Spanish Imperial Eagle Aquila Adalberti Research Articles

Does translocation affect short‐term survival in a long‐lived species, the Spanish imperial eagle?

AbstractIn long‐lived species, usually with deferred maturity and low release numbers, the success of conservation translocations is especially sensitive to survival and site fidelity during the prolonged establishment phase. Concurrent management actions usually implemented in threatened populations may also influence translocation‐related responses. Therefore, the specific performance of translocations should be ideally assessed on vital rates of comparable management groups while accounting for other conservation actions. In this study, we analysed the impact of translocation on the short‐term survival and the relative contribution of electrocution on power pylons by comparing translocated and wild‐reared radio‐tracked juvenile Spanish imperial eagles Aquila adalberti before and after the implementation of further long‐term conservation actions of the species and other generic measures in southern Spain. A multi‐event multi‐state capture–recapture approach accounting for tag retention was applied on a long‐term dataset of resightings and dead recoveries throughout the first 2 years after fledging. Survival probability was similar in translocated and contemporary non‐translocated eagles, but higher than in wild‐reared birds before the generalization of multi‐action plans from the 1980s. It was also higher during early dispersal than in the post‐fledging dependence period. Although mortality due to electrocution decreased in absolute terms after implementing anti‐electrocution measures, its relative contribution as the main mortality source did not differ between translocated and wild‐reared birds or between periods (65%). Our results show that when translocations are properly implemented, observed vital rates of released animals can be comparable to those of wild‐reared individuals subjected to similar generic management addressing mortality sources. As for any management action, post‐release monitoring should play a key role in translocation programmes, with emphasis on comparative studies analysing potential factors affecting survival under active management contexts.

Spatial behaviour of Spanish Imperial Eagle Aquila adalberti juveniles during the dependence period revealed by high-resolution GPS tracking data

During the post-fledging dependence period, young raptors are particularly vulnerable as they are still developing flying, hunting and social skills, and collecting spatial environmental information required for emancipation. We studied the use of space during this critical period by the highly threatened Spanish Imperial Eagle Aquila adalberti, using high temporal and spatial resolution tracking devices. Ten juveniles from four different nests were tracked throughout their dependence period with GPS/GSM tags. This period lasted until a median age of 137 days (93–153). Juveniles remained within a stable home range with a median of 232 ha, but when 90–100 days old began making exploratory movements that became more frequent and extensive with the approach of emancipation. These movements had a median of 22.2 km and were usually completed within the same day, but this exploratory behaviour proved to be quite plastic, as one movement was 1291 km long and lasted 4.5 days. Moreover, three individuals that were apparently forced to emancipate particularly early seem to have virtually skipped the exploratory phase. The length of the movements varied among years within the same nest, and those of females were about twice those of males. Most movements were towards the NW, and the outgoing flight was usually against the wind. This behaviour may keep animals from flying so far from the nest that they are then unable to return. The high spatial and time resolution provided by GPS tracking resulted in data that highlight the conservation importance of the nest site until well after fledging. Measures to minimize nest disturbance should thus be extended until early October. Moreover, data show that so much of the activity of the still inexperienced juveniles is concentrated in the small pre-emancipation range that is critical to protecting its habitats and eliminating all anthropogenic sources of mortality.

Better nutritional condition changes the distribution of juvenile dispersal distances: an experiment with Spanish imperial eagles

We investigated the distribution of juvenile dispersal distances of a territorial long‐lived species with deferred maturity, the Spanish imperial eagle Aquila adalberti. Here we used a reintroduction program as an experimental approach to test predictions of different hypotheses about the distribution of juvenile dispersal distances: competition and wandering behavior. We determined maximal juvenile dispersal distances of 59 young eagles; 1) 30 wild non‐manipulated individuals, and 2) 29 tranlocated young under an ad libitum feeding program, released with adults breeding in the area. The competitive displacement hypothesis predicts a leptokurtic distribution of distances in wild non‐manipulated young as well as in released young. Under the ‘wandering’ hypothesis, however, a leptokurtic distribution is expected in wild young but a normal distribution would be expected in young released (with adults in the release area), owing to a general improvement in the nutritional status of released young that have been fed ad libitum, as is usual in reintroduction programs. Additionally, a negative relationship between hatching date and dispersal distances is expected in wild young but no relationship in released young under ad libitum feeding. Mean maximum dispersal distances for all the juvenile eagles was 142.8 km. No differences between sexes were found, nor between populations or between wild and reintroduced young. Wild young distances were not normally distributed, being closer to a Poisson distribution. In contrast, released young with adults (under ad libitum feeding) showed a normal distribution. Wild birds showed a significant negative relationship between dispersal distance and hatching date, with young that hatched late in the season dispersing shorter distances. However, released young under ad libitum feeding showed no significant relationship between hatching date and dispersal distance. These results support the ‘wandering’ hypothesis as the main driver of the distribution of dispersal distances.

