The effects of habitat quality and female size on the productivity of the lesser spotted eagle Aquila pomarina in the light of the alternative prey hypothesis

Habitat quality is an important but insufficiently understood concept in ecology and conservation biology, due to geographic and temporal variation as well as interaction with individual quality. In 1994–2002, we studied the Estonian population of the lesser spotted eagle Aquila pomarina in order to (1) explore the relative contributions of habitat and female size in reproductive success; (2) check for a switch to alternative prey in vole‐poor years and the relevant variation in annual habitat quality as confirmed in the common buzzard Buteo buteo in the same area. We measured five landscape variables, the number of neighbouring conspecifics and the relative size of the female according to large moulted feathers in 77 nesting territories, and related this to the eagles’ productivity in vole‐rich and vole‐poor years. Nesting lesser spotted eagles benefited from heterogeneous landscapes and suffered from the neighbourhood of conspecifics. There was no evidence that different‐sized females used different habitats. In general, female size was positively related to productivity in vole‐poor but not vole‐rich years, but in the presence of competitors, large size appeared to be disadvantageous. The mean annual productivity of the eagle was well correlated with that of the buzzard, both having peaks after every three years. In contrast to the buzzard, the share of voles in the eagle's diet and its habitat quality did not differ significantly between good and poor years. We concluded that despite a superficial ecological similarity to the buzzard, the lesser spotted eagle did not behave as predicted by the alternative prey hypothesis, but the study confirmed that annual variation in prey utilization and relative habitat quality are parts of the same functional response. Non‐switching to alternative prey may be related to a historical foraging strategy, used by the eagles before they spread to agricultural landscapes, since the current effects of body size strongly suggested food shortage in vole‐poor years.     

Activity patterns at the Arctic Circle: nocturnal eagle owls and interspecific interactions during continuous midsummer daylight

Circadian rhythms result from adaptations to biotic and abiotic environmental conditions that cycle through the day, such as light, temperature, or temporal overlap between interacting species. At high latitudes, close to or beyond the polar circles, uninterrupted midsummer daylight may pose a challenge to the circadian rhythms of otherwise nocturnal species, such as eagle owls Bubo bubo. By non‐invasive field methods, we studied eagle owl activity in light of their interactions with their main prey the water vole Arvicola amphibius, and their competitor the white‐tailed eagle Haliaeetus albicilla during continuous midsummer daylight on open, treeless islands in coastal northern Norway. We evaluated circadian rhythms, temporal overlap, exposure, and spatial distribution. The owls maintained a nocturnal activity pattern, possibly because slightly dimmer light around midnight offered favourable hunting conditions. The eagles were active throughout the 24‐h period as opposed to the strictly diurnal rhythm reported elsewhere, thus increasing temporal overlap and the potential for interference competition between the two avian predators. This may indicate an asymmetry, with the owls facing the highest cost of interference competition. The presence of eagles combined with constant daylight in this open landscape may make the owls vulnerable to interspecific aggression, and contrary to the available literature, eagle owls rarely exposed themselves visually during territorial calls, possibly to avoid detection by eagles. We found indications of spatial segregation between owls and eagles reflecting differences in main prey, possibly in combination with habitat‐mediated avoidance. Eagle owl vocal activity peaked in the evening before a nocturnal peak in visual observations, when owls were active hunting, consistent with the hypothesis of a dusk chorus in nocturnal bird species. The owls may have had to trade‐off between calling and foraging during the few hours around midnight when slightly dimmer light reduced the detection risk while also providing better hunting conditions     

Rabbit (Oryctolagus cuniculus) abundance and protected areas in central-southern Spain: why they do not match?

European rabbits (Oryctolagus cuniculus), a keystone species in the Iberian Mediterranean ecosystem, are the staple prey of the Iberian lynx (Lynx pardinus) and the Spanish imperial eagle (Aquila adalberti). These predators require medium to high rabbit densities and a low degree of human disturbance. We compared rabbit abundances in areas of central-southern Spain under three levels of protection and management: protected areas, intensively managed (nonprotected) hunting estates, and other nonprotected areas. We used pellet abundance indices to estimate rabbit density in 118 surveys conducted during the summers of 2002 and 2003. We observed greater rabbit abundance in intensively managed hunting estates compared to protected areas and other nonprotected areas, perhaps because policy makers did not consider rabbit numbers when selecting priority areas. Alternatively, differences in game management practices (e.g., predator control or habitat management) may explain the higher rabbit densities observed in managed hunting estates. Our results suggest that the best feeding conditions for the Iberian lynx and the Spanish imperial eagle occur in intensively managed hunting areas, where such predators are frequently persecuted. The conservation of these endangered predators may require efforts to increase rabbit densities in protected areas.     

