Bonelli's Eagle Hieraaetus Fasciatus Research Articles

Predator–prey relationships in a Mediterranean vertebrate system: Bonelli’s eagles, rabbits and partridges

How predators impact on prey population dynamics is still an unsolved issue for most wild predator-prey communities. When considering vertebrates, important concerns constrain a comprehensive understanding of the functioning of predator-prey relationships worldwide; e.g. studies simultaneously quantifying 'functional' and 'numerical responses' (i.e., the 'total response') are rare. The functional, the numerical, and the resulting total response (i.e., how the predator per capita intake, the population of predators and the total of prey eaten by the total predators vary with prey densities) are fundamental as they reveal the predator's ability to regulate prey population dynamics. Here, we used a multi-spatio-temporal scale approach to simultaneously explore the functional and numerical responses of a territorial predator (Bonelli's eagle Hieraaetus fasciatus) to its two main prey species (the rabbit Oryctolagus cuniculus and the red-legged partridge Alectoris rufa) during the breeding period in a Mediterranean system of south Spain. Bonelli's eagle responded functionally, but not numerically, to rabbit/partridge density changes. Type II, non-regulatory, functional responses (typical of specialist predators) offered the best fitting models for both prey. In the absence of a numerical response, Bonelli's eagle role as a regulating factor of rabbit and partridge populations seems to be weak in our study area. Simple (prey density-dependent) functional response models may well describe the short-term variation in a territorial predator's consumption rate in complex ecosystems.

Repeated sequence homogenization between the control and pseudo‐control regions in the mitochondrial genomes of the subfamily Aquilinae

In birds, the noncoding control region (CR) and its flanking genes are the only parts of the mitochondrial (mt) genome that have been modified by intragenomic rearrangements. In raptors, two noncoding regions are present: the CR has shifted to a new position with respect to the "ancestral avian gene order," whereas the pseudo-control region (PsiCR) is located at the original genomic position of the CR. As possible mechanisms for this rearrangement, duplication and transposition have been considered. During characterization of the mt gene order in Bonelli's eagle Hieraaetus fasciatus, we detected intragenomic sequence similarity between the two regions supporting the duplication hypothesis. We performed intra- and intergenomic sequence comparisons in H. fasciatus and other falconiform species to trace the evolution of the noncoding mtDNA regions in Falconiformes. We identified sections displaying different levels of similarity between the CR and PsiCR. On the basis of phylogenetic analyses, we outline an evolutionary scenario of the underlying mutation events involving duplication and homogenization processes followed by sporadic deletions. Apparently, homogenization may easily occur if sufficient sequence similarity between the CR and PsiCR exists. Moreover, homogenization itself allows perpetuation of this continued equalization, unless this process is stopped by deletion. The Pandionidae and the Aquilinae seem to be the only two lineages of Falconiformes where homology between both regionsis still detectable, whereas in other raptors no similarity was found so far. In these two lineages, the process of sequence degeneration may have slowed down by homogenization events retaining high sequence similarity at least in some sections.

Breeding abundance of threatened raptors as estimated from occurrence data

A model derived from the negative binomial distribution (NBD) has been proposed to solve the problem of predicting abundance of species from occurrence maps. The viability of NBD was explored for predicting the breeding abundance of five threatened species of raptor: Bonelli's Eagle Hieraaetus fasciatus, Golden Eagle Aquila chrysaetos, Peregrine Falco peregrinus, Lanner Falco biarmicus and Lesser Kestrel Falco naumanni. First, the accuracy of the NBD was tested in a reference area where the species abundance and occurrence were known through intensive field surveys. Next, an estimation of regional abundance derived from NBD was made for each species. These estimates were then compared to the existing regional data for the five raptors. The spatial distributions of the species were strongly aggregated, with F. peregrinus correctly showing the most widespread area of occupancy. The NBD gave a good approximation of the breeding abundance of the raptors, but tended to overestimate real data, particularly the regional data for falcons. Difficulties in species detection, insufficient sampling effort (F. biarmicus) or data collected over long time spans when population size increased (F. naumanni) may have reduced the NBD's resolution power. The ability of the method to predict local abundances over large areas from readily available presence‐absence data, with relatively low fieldwork effort, could have considerable applications in conservation biology.