Behavioral evidence of hunting and foraging techniques by a top predator suggests the importance of scavenging for preadults

Scavenging may be a regular feeding behavior for some facultative raptor species occupying low quality habitats and/or with little experience in hunting techniques. However, its importance has been largely underestimated due to methodological limitations in identifying the real proportion in the diet. Here, through direct observations, we assessed the hunting and foraging success of the threatened Spanish imperial eagle Aquila adalberti determining the influence of age, sex, breeding status, habitat quality, prey type, and landscape characteristics. From 465 observations, Spanish imperial eagles used hunting in flight (42%), scavenging (30%), hunting from a perch (16%) and kleptoparasitism (12%). Our model suggests that Prey size and Prey type best explain hunting success, followed by Landscape and Sex. Our findings suggest that Spanish imperial eagles increase hunting success with age, with scavenging and kleptoparasitism regularly used as juveniles. The absence of relationships with any of the variables considered suggests that kleptoparasitism is an opportunistic behavior used sporadically. Scavenging is also independent of habitat quality and landscape characteristics. Accordingly, low prey density is not a driver of carrion use for preadult individuals, suggesting that a lack of hunting ability obliges this age‐class to use this alternative feeding technique regularly. As a result, the threatened Spanish imperial eagle population is also prone to mortality related to the illegal use of poison baits and, potentially, veterinary drugs (i.e., diclofenac).

Floater interference reflects territory quality in the Spanish Imperial Eagle Aquila adalberti: a test of a density‐dependent mechanism

We report on an 11‐year study of floater interference in a population of Spanish Imperial Eagles Aquila adalberti. We analysed changes over the years in the productivity of 15 territories to test predictions of two hypotheses of density‐dependent productivity in relation to the presence of floaters (birds without territories). According to the ‘interference' hypothesis, the frequency of intrusion by floaters increases with density, resulting in a decrease in productivity. Thus, in a high‐density population a negative relationship between floater intrusions and productivity of the territory is expected. In contrast, under the ‘habitat heterogeneity' hypothesis, as density increases a higher proportion of individuals is forced to occupy lower quality habitats. Support of this hypothesis requires that floaters detect differences in quality among territories and preferentially visit the better quality territories. Consequently, a positive relationship between floater intrusions and productivity is expected. Results showed that floaters tended to visit their natal area at the beginning of the breeding season. Among floater Eagles, males made significantly more intrusions per day than did females, but females stayed in the natal population for longer each year than males. Floater intrusions and productivity were highly positively correlated, supporting the ‘habitat heterogeneity' hypothesis; individuals were apparently able to assess the quality of a territory and, at the frequencies observed, their interference with the breeding pair had no obvious negative effect on productivity.

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

The population of the Vulnerable Spanish imperial eagle Aquila adalberti has experienced a gradual recovery from 38 pairs (1974) to 198 (2004). We analysed the spatial and temporal variation of the demographic parameters for 1981–2004. Annual productivity was 1.19–1.32 chicks per female and adult survival rate 0.918–0.986. Survival during the post-fledging period was 0.894 and the annual survival rate of the dispersing individuals was 0.561. Three phases of population evolution were distinguished: growth (1981–1993), stability or slight decrease (1994–1999) and growth (2000–2004). Variation in adult survival seems to explain this pattern for the first two periods. However, a large disparity between the observed growth rate and the modelled population growth in 2000–2004 is best explained if we assume that a decrease in the age of recruitment took place. This is supported by the recent increase in the frequency of non-adult birds in breeding pairs. The survival of unpaired eagles in dispersal areas is becoming more important for the maintenance of current population growth. Spatial variation of adult survival and breeding success is not congruent with the observed growth rate of the population, which suggests the existence of an important flow of individuals between populations. These results highlight the importance of addressing the conservation of the species from a global perspective and the need to focus on adult survival in breeding territories and on non-adult survival in dispersal areas.

