A demographic description of the recovery of the Vulnerable Spanish imperial eagle Aquila adalberti

Moderate evidence for the sex-dependent effect of poisoning on adult survival in a long-lived raptor species

Survival rate is usually the greatest contributor to population growth in long-lived species, and its accurate estimation along with the evaluation of the factors influencing it is therefore essential for effective conservation. Here, we studied the survival of breeding eastern imperial eagles Aquila heliaca in Hungary between 2011 and 2022 and investigated the effect of poisoning, the leading known anthropogenic cause of mortality. We used the Cormack-Jolly-Seber mark-recapture model to estimate annual apparent survival and encounter probabilities based on the capture histories of 208 males and 411 females. We obtained encounter data from the DNA profiles of shed feathers collected at the nest sites, which we also supplemented with presences inferred from parentage analysis. The most supported model estimated a constant 91.6% annual survival over the study period, but models including the effect of sex and poisoning rate on survival had similar support. Sex difference in survival was less than 1% on average, but the survival of males decreased more with poisoning rate than the survival of females. However, due to smaller encounter probabilities, the estimates for males were less precise compared to females. Males may be more at risk from poisoning than females not only due to their more active foraging behaviour during incubation and chick-rearing but also due to their smaller body size. Apart from providing direct practical information for the conservation management of imperial eagles, our results also highlight the importance of long-term studies for estimating population parameters of birds of prey.

Demographic modeling to fine-tune conservation targets: importance of pre-adults for the decline of an endangered raptor

Large, long-lived species with slow life histories and protracted pre-breeding stages are particularly susceptible to declines and extinction, often for unknown causes. Here, we show how demographic modeling of a medium-sized raptor, the Red Kite Milvus milvus, can aid to refocus conservation research and attention on the most likely mechanisms driving its decline. Red Kites' survival and reproduction increased through three sequential stages for 1-2, 3-6, and 7-30 yr of age, mainly corresponding to individuals that are dispersing, attempting to gain a territory, and breeding. As typical of long-lived species, elasticities were highest for adult (≥7 yr old) survival, but this was high, with little scope for improvement. Instead, the declines were driven by an extremely low survival of pre-adults in their first years of life, which weakened the whole demographic system by nullifying the offspring contribution of adults and curtailing their replacement by recruits. For example, 27 pairs were necessary to generate a single prime age adult. Simulation of management scenarios suggested that the decline could be halted most parsimoniously by increasing pre-adult survival to the mean levels recorded for other areas, while only the synergistic, simultaneous improvement of breeding success, adult and pre-adult survival could generate a recovery. We propose three actions to attain such goals through selective supplementary feeding of both breeding and non-breeding individuals, and through mortality improvement by GPS remote-sensing devices employed as surveillance monitoring tools. Our results show how improving demographic models by using real, local vital rates rather than "best guess" vital rates can dramatically improve model realism by refocusing attention on the actual stages and mortality causes in need of manipulation, thus building precious time and resources for conservation management. These results also highlight the frequent key role of pre-adult survival for the management of long-lived species, coherent with the idea of demographic systems as integrated chains only as strong as their weakest link.

Disease-mediated bottom-up regulation: An emergent virus affects a keystone prey, and alters the dynamics of trophic webs

Emergent diseases may alter the structure and functioning of ecosystems by creating new biotic interactions and modifying existing ones, producing cascading processes along trophic webs. Recently, a new variant of the rabbit haemorrhagic disease virus (RHDV2 or RHDVb) arguably caused widespread declines in a keystone prey in Mediterranean ecosystems - the European rabbit (Oryctolagus cuniculus). We quantitatively assess the impact of RHDV2 on natural rabbit populations and in two endangered apex predator populations: the Iberian lynx (Lynx pardinus) and the Spanish Imperial eagle (Aquila adalberti). We found 60–70% declines in rabbit populations, followed by decreases of 65.7% in Iberian lynx and 45.5% in Spanish Imperial eagle fecundities. A revision of the web of trophic interactions among rabbits and their dependent predators suggests that RHDV2 acts as a keystone species, and may steer Mediterranean ecosystems to management-dependent alternative states, dominated by simplified mesopredator communities. This model system stresses the importance of diseases as functional players in the dynamics of trophic webs.