Population dynamics and spatial distribution of Griffon Vultures (Gyps fulvus) in Portugal

AbstractThe global decrease of vulture populations has been attributed to several factors, such as food availability, poisoning, human disturbance, or habitat suitability. We studied the effect of factors that vary both spatially and temporally on the nest site distribution of the Griffon Vulture Gyps fulvus in northeast Portugal, and influence the population dynamics of these cliff-dwelling birds. Several demographic parameters were studied in the field, and the age structure of the population was determined. Additionally we investigated how food availability affected population fluctuations. The vulture population was most sensitive to juvenile mortality according to our model results. Our models showed that a decrease in the present food supply resulted in a decrease in vulture abundance and, moreover, negatively affected the spatial distribution of the species by decreasing the number of breeding pairs per colony. The total quantity of sheep and goat biomass, a greater distance to the nearest unpaved road, and the presence of Egyptian Vultures Neophron percnopterus at the breeding colonies showed a significant positive relationship to the numbers of Griffon Vulture breeding pairs. It is recommended that the available food supply in the most important foraging areas should be monitored and protected. Furthermore, to manage the community of cliff breeding raptors adequately, future research should identify and protect the most suitable breeding areas for each species. Human disturbance should be kept to a minimum by protecting breeding areas, especially in an area like Riba-Côa where species such as the Egyptian Vulture, Bonelli's Eagle Hieraaetus fasciatus and Griffon Vulture breed so close to each other.

Density dependence hypotheses and the distribution of fecundity

1. Beja & Palma (2008, Journal of Animal Ecology, 77, doi: 10.1111/j.1365-2656.2007.01312.x) attempt to provide a critical analysis of the effectiveness and limitations of a previously published method (Ferrer et al. 2006, Journal of Animal Ecology, 75, 111-117.) to discriminate between Habitat Heterogeneity Hypothesis and the Individual Adjustment Hypothesis using real data from a Bonelli's eagle Hieraaetus fasciatus population. 2. They conclude that significant and strong correlations between mean and CV or skewness are expected under a biologically plausible assumption about brood size distribution, and that the two hypotheses cannot therefore be distinguished. 3. A major concern we have with their paper centres on this biologically plausible brood-size distribution. They used the same quasi-Poisson distribution of brood sizes (typical for a saturate population under Habitat Heterogeneity Hypothesis) for both families of simulations. So, is not surprising that both groups gave similar results. 4. They argued that this approach was 'empirical', free of theoretical assumptions. But in testing between hypotheses, what we are looking for is precisely the differences among theoretical brood-size distributions predicted under the two hypotheses. 5. Summarizing, with the same mean fecundity at high densities, both hypotheses must have different brood-size distributions. So the use of a single left-skewed distribution, typical of a real saturated population (most likely under Habitat Heterogeneity Hypothesis) in attempts to distinguish between the two hypotheses by re-sampling several times on the same left-skewed distribution, as done by Beja & Palma, is clearly inappropriate.

Breeding performance, age effects and territory occupancy in a Bonelli's Eagle Hieraaetus fasciatus population

Bonelli's Eagle Hieraaetus fasciatus is one of the most endangered birds of prey in Europe. Despite mounting interest and research, several questions regarding the conservation implications of territory occupancy and site‐dependent population regulation remain insufficiently explored for this species. Here, we report on a 12‐year study of the territorial structure of a Bonelli's Eagle population in southeastern Spain. No signals of population decline were found in the breeding population, as mean annual productivity was stable and the presence of mixed‐age pairs in the population decreased with the years. However, the average proportion of subadults occupying territories was larger than that observed in other Spanish populations. Contrary to the predictions of a despotic distribution model, we found no significant relationship between occupancy rates and breeding parameters. Our results showed significant variations in productivity attributable to differences in the quality of individuals (i.e. mixed versus adult pairs), but no variability among territories per se (i.e. caused by habitat heterogeneity). Moreover, coexistence with intraguild species did not have any significant effect on productivity, although the proximity of Eagle Owls Bubo bubo affected the occupation rate of territories. Finally, our population does not appear to experience site‐dependent population regulation, as a positive relation between mean annual productivity and density was found. The threat posed by changes in land use in the study area leads us to suggest that strict protection of current territories is necessary to ensure population persistence, and we suggest that a significant population increase is only likely if new or deserted territories become available.