Breeding of Non-Adults and Effects of Age on Productivity in the Spanish Imperial EagleAquila adalberti

We studied annual and geographical variation in the proportion of breeding non-adult (<4.5 years old) Spanish Imperial Eagles Aquila adalberti in relation to the species' recent demographic recovery. The proportion of non-adult breeders in the populations studied increased significantly during 1983–2004 in parallel with the population increase. Non-adult pairs were more often associated with ‘young’ territories (occupied recently) than with territories that had already been occupied for longer time (mostly used by adult pairs). Adult pairs produced significantly more chicks than mixed adult— non-adult pairs, which in turn had a higher productivity than pairs in which both male and female were non-adult. Adult pairs attempted breeding more often than non-adult pairs, and clutch and brood size were larger. The poorest reproductive output was found in pairs with non-adult males. These differences were not associated with differences in fledging rates. Once the influence of territories was controlled for (com...

Brood‐switching in Eagle Owl Bubo bubo fledglings

Nest-switching, the (permanent or temporary) intrusion of one or several foreign young in another brood (which gener ally occurs when fledglings are capable of flight but are not yet completely independent of parents) and their sub sequent adoption by a foster family (parents + native young), has been recorded in many bird species (Riedman 1982). These include several birds of prey (Ospreys Pandion haliaetus , Poole 1982, Gilson & Marzluff 2000; Eurasian Sparrowhawks Accipiter nisus , Wyllie 1985; American Kestrels Falco sparverius , Lett & Bird 1987; Egyptian Vultures Neophron percnopterus , Donazar & Ceballos 1990; Black Kites Milvus migrans and Red Kites Milvus milvus , Bustamante & Hiraldo 1990; Northern Goshawks Accipiter gentilis , Kenward et al. 1993; Spanish Imperial Eagles Aquila adalberti , Ferrer 1993; Lesser Kestrels Falco naumanni , Tella et al. 1997; Montagu’s Harrier Circus pygargus , Arroyo & Garcia 2002) and owls (e.g. Barn Owl Tyto alba , Roulin 1999). One of the most intriguing aspects of this form of alloparental care (Riedman 1982) is the investment of resources by birds into non-genetic offspring, instead of allocating breeding effort exclusively to their own genetic contribution to future generations. Such behaviour seems incompatible with classic evolutionary theory, because it apparently violates the Darwinian principle by which selection does not act to benefit competing genotypes. To explain adoptions in birds, several hypotheses have been invoked and tested. The potential explanations for nest-switching and alloparental care form two main groups (see reviews in Redondo et al. 1995, Bize et al. 2003). First, there may be adaptive explanations such as (1) benefit to fledglings (e.g. better care than in the natal nest, acquisition of a dominant rank within a younger brood, reduction of the ectoparasite load), and (2) kin-selected benefits from a parental perspective when young switch to nests of related adults, diluting predation risk, or increasing breeding experience of related individuals. Secondly, there may be non-adaptive explanations such as reproductive errors or adoptions with negligible costs which do not affect reproductive success or survival of the foster parent. The different intensities of selection pressure for intruder chicks (surviving vs. dying) and foster parents (cost of investment in unrelated fledglings) can also be conceived of as an arms race (Pierotti & Murphy 1987). Under such a scenario, chicks are likely to ‘win’ (and consequently gain adoption) because the selection pressure is stronger than on foster parents to discriminate and reject intruders. Here, we report detailed observations of two cases of nest-switching in radiotagged Eagle Owl Bubo bubo fledglings and discuss them in an evolutionary context. Because nest-switching occurred during the post-fledging dependence period (i.e. owlets were no longer in the nest because they abandon it completely), hereafter we consider it more appropriate to replace the term nest-switching with brood-switching.