Spatial and temporal movements in Pyrenean bearded vultures (Gypaetus barbatus): Integrating movement ecology into conservation practice

Understanding the movement of threatened species is important if we are to optimize management and conservation actions. Here, we describe the age and sex specific spatial and temporal ranging patterns of 19 bearded vultures Gypaetus barbatus tracked with GPS technology. Our findings suggest that spatial asymmetries are a consequence of breeding status and age-classes. Territorial individuals exploited home ranges of about 50 km², while non-territorial birds used areas of around 10 000 km² (with no seasonal differences). Mean daily movements differed between territorial (23.8 km) and non-territorial birds (46.1 km), and differences were also found between sexes in non-territorial birds. Daily maximum distances travelled per day also differed between territorial (8.2 km) and non-territorial individuals (26.5 km). Territorial females moved greater distances (12 km) than males (6.6 km). Taking into account high-use core areas (K20), Supplementary Feeding Sites (SFS) do not seem to play an important role in the use of space by bearded vultures. For non-territorial and territorial individuals, 54% and 46% of their home ranges (K90), respectively, were outside protected areas. Our findings will help develop guidelines for establishing priority areas based on spatial use, and also optimize management and conservation actions for this threatened species.

Assessing the impact of removal scenarios on population viability of a threatened, long-lived avian scavenger

The removal of eggs or chicks from wild populations to create captive populations, reinforce free-ranging populations or reintroduce species into the wild is a restoration tool that requires an assessment of potential detrimental effects upon the donor population. This is an absolute prerequisite when wild donor populations are scarce and small. Here, we forecast the population trend of the largest European population of the bearded vulture (Gypaetus barbatus) over the next 30 years under different demographic and management scenarios (removal of eggs, chicks or fledglings). Projections derived from the combination of a PDP model (Population Dynamic P-system) and a Box-Behnken design would lead to a decline in 77% of all 57 scenarios analysed. Among the 13 scenarios predicting a population increase, only 4 seem realistic in terms of growth rate (0.04%-1.01%), at least if current age at first breeding and productivity would remain constant over time. Our simulations thus suggest that most extraction scenarios would have detrimental effects on the demography of the donor population. Release of captive-born young or removal of only the second hatched chick for subsequent captive rearing and translocation into the wild appear to represent much better supplementation and reintroduction options in this threatened species.

Trade of threatened vultures and other raptors for fetish and bushmeat in West and Central Africa

Diurnal raptors have declined significantly in western Africa since the 1960s. To evaluate the impact of traditional medicine and bushmeat trade on raptors, we examined carcasses offered at markets at 67 sites (1–80 stands per site) in 12 countries in western Africa during 1990–2013. Black kite Milvus migrans and hooded vulture Necrosyrtes monachus together accounted for 41% of 2,646 carcasses comprising 52 species. Twenty-seven percent of carcasses were of species categorized as Near Threatened, Vulnerable or Endangered on the IUCN Red List. Common species were traded more frequently than rarer species, as were species with frequent scavenging behaviour (vs non-scavenging), generalist or savannah habitat use (vs forest), and an Afrotropical (vs Palearctic) breeding range. Large Afrotropical vultures were recorded in the highest absolute and relative numbers in Nigeria, whereas in Central Africa, palm-nut vultures Gypohierax angolensis were the most abundant vulture species. Estimates based on data extrapolation indicated that within West Africa 73% of carcasses were traded in Nigeria, 21% in Benin and 5% elsewhere. Offtake per annum in West Africa was estimated to be 975–1,462 hooded vultures, 356–534 palm-nut vultures, 188–282 Rüppell's griffons Gyps rueppellii, 154–231 African white-backed vultures Gyps africanus, 143–214 lappet-faced vultures Torgos tracheliotos, and 40–60 crowned eagles Stephanoaetus coronatus. This represents a sizeable proportion of regional populations, suggesting that trade is likely to be contributing significantly to declines. Stronger commitment is needed, especially by governments in Nigeria and Benin, to halt the trade in threatened raptors and prevent their extirpation.

Is wedge-tailed eagle, Aquila audax, survival and breeding success closely linked to the abundance of European rabbits, Oryctolagus cuniculus?