Population size, breeding performance and territory quality of Bonelli's Eagle Hieraaetus fasciatus in eastern Spain

Capsule A five-year monitoring study is described of overall population stability and differential reproductive success in relation to habitat heterogeneity. Aims To assess the effect of the altitude and orientation on laying date and breeding performance, and analyse the effect of territory quality as a likely factor that could be regulating the population. Methods We monitored a population of 28–33 pairs, from 2002 to 2006, counting a total of 131 breeding attempts. Results Territories located at lower altitude showed higher mean fecundity than those located at higher altitude. The mean laying date was February 18 ± 16 days. Laying date was positively correlated with nest altitude, the coastal pairs laying earlier than those in mountainous regions. Pairs located at lower altitudes showed higher mean fecundity than those located at higher altitudes. There was no preference in mean orientation either in breeding performance or in relationship to nest altitude. We did not find a difference in breeding performance between territories classed as being at high density and those classed as being at low density. Conclusion The population has remained stable since the first national census was conducted 17 years ago. Our results could be explained in the light of the Habitat Heterogeneity Hypothesis. We suggest a differential reproductive success in relation to habitat heterogeneity.

Limitations of methods to test density‐dependent fecundity hypothesis

1. Two main hypotheses are usually invoked to explain density dependence in fecundity: the habitat heterogeneity hypothesis (HHH) and the individual adjustment hypothesis (IAH). Although simple methods have been proposed to discriminate between the two hypotheses, their adequacy was tested for only a limited set of real and model populations. 2. In a computer simulation study based on a stochastic territory-based approach, Ferrer, Newton & Casado (2006, Journal of Animal Ecology, 75, 111-117) argued that a strong negative relationship between mean fecundity and its skewness in stable or increasing populations provides critical support for HHH, as this relationship should be lacking under IAH. A negative relationship between mean fecundity and its coefficient of variation (CV) was predicted under both hypotheses, although with a lower slope under IAH. 3. We used a comparable simulation approach, with model populations parameterized from an increasing Bonelli's eagle Hieraaetus fasciatus population (1992-2006), to show that both HHH and IAH can produce indistinguishable relationships between mean fecundity and both its CV and its skewness. 4. Strong negative correlations between the mean and both its CV and its skewness can emerge as statistical artifacts under biologically plausible assumptions, and so they may be largely inadequate to infer mechanisms underlying density dependence in demographic parameters.

Green plant material versus ectoparasites in nests of Bonelli's eagle

AbstractThe greenery and arthropod nest composition has been studied in a population of Bonelli's eagle Hieraaetus fasciatus in south‐east Spain, relating them to the plant availability within territories and breeding success of pairs. Greenery was invariably from trees and shrubs, with pine and oak species accounting for 78% of the nest composition in weight. All eagle pairs with Pinus pinaster availability in territories actively selected it for nest greenery. This pine species is characterized by a high level of aromatic compounds, particularly β‐pinene, highly repellent for insects. The amount of pine greenery in the nest was correlated with a lower presence of ectoparasites in that nest (blow fly larvae, Protocalliphora), and higher breeding success of pairs. We discuss whether a coevolutionary process between parasites and their hosts has guided a particular nesting strategy of the eagles to improve their breeding success.