Non‐independence of demographic parameters: positive density‐dependent fecundity in eagles

Summary 1 Using information on the Donana population of Spanish imperial eagles Aquila adalberti from 1959 to 2004, we present strong empirical support to theoretical models on the regulation of population trajectories by the relationships between breeder mortality and floater availability. 2 During the study period, the eagle population showed three distinct phases: (i) a population increase with negative density-dependent fecundity; (ii) a period of stability without any relationship between density and fecundity; and (iii) a population decrease with a positive relationship between density and fecundity. 3 A dramatic increase in annual adult mortality due to an increase in poisoning in hunting areas surrounding the Donana National Park was recorded. The use of poison against generalist predators accounted for more than 54% of the total number of breeding eagles found dead since 1990, increasing annual adult mortality from 6·07 to 12·01%. 4 This high mortality reduced the population annually by 6% during the 1992–2004 period. Also, the population changed from a negative to a positive relationship between density and fecundity (Allee effect). These trends made the population approach extinction due to the double effect of increasing breeder mortality and low availability of floaters. 5 A supplementary feeding programme established in 1990 did not increase fecundity because it was a consequence of high adult mortality rather than low food availability. The high mortality of adults increased the risk of breeding failure and also decreased the availability of floaters, decreasing the likelihood of mate substitution. 6 Synthesis and applications. The main target in species conservation management plans is often a single demographic parameter (typically, fecundity in raptor populations). Our research demonstrates, however, that demographic parameters must not be considered as independent variables when formulating management programmes. The essential relationship between adult mortality and the availability of floaters must be better understood to avoid the risk of implementing inefficient management strategies. Although fecundity was low in this eagle population, it was a result of high adult mortality rates. Consequently, management must aim to correct the unusual mortality and recover the floater population.

Status and habitat changes in the endangered Spanish Imperial Eagle Aquila adalberti population during 1974–2004: implications for its recovery

AbstractThe distribution and abundance of Spanish Imperial Eagle Aquila adalberti populations between 1974 and 2004 were determined using information from national censuses. Its breeding area occupies the south-western quadrant of the Iberian Peninsula (Spain and Portugal) and is composed of 13 nuclei and 5 subpopulations. Since 1974, population levels in all nuclei, except the one in Doñana, have expanded. The non-breeding dispersion area, according to sightings of juvenile and immature individuals in quadrants of 10 x 10 km, coincided with that of the breeding area. Bibliographical information showed that halfway through the 19th century the Spanish Imperial Eagle was considered abundant, at least locally; and most cited breeding areas were in relatively human-occupied plains. Towards the end of the 19th century the population became scarce; remaining so for most of the 20th century, with remote mountain ranges being the most cited breeding habitats. The comparison between the data from the first census, in 1974, that located 38 territorial pairs, and the 2004 census that located 198 pairs, shows that: 1) percentages of pairs in plains have increased, while those in mountains have decreased; 2) the trophic quality of the habitat, based on rabbit abundance, has decreased, and 3) numbers of nests in both protected areas and on private ground have increased significantly. The type of land ownership did not seem to affect breeding performance. Populations have increased more outside protected areas than within, despite the availability of potential habitat. In the past century, legal protection and attitude changes towards this eagle seem to have been influential in preventing its extinction. At present, habitat management seems also to be an important factor in its continuing recovery.

Causes and spatio-temporal variations of non-natural mortality in the Vulnerable Spanish imperial eagle Aquila adalberti during a recovery period

AbstractThe analysis of 267 records of non-natural mortality of the Spanish imperial eagle Aquila adalberti over a 16-year period (1989-2004) shows an annual rate of 15.1 individuals found dead per year and that electrocution (47.7%) and poisoning (30.7%) were the most frequent causes of mortality. Most cases (91.7%) were of human origin, and of those 92.3% were accidental. Just over half (50.2%) were related to the transmission of electricity and human activity (collisions and electrocution), and 40.7% related to game practices and livestock protection (control of predators). No differences between sexes were found but subadults were electrocuted more frequently than expected whilst adults were poisoned more frequently. In breeding areas poison was the most frequent cause of mortality, whereas electrocution was the most common cause of death in dispersal areas. Poisoning occurred more frequently than expected in the Northern and Southern regions compared to the West-Central region. Electrocution was significantly more frequent in the West-Central region, and less common in the Southern region. The increase in electrocutions over the last few years is associated with previous non-permanent corrections on electricity power lines, whilst the increase in the cases of poisoning appears to be associated with the use of illegal poison in predator control by small game practices and for livestock protection. Permanent corrections in power lines and more research and awareness effort in the small game sector are recommended to reduce human-induced mortality in this Vulnerable species.