Context Some ecologists argue that nesting success and abundance of wedge-tailed eagles (Aquila audax) are strongly linked to the abundance of introduced wild rabbits (Oryctolagus cuniculus). Consequently, concerns were expressed about eagle population viability when the biological control agent rabbit haemorrhagic disease virus (RHDV) heavily reduced rabbit numbers. However, observations following the spread of rabbit haemorrhagic disease (RHD) in Australia and Spain (where Aquila adalberti is an equivalent of A. audax) question this assertion. Eagle numbers did not fall even though rabbits declined regionally by up to 90% in both countries. Aims To reconsider the assumption of a strong link between rabbit abundance and wedge-tailed eagle breeding and population maintenance. Dispelling misconceptions, if any, about the eagles’ dependence on rabbits would benefit the future management of both eagles and rabbits. Methods We reviewed the literature associated with claims that eagles were heavily dependent on rabbits and asked whether these views could be substantiated given the lack of changes in eagle abundance following the spread of RHD. Data on eagle egg-clutch size and nesting success were also reviewed. Conclusions There is little evidence that eagles depend heavily on rabbits as prey. Instead, as rabbits decline, more kangaroos, reptiles and birds are eaten, partly because more native prey becomes available. Eagles have a high proportion of rabbits in their diets mainly where degradation of natural ecosystems, including that caused by rabbits, results in native prey being rare or unavailable. There has been minimal variation in average clutch size following major perturbations in rabbit population size. Implications Rather than perpetuating the idea that high populations of rabbits are needed for wedge-tailed eagle conservation, resources would be better re-directed into understanding continental-scale eagle population dynamics. This would provide a more rational framework to assist decisions on future biological control agents for rabbits.

Uneven large-scale movement patterns in wild and reintroduced pre-adult bearded vultures: conservation implications

After the quasi-extinction of much of the European vertebrate megafauna during the last few centuries, many reintroduction projects seek to restore decimated populations. However, the future of numerous species depends on the management scenarios of metapopulations where the flow of individuals can be critical to ensure their viability. This is the case of the bearded vulture Gypaetus barbatus, an Old World, large body-sized and long-lived scavenger living in mountain ranges. Although persecution in Western Europe restrained it to the Pyrenees, the species is nowadays present in other mountains thanks to reintroduction projects. We examined the movement patterns of pre-adult non-breeding individuals born in the wild population of the Pyrenees (n = 9) and in the reintroduced populations of the Alps (n = 24) and Andalusia (n = 13). Most birds were equipped with GPS-GSM radio transmitters, which allowed accurate determination of individual dispersal patterns. Two estimators were considered: i) step length (i.e., the distance travelled per day by each individual, calculated considering only successive days); and ii) total dispersal distance (i.e., the distance travelled between each mean daily location and the point of release). Both dispersal estimators showed a positive relationship with age but were also highly dependent on the source population, birds in Andalusia and Alps moving farther than in Pyrenees. Future research should confirm if differences in dispersal distances are the rule, in which case the dynamics of future populations would be strongly influenced. In summary, our findings highlight that inter-population differences can affect the flow of individuals among patches (a key aspect to ensure the viability of the European metapopulation of the endangered bearded vulture), and thus should be taken into account when planning reintroduction programs. This result also raises questions about whether similar scenarios may occur in other restoration projects of European megafauna.

Demographic consequences of poison-related mortality in a threatened bird of prey

Evidence for decline or threat of wild populations typically comes from multiple sources and methods that allow optimal integration of the available information, representing a major advance in planning management actions. We used integrated population modelling and perturbation analyses to assess the demographic consequences of the illegal use of poison for an insular population of Red Kites, Milvus milvus. We first pooled into a single statistical framework the annual census of breeding pairs, the available individual-based data, the average productivity and the number of birds admitted annually to the local rehabilitation centre. By combining these four types of information we were able to increase estimate precision and to obtain an estimate of the proportion of breeding adults, an important parameter that was not directly measured in the field and that is often difficult to assess. Subsequently, we used perturbation analyses to measure the expected change in the population growth rate due to a change in poison-related mortality. We found that poison accounted for 0.43 to 0.76 of the total mortality, for yearlings and older birds, respectively. Results from the deterministic population model indicated that this mortality suppressed the population growth rate by 20%. Despite this, the population was estimated to increase, albeit slowly. This positive trend was likely maintained by a very high productivity (1.83 fledglings per breeding pair) possibly promoted by supplementary feeding, a situation which is likely to be common to many large obligate or facultative European scavengers. Under this hypothetical scenario of double societal costs (poisoning of a threatened species and feeding programs), increasing poison control would help to lower the public cost of maintaining supplementary feeding stations.