Bonelli's Eagle Hieraaetus fasciatus juvenile dispersal: hourly and daily movements tracked by GPS

Capsule Birds cover daily distances not normally exceeding 20 km during the initial phase of dispersal, with the daily peak of movement/activity in the afternoon.

Identifying potentially suitable nesting habitat for golden eagles applied to ‘important bird areas’ design

AbstractGeographic information systems (GIS)‐based habitat‐suitability modelling is becoming an essential tool in conservation biology. A multi‐scale approach has been proposed as a particularly useful way to identify different factors affecting habitat preferences. In this paper, we developed predictive models of potentially suitable habitat for golden eagles Aquila chrysaetos at three spatial scales in a representative Mediterranean area on the Iberian Peninsula. We used logistic regression through a generalized linear model (GLM) to model golden eagle breeding habitat preferences. The best‐occurrence GLM models were those that involved topographic factors as independent predictors. Golden eagles seemed to prefer rugged and higher places of the study area for nesting. Climatic factors identified cold temperatures in January and temperate ones in July as the best predictors of eagles' occurrence. This was also higher in places with less agricultural areas and higher surface of pine forests. The distribution of potentially suitable area matches the distribution of mountain ranges, mainly in inner sectors of the study area. In contrast, potentially suitable nest sites in coastland areas remain unoccupied by golden eagles. Avoidance of coastland places for nesting may be due to the synergistic effects of human avoidance and the occurrence of potential competitors, like the endangered Bonelli's eagle Hieraaetus fasciatus. When mapped at a fine spatial resolution, the best GLM model identified large areas that fall outside the current network of protected areas. We therefore propose three new important bird areas for the region.

Why do raptors take domestic prey? The case of Bonelli's eagles and pigeons

Summary Predator conservation management requires detailed understanding of the ecological circumstances associated with predation, especially that on economically valuable prey. We examined the mechanisms behind Bonelli's eagle Hieraaetus fasciatus predation on prey of domestic origin, using dietary data from 22 pairs breeding in south‐west Portugal (1992–2001) together with information on landscape composition and prey availability. Numerically, 42·7% (37·7% in biomass) of eagle prey comprised domestic species, about 70% of which were rural pigeons Columba livia and the remainder were racing pigeons Columba livia and domestic fowl Gallus gallus. Rabbits Oryctolagus cuniculus, red‐legged partridges Alectoris rufa and jays Garrulus glandarius were the most frequent wild prey (43·1%; 50·8% in biomass). This dietary pattern was remarkably stable over a decade, but within each year the intake of pigeons almost halved over the course of the breeding season. Landscape composition significantly affected the dietary proportion of wild and domestic prey items. This was particularly evident in territories dominated by eucalyptus Eucalyptus globulus plantations, where there was reduced consumption of rural pigeons and partridges, an increased intake of minor avian prey items and greater diversity in the diet overall. Bonelli's eagles showed type II functional responses while preying on the most important wild (rabbit) and domestic (rural pigeon) prey, although the former was much stronger. Eagle predation on rabbits declined with increasing abundance of pigeons, and vice versa, but there was no switching in the traditional sense, as selection between these two species was inversely frequency dependent. Synthesis and applications. Predation by Bonelli's eagle on domestic pigeons results from a combination of high vulnerability of the pigeons to eagles and a shortage of key wild prey such as rabbits and partridges, especially during the early breeding season. Given the relatively low economic value of rural pigeons and their importance in the diet of Bonelli's eagles, they could probably be used as a conservation tool to enhance food resources in breeding territories and to deflect predation from more valuable prey such as partridges and racing pigeons.

Condition of large brood in Bonelli's Eagle Hieraaetus fasciatus

Capsule Young body condition is affected by the interaction of environment (rainfall) and brood size. Aims To investigate factors affecting offspring condition using levels of urea in plasma. Methods We used generalized linear mixed models (GLMMs) with the levels of urea in plasma as the dependent variable and laying date, brood size, sex and year as the explanatory ones. Results Brood size had a significant effect on offspring condition only during a year of adverse weather (heavy rainfall). During this period, young from broods of two were in poor condition compared with single broods. Conversely, brood size had no effect in the other two years analysed. Neither sex nor laying date had a significant effect on young condition. Offspring condition was not related to first-year survival. Conclusion There is a trade-off between reproduction (brood size) and offspring condition only in years of adverse environmental conditions.