Human disturbance and the conservation of the Spanish imperial eagle: a response to Gonzalez et al. (2006)

González et al. (2006) analysed the influence of human intrusions on the behavior of breeding Spanish imperial eagles Aquila adalberti and concluded that hatching rate was negatively correlated with the frequency of intrusions. Following this result, they suggested the adoption of a 500-m-radius exclusion zone around every nest with no human activities allowed during the breeding season and an outer 800-m-radius buffer zone where some activities would be allowed by permission. This is an interesting and valuable study. Nevertheless, some considerations should be made before the proposal could be adopted. First, González et al. may have underestimated the fact that eagles can habituate to non-lethal human activities. They did report, however, a negative relationship between the probability of a flight reaction and the frequency of human activities, with reactions being less frequent in areas with more human activities. This critical result points out that eagles are indeed able to habituate to human presence, as other raptors do where they are not persecuted (e.g. ospreys Pandion haliaetus in North America, Bird et al. (1996). Furthermore, this agrees with previous studies on responses of breeding Spanish imperial eagles to human intruders (Ferrer, 1990; Ferrer, García & Cadenas, 1990). These studies showed reinforcement in nest-defense by eagles in the same territories over the years. The legal protection of all birds of prey from the late 1970s in Spain and, especially the growth of public awareness for endangered species, resulted in the reinforcement of the eagles' aggressive tendencies by non-destructive human intruders, including bird-banders or veterinarians. This change in selective pressure, coupled with the fact that the Spanish imperial eagle is a raptor possessing a long reproductive life, would facilitate a decrease in approaching distance to the intruders. Second, we have concerns with the statistical analyses. González et al. reported a significant negative correlation between hatching rate and per cent of days with intrusions during the incubation period (rs=−0.63, P<0.05). However, using data from their table 5, rs=−0.627, P=0.052, only marginally significant under α=0.05 (using Statistica software as well as SPSS software). The authors also reported a non-significant relationship between hatching rate and average intrusions per day (rs=−0.52, P>0.05), which was in fact significant using the raw data they provide (i.e. rs=−0.677, P=0.031). Nonetheless, excluding territory M-18, with only six observation days, all significances disappear. Taking into account that the relation between disturbances and hatching rate is the basis for the proposed 800-m-radius buffer area, this proposal does not seem to be well supported. In addition, it seems to be unnecessarily conservative, as González et al. state that imperial eagles never respond to human intrusions occurring >600 m away from their nest site. The extra distance from 600 to 800 m covers 88 Ha, or 43.7% of the total suggested area. Lastly, in long-lived species such as the Spanish imperial eagle, fecundity is the demographic parameter with the lowest effect in the short term on the annual per cent of population change (Ferrer & Calderón, 1990; Ferrer & Hiraldo, 1991; Ferrer, 2001). A hypothetical increase of 5% in fecundity due to the establishment of the human-free area would increase the population 0.09% year−1. A 5% increase in adult survival, however, would increase the population 1.6% year−1, or 17.7 times faster. Results obtained protecting dangerous power lines have shown that even higher annual increases are possible (Ferrer & Hiraldo, 1991). Considering that the Spanish imperial eagle population has been on the increase during the last 10 years, and that more than 60% of the new pairs established themselves in private properties, it seems counter-productive to impose restrictions to land owners. The proposed management action is not warranted from a scientific point of view, and may even be detrimental if the presence of eagles diminished owners' income. There is the risk that eagles were harassed or their nest destroyed (even if unlawful) if otherwise legal activities such as hunting, cork harvesting or livestock raising were curtailed. In conclusion, although we recognize the interest and utility of this study, and we agree to improve eagle fecundity avoiding human interference; we believe that the usefulness of such a buffer area excluding human presence has not been sufficiently demonstrated by Gonzalez et al. It is expected to have a small effect on eagle population growth and probably a negative impact in public opinion about eagles. On the other hand, eagle's habituation to human presence can lead to a huge increase in potential nesting habitat for this species, and there are already examples of eagles successfully raising young <200 m away from picnic areas and roads (M. Ferrer pers. obs.). The adoption of large exclusion areas in otherwise humanized landscapes must be exerted with caution in a world where conservation should not always equate to exclude us humans.