Minimising mortality in endangered raptors due to power lines: the importance of spatial aggregation to optimize the application of mitigation measures

Electrocution by power lines is one of the main causes of non-natural mortality in birds of prey. In an area in central Spain, we surveyed 6304 pylons from 333 power lines to determine electrocution rates, environmental and design factors that may influence electrocution and the efficacy of mitigation measures used to minimise electrocution cases. A total of 952 electrocuted raptors, representing 14 different species, were observed. Electrocuted raptors were concentrated in certain areas and the environmental factors associated with increased electrocution events were: greater numbers of prey animals; greater vegetation cover; and shorter distance to roads. The structural elements associated with electrocutions were shorter strings of insulators, one or more phases over the crossarm, cross-shaped design and pylon function. Of the 952 carcasses found, 148 were eagles, including golden eagle (Aquila chrysaetos), Spanish imperial eagle (Aquila adalberti) and Bonelli's eagle (Aquila fasciata). Electrocuted eagles were clustered in smaller areas than other electrocuted raptors. The factors associated with increased eagle electrocution events were: pylons function, shorter strings of insulators, higher slopes surrounding the pylon, and more numerous potential prey animals. Pylons with increased string of insulators had lower raptor electrocution rates than unimproved pylons, although this technique was unsuccessful for eagles. Pylons with cable insulation showed higher electrocution rates than unimproved pylons, both for raptors and eagles, despite this is the most widely used and recommended mitigation measure in several countries. To optimize the application of mitigation measures, our results recommend the substitution of pin-type insulators to suspended ones and elongating the strings of insulators.

Solving man-induced large-scale conservation problems: the Spanish imperial eagle and power lines

Background

Man-induced mortality of birds caused by electrocution with poorly-designed pylons and power lines has been reported to be an important mortality factor that could become a major cause of population decline of one of the world rarest raptors, the Spanish imperial eagle (Aquila adalberti). Consequently it has resulted in an increasing awareness of this problem amongst land managers and the public at large, as well as increased research into the distribution of electrocution events and likely mitigation measures.

Methodology/Principal Findings

We provide information of how mitigation measures implemented on a regional level under the conservation program of the Spanish imperial eagle have resulted in a positive shift of demographic trends in Spain. A 35 years temporal data set (1974–2009) on mortality of Spanish imperial eagle was recorded, including population censuses, and data on electrocution and non-electrocution of birds. Additional information was obtained from 32 radio-tracked young eagles and specific field surveys. Data were divided into two periods, before and after the approval of a regional regulation of power line design in 1990 which established mandatory rules aimed at minimizing or eliminating the negative impacts of power lines facilities on avian populations. Our results show how population size and the average annual percentage of population change have increased between the two periods, whereas the number of electrocuted birds has been reduced in spite of the continuous growing of the wiring network.

Conclusions

Our results demonstrate that solving bird electrocution is an affordable problem if political interest is shown and financial investment is made. The combination of an adequate spatial planning with a sustainable development of human infrastructures will contribute positively to the conservation of the Spanish imperial eagle and may underpin population growth and range expansion, with positive side effects on other endangered species.

Assessing the risk of lead exposure for the conservation of the endangered Pyrenean bearded vulture (Gypaetus barbatus) population

Acute and chronic lead (Pb) poisoning have been recognized as some of the most important causes of mortality for raptors worldwide. We simultaneously examined the recent, medium-term and long-term lead exposure of the endangered bearded vulture (Gypaetus barbatus) from the Pyrenees (northern Spain and southern France). One hundred and one blood samples from 87 captured individuals and tissue samples (liver and bone) from a further 43 dead individuals were analyzed for lead residues. The majority of individuals examined had very low lead concentrations in blood, liver and bone. However, two individuals showed elevated blood Pb levels, two individuals showed liver lead concentrations indicative of excessive lead exposure and one individual showed bone lead levels indicative of chronic lead poisoning, suggesting that the Pyrenean population is not free from the risk of poisoning. We found that Pb exposure was significantly higher in adult individuals as well as in the northern (France) and eastern (Catalonia) range of their distribution. These differences could be related to different feeding habits between age classes (pre-adults are more linked to supplementary feeding sites) and differences in hunting practices between regions (in some regions, carcasses and offal of game animals are not retrieved). Blood, liver and bone lead levels found were slightly higher during the hunting season than outside of the hunting season. Lead presents an unnecessary threat to adult birds and the only way to remove this risk is to ban all hunting with lead within the range of distribution of the endangered bearded vulture. Acute and chronic lead poisoning should be considered in differential diagnosis in any diseased or injured wild bearded vulture, especially subadult and adult individuals, and the potential risk of Pb poisoning should be considered in future reintroduction programs.