Demography and habitat availability in territorial occupancy of two competing species

Although metapopulation dynamics have become the focus of considerable theoretical research, little attention has been paid to its role when examining the coexistence of species. When two or more species live in the same patch network, interspecific interactions may affect their dispersal, colonization and extinction rates, and it may be possible to incorporate competition affecting these parameters in metapopulation models. Here, we extend the territorial occupancy model proposed by Lande to competing species. Our model estimates an equilibrium proportion of habitat occupancy as a function of life‐history parameters, dispersal behavior, habitat suitability and interspecific interactions. Moreover, it could prove to be useful as a tool in the assessment of potential management decisions. We apply the model to the golden Aquila chrysaetos and the Bonelli's eagle Hieraaetus fasciatus, two territorial raptors that coexist in the Mediterranean region, sharing food and nesting habitats. Over the last twenty years, while the golden eagle has maintained and, in some cases, increased its breeding numbers, Bonelli's eagle has suffered a marked decline, with many territories abandoned by the latter now occupied by the former. This suggests that the dynamics of these species could be influenced by interspecific competition. The model identified the relative importance of competition (stable equilibrium that allows long‐term coexistence) and predicted that, when habitat overlap is slight as in the study area, intraspecific dynamics are much more important for the persistence of each species than interspecific ones. Our results suggest that the improvement of territorial bird survival and productivity are the most urgently needed actions to be undertaken in the case of the golden eagle, while for Bonelli's eagle efforts should be focused on improving territorial and non‐territorial bird survival. As habitat conservation measures, the proportion of suitable exclusive habitat should be increased for both species.

Physical, environmental and human factors influencing productivity in Bonelli's eagle Hieraaetus fasciatus in Granada (SE Spain)

We analysed the productivity of the Bonelli's eagle Hieraaetus fasciatus in relation to 26 variables that account for physiography, level of human presence, land-use, climate, laying date, and nest orientation, in the province of Granada (SE Spain), where a stable population exhibits the maximum productivity value for the species in Europe. Productivity positively correlated with cliff availability within territories and closeness of nest site to SE orientation. Cliff availability permitted some pairs to choose the best nest site orientation, thereby favouring high productivity. Nests in the preferred orientation (SE) had 35% higher productivity than those towards non-preferred orientations. Higher productivity in the best orientation nests might be related to warmer conditions. High productivity in the study area could offset the main problem for Bonelli's eagle conservation in Europe, that is, the high preadult and adult mortality. Actions required to ensure the recovery of neighbouring populations must focus on the preservation of high cliff availability and the diminution of preadult mortality in southeastern Spain. This would favour restocking of the decreased northern populations by the juvenile dispersion.

Predicting the implications of conservation management: a territorial occupancy model of Bonelli's eagle in Murcia, Spain

Bonelli's eagle Hieraaetus fasciatus is a for the species. The model, which assumes demographic stability, estimates an equilibrium proportion of habitat threatened cliC-nesting raptor mainly distributed in the coastal regions of the Mediterranean. The European occupancy as a function of life history parameters, dispersal behaviour and habitat suitability. In spite of population has declined and the species is considered Vulnerable in Spain. Before eCective conservation pro- some limitations the model identified the most important conservation measures required for an increase in the grammes can be proposed the causes of this decline need to be considered. In a monitoring programme of the proportion of territories occupied: the improvement of the survival of both territorial and non-territorial birds, population of Bonelli's eagle in Murcia carried out since 1983 we observed a marked decline, followed by a period and an increase in the proportion of suitable habitat. of stability that began in 1990. Using demographic data (territorial bird survival, non-territorial bird survival, Keywords Bonelli's eagle, demography, Hieraaetus fasciatus, management, Mediterranean, Spain, survival, and fecundity) for 1990-1998 we applied a territorial occupancy model to evaluate management alternatives territorial occupancy model.