Can buffer zones have detrimental effects on the conservation of Spanish imperial eagles?: a response from Gonález et al.

We recently published the first study that empirically quantifies the reaction of the threatened Spanish imperial eagles Aquila adalberti towards a variety of human activities around the nest, and an evaluation of its influence on breeding success, with the aim of recommending buffer zones in order to enhance the Spanish imperial eagle’s reproductive success (Gonzalez et al., 2006). A reply of Ferrer et al. (2007) agrees with the interest and utility of our study, although they argued that we ‘underestimated that eagles can habituate to non-lethal human activities’, suggesting that ‘it seems counter-productive to impose restrictions on landowners and may even be detrimental if the presence of eagles diminished owners’ income’. Ferrer et al. speculate that a change in selective pressure (based on reinforcement of nest defence behaviour with increasing intruder frequency, Ferrer, Garcia & Cadenas, 1990) linked with the long reproductive life of the species would facilitate a decrease in the distance at which eagles would flush. This is not necessarily so. It could facilitate a decrease in the distance of attacks towards the intruders (as they state), but not necessarily a decrease in the distance from the intruders at which eagles would jump from the nest. Additionally, it is not clear whether the change in selective pressure indeed has the consequences predicted by Ferrer et al. (1990). For example, no further increase in aggressive behaviour of Spanish imperial eagles towards nest visitors has been observed, as Ferrer et al. (1990) expected. The small sample size in which their study was based may imply that those results were more individualdependent than applicable to the whole of the species. These authors argue that it would be more helpful to get the eagles used to human intrusions, assuming that if human presence were to be excluded from the territories, eagles would become increasingly sensitive to human presence. However, even if our results showed that some habituation does exist, this habituation was not enough to prevent a decrease in reproductive success, and so we disagree with the fact that it would be better to get eagles used to humans (the risk that it could create a decrease in productivity before the predicted ‘total’ habituation is not worth taking, in our opinion). Secondly, these authors minimize the importance of the effect of human activities on the eagles’ hatching rate because, according to them, the relation is only ‘marginally significant’. According to their re-analyses, the probability of the correlation between hatching rate and number of days with intrusions is P=0.052 with r= 0.627 (being rounded off in Gonzalez et al. (2006) as r= 0.63, Po0.05, n=10). The discrepancy may arise because of different ways of estimating the significance of r in different statistical software. Following the Sokal & Rohlf (1981) tables, with n=10, P is significant (0.05) when r40.576. Furthermore, Siegel & Castellan (1988) indicate that when testing for the significance of Spearman’s correlations, a special technique has to be used for a small sample size. According to that ouvrage, with n=10, P is significant (0.05) when r40.564. Thus, we reassert that there exists a significant trend for the hatching success to decrease with increasing intrusions in the eagle’s territories. And, as they are right to point out, there is also a significant (r= 0.677, P=0.02) and negative relationship between the hatching rate and intrusions per day, which also reaffirms our results. Ferrer et al. also argue that the significance disappears when excluding one of the territories, and so they conclude that the relationship is not important enough to support a proposal of a buffer around the nest. As Ferrer et al. know, it is difficult enough to have large sample sizes with these species (in terms of number of territories monitored). Even if significance decreased when excluding one of the nests, that could simply mean that the remaining sample size would not be large enough to detect significant results at 0.05 (a b test would be needed). Nevertheless, we have re-analysed the correlation eliminating the M-18 territory as suggested, and contrary to their arguments, the correlation is still significant (Po0.05) between the hatching rate and both variables (r= 0.6129 and r= 0.6129). Finally, Ferrer et al. (2007) argue that an increase in fecundity is less important than a decrease in survival (which

A long-term large-scale study of the breeding biology of the Spanish imperial eagle (Aquila adalberti)

