Most studies on the EIE food composition covering different parts of the species distribution range analyse the breeding season, while data about the winter diet are scarce. To the best of our knowledge, this is the first study detailing the differences in EIE's dietary preferences between the breeding and the winter periods. We identified 4891 specimens belonging to 196 different taxa, which represents the most comprehensive study considering the diet diversity of this threatened species. Mammals represented the largest proportion of the diet, followed by birds and reptiles. Northern White-breasted Hedgehog was the most common prey, accounting for 25.7% of the total prey caught and 26.75% of the biomass. The European Souslik was the second most important prey with 14.35% participation in the eagle's diet, but with a 3.75% contribution to the biomass. As we predicted, prey composition and main prey species varied spatially and seasonally. Modelling differences in the EIE diet, we found that the "territory effect" had the strongest impact on the dietary variations. Diet diversity differed significantly between regions (F = 12.6, df = 4, p = 0.01). During the breeding season, eagles fed mainly on Hedgehogs (29.88%), Sousliks (16.85%) and Storks (7.74%), while the winter diet was predominantly small rodents (44.17%) and songbirds (13.96%). We found that top predators, such as EIE, have successfully adapted to a novel food source, which is abundant in the area. The detected flexibility in the diet of the species and its ability to switch to alternative prey, if available, when the primary prey decreased, should be considered when planning species conservation efforts. Investigating the temporal change of the main prey in the eagle's diet is also crucial for further species conservation measures.
Habitat alteration is a widespread threat severely affecting large raptors because of their low density and the huge area they inhabit. In this study, we assessed whether human-driven habitat ...alterations mediated dietary shifts of apex predators, focusing on the Eastern imperial eagle (Aquila heliaca). Following a bottom-up conception (before–after), we evaluated the effect of grassland change on the eagle’s dietary shift and breeding success. Land use patterns underwent a significant transformation over the study period, creating a large decrease in grasslands. The territories lost an average of 25.79% of their grasslands. Habitat alteration mediated dietary shifts, but had no reproductive consequences for eagles. Eagles became 1.90 times more likely to predate on northern white-breasted hedgehog and 1.62 times more likely to forage on white stork in the period after grassland alteration. The frequency of tortoises also increased, and they were 4.04 times more likely to be predated on in the years after transformation. Conversely, brown hare was 0.51 times less likely to be consumed in the grassland loss period, while this likelihood was 0.54 times lower for rodents and 0.64 times lower for the European souslik. Doves, meanwhile, were 2.73 times more likely to be predated on in the years following grassland destruction. We found that the presence and biomass of songbirds correlated negatively with the breeding success of eagles, and biomass supply from European souslik was negatively associated with breeding success, while the white stork’s presence and biomass resulted positively in more progeny. Diet diversity did not have an effect on the eagle’s reproduction. The responses of these eagles may vary across territories, depending on how they rank their prey, as the territory effect was a powerful factor shaping dietary shifts for this top predator. Our results offer new evidence of the link between habitat alteration, dietary shifts, and reproductive success, contributing to our understanding of the enigmatic mechanism through which an apex predator successfully adapts to large-scale land use pattern transformation by increasing dietary specialization. We recommend restoration of habitat complexity, including preservation of field margins, grassland patches with scattered small shrub formations, and grassland margins between medium-sized arable lands, promotion of measures for traditional grassland management through gradual grazing, and a ban on the use of shredders.
The Eastern Imperial Eagle (EIE) is a large-sized, globally threatened species with a wide distribution. The species is extremely vulnerable and sensitive to human disturbance and activities in the ...vicinities of its nests. We analyzed the effect of human disturbance in two territories of Eastern Imperial Eagles from Bulgaria in 2008–2009. We recorded 375 cases of different types of human activity in the vicinities of the two surveyed nests – 60 cases in nest A and 315 in nest B. The most common activity around the studied nests was the passing of light motor vehicles (n=100 cases). Our results highlight that the probability of EIE’s reaction is dependent on the type of activity, distance from the nest and the duration of the activity. However, eagles’ reaction is independent from the number of intruders. We found that with the decrease of the distance to the nest, the reaction progresses and is more acute. We found statistical differences between the distance belts and the majority of alert and flight reactions that were recorded at distances up to 300 m from the nests. We reported that humans walking around nests (mainly hunters, fishermen, tourists, people illegally extracting sand in the close vicinities of the nests) result in a large number of reactions of flight off by the eagles thus, leaving the nest unattended. More research on a large scale on this topic is needed including more accurate measures to address human disturbance in EIE territories. The findings will be applied to ensure higher breeding rates and species conservation.
Roosting behaviour during the pre-breeding season is an important part of the Eurasian Griffon Vulture Gyps fulvus life cycle. We demonstrate the importance of roost site selection and its relation ...to the breeding distribution of the Griffon Vulture.
To quantify the distribution, population size, age structure and roosting site selection of the Eurasian Griffon Vulture in the Eastern Rhodopes, Bulgaria during the pre-breeding season.
We used correlation analysis and generalized linear mixed models to examine the relationship between the vultures use of roost sites and their distribution in relation to topographic variables. We additionally used GPS telemetry data from 25 birds to study and compare the use of roosting cliffs.
During the 14-year survey period the number of roosting Eurasian Griffon Vultures gradually increased from 25 birds in 2005 to 201 in 2018. We recorded 16 cliffs used by 124 ± 51 (sd) vultures for roosting. The number of non-adult vultures increased by 25% and reached 41% of all the recorded birds in the last year of the survey. We found that the number of breeding pairs correlated positively with the number of roosting vultures (r
= 0.85) and that roosting cliff selection was determined by cliff length and height, distance to the nearest feeding site, and whether the cliff was used for breeding.
Cliffs regularly used by vultures in the pre-breeding season, were afterwards used for breeding. Thus, cliffs used in the pre-breeding season for roosting could be used as an index for the breeding distribution of the species, and should be protected to sustain the species in and out of the breeding season.
Assessing whether conservation management actions are effective requires a good understanding of the demographic parameters that contribute to the population growth rate. Among the key demographic ...parameters influencing a population, immigration is one of the most difficult to measure empirically but may mask or accentuate the effects of conservation measures. We use an integrated population model to assess whether a population increase of a large raptor species can be explained by high fecundity and survival resulting from local conservation measures, or whether immigration may have contributed to population growth. We monitored the Eastern Imperial Eagle (Aquila heliaca) population in Bulgaria from 1998 to 2014, and tracked juveniles using satellite transmitters to estimate survival probability over the first three years of life. We used intensive territory monitoring of breeding birds to estimate survival probabilities and fecundity of birds older than 3 years. The Imperial Eagle population in Bulgaria increased by about 11 % per year between 1998 and 2014 (λ = 1.111, 95 % credible interval 1.076–1.156). While local conservation measures have succeeded in reducing nest loss and the mortality of adults (adult survival = 0.924; 0.887–0.955), high mortality of juveniles during their first year of life resulted in only 12 % of fledglings surviving to adult age. Based on these survival probabilities and the estimated fecundity of breeding pairs in Bulgaria (1.063; 0.932–1.203), some immigration may have contributed to the population growth in Bulgaria. Because the integrated population model accounts for all the uncertainty associated with disparate data sources, the estimated immigration rate was too imprecise (0.143–2.862 young birds per breeding pair) to quantify the relative importance of immigration. Future conservation measures for Imperial Eagles need to focus on improving the survival of juvenile birds, particularly the reduction of electrocution risk in Eastern Europe and the Middle East.
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