Factors influencing the decline of a Bonelli's eagle Hieraaetus fasciatus population in southeastern Spain: demography, habitat or competition?

We investigated three possible causes of territory desertion among Bonelli's eagles Hieraaetus fasciatus in Murcia (southeastern Spain): low demographic parameters, low habitat quality and competition with Golden eagles Aquila chrysaetos. From 1983 to 1997, we surveyed a Bonelli's eagle population. Abandoned and occupied territories were compared to find differences in demographic parameters (flight rate, productivity and mortality) or habitat characteristics. Mortality was significantly higher in abandoned territories. Abandoned territories also had larger areas of forest and extensive agriculture, while occupied territories had more shrublands. Competition with Golden eagles was not a determinant of territorial abandonement but interacted with human persecution of the species. Management implications are discussed.

Causes of death in different areas for Bonelli's Eagle Hieraaetus fasciatus in Spain

The Spanish Bonelli's Eagle populations have decreased markedly because of high mortality. We recorded 424 cases of dead eagles between 1990 and 1998 in Spain which after cross-comparison corresponded to 377 individuals. Electrocution (55% of deaths), followed by direct persecution (26%) were the main causes of death. No differences in the cause of death were found between sexes. Non-adult eagles mostly died of electrocution whereas adults were mainly the victims of persecution. A log-linear model showed that these differences were associated with a difference in the spatial distribution of age classes, rather than to age or experience per se. Persecution was the main cause of death in breeding areas and electrocution in non-breeding areas. There were differences between regions: electrocution was the main cause in Catalonia and Central Spain (50% and 86% respectively) whereas direct persecution was the main cause in Levant and Northern Spain (52% and 43% respectively). We recommend that steps are taken in order to reduce eagle mortality, taking into account the differences between regions and areas.

Trichomoniasis in a Bonelli's eagle population in Spain.

During 1980-97, trichomoniasis was detected in nestlings of Bonelli's eagle Hieraaetus fasciatus in Catalonia (Spain). In 1993 Trichomonas gallinae was isolated in 36% of nestlings (n = 39) and affected 41% of broods (n = 22). Overall, trichomoniasis was one of the most important single nestling mortality factor, accounting for 22% of total chick mortality, and causing the death of 2% of chicks. Trichomoniasis deaths took place during the second half of the nestling period. The median age at death was 45.5 days. Although the presence of the parasite was not related to the composition of the diet or parental age, pairs that developed the disease ate more pigeons and included more often non-adult birds. At present trichomoniasis apparently has little demographic impact on the Bonelli's eagle population in Catalonia, but the eventual spread of this disease in chicks and its unknown effects on adults might be of concern.

Demography and conservation of western European Bonelli's eagle Hieraaetus fasciatus populations

The basic demographic parameters of six Bonelli's eagle Hieraaetus fasciatus populations in Spain and France were calculated from field data obtained in the last 15 years. Average annual productivity ranged from 0.36 to 1.24 young/pair and average annual adult survival from 84% to 96%. Preadult survival (from fledging to recruitment) was estimated at 10%. All the populations were declining at annual rates ranging from −7.3 to −1.1%. Although no statistically significant disagreement was observed between the predictions of a Leslie matrix model fitted to the population parameters and the real trends, some populations declined faster and others more slowly than expected. These differences were interpreted as a result of differential emigration, recruitment rates or preadult survival not accounted for by the model. Since the intrinsic population growth rate was about four times less sensitive to changes in preadult survival than to changes in adult survival, and about ten times less sensitive to changes in fecundity and predispersal survival, conservation efforts must be primarily directed towards increasing adult and preadult survival. Power line casualties and direct persecution must be eliminated in order to reduce mortality. Priorities on research should address dispersal, mortality and habitat relationships involving these two parameters. Monitoring should be extended to other populations.