We present data from a 17-year study of the population biology of a growing population of Spanish imperial eagles Aquila adalberti across most of its breeding range. The objective of this study was to analyse the effects of age, supplemental feeding and rabbit haemorrhagic disease (RHD) on several breeding parameters of this population of eagles. Average clutch size was 2.2 eggs per clutch, and the average incubation time was 41.7 days per clutch. Fledging occurred an average of 76.8 days after hatching, the length of the fledgling period was not correlated to clutch size. The annual average percentage of pairs laying eggs was 88%. A significant reduction in the percentage of pairs laying eggs in the period 1992–1994 (from 91 to 81%) coincided with most of the eagles’ territories being affected by the rabbit epizootic disease, RHD, which reduced their food supply significantly. Average productivity was 1.23 chicks per monitored territory, average breeding success was 1.40 chicks in a clutch per territory and the average fledging rate was 1.69 chicks per territory with hatching success. The main known causes of breeding failure during incubation were nest collapse and human disturbance. During chick-rearing, total or partial chick losses were mainly caused by siblicide, disease, malnutrition or nest collapse. In 26.2% of the 1372 monitored breeding attempts, at least one of the breeding birds was a subadult (the male in 56.1% of the cases, the female in 15.5% and both sexes in 28.4% of cases). In cases of mixed-aged pairs (n = 205), 70.7% were the result of a substitution, and 29.3% were the result of the forming of a new pair. Egg laying took place significantly earlier and breeding success was higher in territories occupied by adults than in those occupied by subadults. Breeding parameters were higher in high-quality (rabbit-rich) territories than in low-quality (rabbit-poor) territories, but only for those territories occupied by adults. The values obtained in the territories occupied by adults were only significantly higher than in those of the subadults in high-quality territories. Age and territory quality thus simultaneously affected reproductive output.

Parental behaviour of Spanish Imperial Eagles Aquila adalberti: sexual differences in a moderately dimorphic raptor

Capsule The sexes make significantly different contributions in the tasks of nest-building, incubation and food provisioning to chicks. Aims To determine the division of parental activities during breeding (nest-building, incubation, brooding, food provisioning and feeding). Methods Between 1991 and 1998 focal observations were made at nests using 20–60× telescopes. During the incubation period, 11 pairs were studied involving a total of 2812 observation hours over 269 days. During the chick-rearing period, seven pairs were studied involving a total of 5499 observation hours spread over 503 days. The birds were sexed and identified individually on the basis of size, position during copulation, vocalization and plumage differences. Results Significant differences were found between the sexes in parental behaviour. Males scarcely participated in incubation and contributed significantly more than the females only in food provisioning, both during incubation (food transfers to feed the female) and during chick-rearing (to feed the chicks). Females were responsible for most of the incubation, nest-building (during incubation and chick-rearing), brooding, shading and feeding of chicks. Nest attendance by females decreased over time. In both sexes, food provisioning increased with brood size. Conclusion Intersexual differences are discussed in the context of the reversed sexual size dimorphism and parental investment strategies. We suggest that in Spanish Imperial Eagles reversed sexual size dimorphism is best explained by the prey capture difficulty hypothesis, than by parental role division during reproduction.

Effective natal dispersal and age of maturity in the threatened Spanish Imperial Eagle Aquila adalberti: conservation implications

Capsule Dispersive capacity appears sufficient to establish natural re-occupation of the former range. Aims To document data on effective natal dispersal and age of maturity in the increasing, but fragmented, Spanish Imperial Eagle population. Methods Between 1990 and 2002, 84 nestlings from six of the 14 breeding nuclei of the species' range in Spain were tagged with metal and plastic leg-rings and radiotagged with VHF and satellite transmitters. Birds and breeding pairs were monitored annually throughout the distribution range for evidence of breeding status, movements, juvenile dispersal areas and behaviour prior to first-time pairing, establishment of a territory, and breeding. Results The mean effective natal dispersal distance was 101.2 km and the median was 80 km (range 3–310, n = 12). Depending on how philopatry was defined, between 42 and 50% of individuals were considered philopatric. The majority of the philopatric individuals bred for the first time on the border of the breeding distribution range. Birds tend to breed first in the last area they inhabited. During their first breeding attempt, most eagles lay eggs and most have subadult plumage at this time. Birds were first recorded as being paired and territorial at a mean age of 3.4 years (n = 9), first egg-laying occurred at 4.5 years (n = 10) and first successful breeding at five years (n = 6). The earliest known breeding age was two years in females and three years in males. Conclusion The dispersive capacity of Spanish Imperial Eagles appears sufficient to enable natural re-occupation of their former range. These results have important implications for the species conservation strategy, because the chance of contacting other breeding nuclei is greater than previously estimated and the natal dispersal distance has a relatively high colonizing capacity.

Floater Dynamics Can Explain Positive Patterns of Density‐Dependent Fecundity in Animal Populations

After some 70 years of debate on density-dependent regulation of animal populations, there is still poor understanding of where spatial and temporal density dependence occurs. Clearly defining the portion of the population that shapes density-dependent patterns may help to solve some of the ambiguities that encircle density dependence and its patterns. In fact, individuals of the same species and population can show different dynamics and behaviors depending on their locations (e.g., breeding vs. dispersal areas). Considering this form of intrapopulation heterogeneity may improve our understanding of density dependence and population dynamics in general. We present the results of individual-based simulations on a metapopulation of the Spanish imperial eagle Aquila adalberti. Our results suggest that high rates of floater mortality within settlement areas can determine a shift in the classical relationship (from negative to positive) between the fecundity (i.e., fledglings per pair) and density (i.e., number of pairs) of the breeding population. Finally, we proved that different initial conditions affecting the breeder portion of the population can lead to the same values of fecundity. Our results can represent a starting point for new and more complex approaches studying the regulation of animal populations, where the forgotten and invisible component--the floater--is taken into account.

Can prey behaviour induce spatially synchronic aggregation of solitary predators?

Spatio‐temporal patterns of predator aggregations within their settlement areas (i.e. temporary settling zones used during dispersal or, more generally, foraging patches) were studied. By integrating the main behavioural rules of juveniles of Spanish imperial eagles Aquila adalberti during dispersal with the temporal availability of the eagles’ main prey, we have developed several individual‐based models under different simulation scenarios. The results suggest that the spatially synchrony of the aggregation patterns that we observed were derived from a combination of behavioural responses of the prey and the distances between available settlement areas. These results are discussed in the context of predator–prey relationships, optimal foraging and self‐organizing processes.

Floater Dynamics Can Explain Positive Patterns of Density-Dependent Fecundity in Animal Populations

After some 70 years of debate on density‐dependent regulation of animal populations, there is still poor understanding of where spatial and temporal density dependence occurs. Clearly defining the portion of the population that shapes density‐dependent patterns may help to solve some of the ambiguities that encircle density dependence and its patterns. In fact, individuals of the same species and population can show different dynamics and behaviors depending on their locations (e.g., breeding vs. dispersal areas). Considering this form of intrapopulation heterogeneity may improve our understanding of density dependence and population dynamics in general. We present the results of individual‐based simulations on a metapopulation of the Spanish imperial eagle Aquila adalberti. Our results suggest that high rates of floater mortality within settlement areas can determine a shift in the classical relationship (from negative to positive) between the fecundity (i.e., fledglings per pair) and density (i.e., number of pairs) of the breeding population. Finally, we proved that different initial conditions affecting the breeder portion of the population can lead to the same values of fecundity. Our results can represent a starting point for new and more complex approaches studying the regulation of animal populations, where the forgotten and invisible component—the floater—is taken into account.

Lead concentrations in bones and feathers of the globally threatened Spanish imperial eagle

Raptors that consume game species may ingest lead fragments or shot embedded in their prey’s flesh. Threatened Spanish imperial eagles Aquila adalberti feed on greylag geese in southern Spain in winter, and often ingest lead shot. We analysed bone and feather samples from 65 Spanish imperial eagle museum specimens collected between 1980 and 1999, to investigate the prevalence of elevated lead concentrations. Four of 34 birds (12%) had very elevated bone lead concentrations. All four birds were young and the concentrations were outliers to the distribution, suggesting probable exposure to lead gunshot. Excluding these elevated lead outliers, bone lead concentrations were correlated with the bird’s age at death. Three of 41 feathers (7%) had elevated lead concentrations, indicative of high exposure during feather formation. When these outliers were omitted, feather lead concentration was correlated with the age of museum specimens, suggesting that a high proportion of feather lead was exogenous, deposited after specimen collection. Therefore, careful interpretation of feather lead concentrations is required to separate endogenous and exogenous lead. We discuss the potential significance of lead poisoning in Spanish imperial eagles and other raptors, and recommend measures for its reduction.