Availability of optimal‐sized prey affects global distribution patterns of the golden eagle Aquila chrysaetos

Climate and landscape change are expected to significantly affect trophic interactions, which will especially harm top predators such as the golden eagle Aquila chrysaetos. Availability of optimal prey is recognized to influence reproductive success of raptors on a regional scale. For the golden eagle, medium‐sized prey species between 0.5 and 5 kg are widely considered to be optimal prey during the breeding season, whereas smaller and larger species are deemed as energetically sub‐optimal. However, knowledge about the effects of optimal prey availability is still scarce on larger scales. To decrease this apparent knowledge gap, we combined biogeographical information on range margins with information about the foraging behaviour and reproductive success of golden eagles from 67 studies spanning the Northern Hemisphere. We hypothesized that availability of optimal prey will affect foraging behaviour and breeding success and, thus, distribution patterns of the golden eagle not only on a local but also on a continental scale. We correlated the diet breadth quantifying foraging generalism, breeding success and proportions of small (< 0.5 kg), medium (0.5–5 kg) and large‐sized (> 5 kg) prey species within the diet with the minimum distance of the examined eagles to the actual species distribution boundary. Closer to the range edge, we observed decreased proportions of medium‐sized prey species and decreasing breeding success of golden eagles. Diet breadth as well as proportions of small and large‐sized prey species increased, however, towards the range edge. Thus, availability of optimal‐sized prey species seems to be a crucial driver of foraging behaviour, breeding success and distribution of golden eagles on a continental scale. However, underlying effects of landscape characteristics and human influence on optimal prey availability has to be investigated in further large‐scale studies to fully understand the major threats facing the golden eagle and possibly other large terrestrial birds of prey.     

The feeding ecology of Bonelli’s eagle (<Emphasis Type="Italic">Aquila fasciata</Emphasis>) floaters in southern Spain: implications for conservation

Although many studies have investigated the feeding habits of Palaearctic raptors, few have analysed non-breeding populations during dispersal. Bonelli’s eagle (Aquila fasciata), a threatened species in Western Europe, has a relatively long and critical dispersal period. We studied feeding habits, prey selection, and the influence of prey density on floater abundance in this species during its dispersal period in southern Spain. Differences were found between the diet of floaters and that of the closest breeding populations. Diet diversity was rather low for floaters, with European wild rabbits (Oryctolagus cuniculus) being the main prey and the only prey positively selected. Moreover, the number of floater Bonelli’s eagles observed in the dispersal areas was positively associated with rabbit abundance. Other prey included red-legged partridges (Alectoris rufa) and pigeons (Columba sp.). We propose measures to match suitable prey availability for floater eagles in settlement areas and the use of such areas as human hunting fields.     

Tetranucleotide microsatellites for aquila and haliaeetus eagles

A unique community of four syntopic eagle species exists in north‐central Kazakhstan. Questions about behaviour and genetics in these four species would benefit from the development of microsatellite markers. We isolated eight polymorphic microsatellite repeats (AAAG)_n from the eastern imperial eagle (Aquila heliaca) genome using a hybridization enrichment technique. These loci revealed moderate diversity in a local population of eastern imperial eagles (observed heterozygosity 0.26–0.78), and were also polymorphic in steppe eagles (A. nipalensis) and white‐tailed sea eagles (Haliaeetus albicilla). These primers may be polymorphic in other species of Aquila and Haliaeetus eagles.     

Density-dependence in the survival and reproduction of Bald Eagles: Linkages to Chum Salmon

During the late 20th Century, due to decreases in both contamination and persecution, bald eagle (Haliaeetus leucocephalus) populations increased dramatically. Currently, mechanisms regulating eagle populations are not well understood. To examine potential regulating processes in the Pacific Northwest, where eagles are no longer primarily regulated by contaminants or direct persecution, we examined bald eagle reproductive success, breeding populations, winter populations, mortality, and salmon stream use. Wintering and breeding eagle populations in south-coastal British Columbia (BC) quadrupled between the early 1980s and the late 1990s, and have since stabilized. Density-dependent declines in reproduction occurred during 1986–2009, but not through changes in site quality. Mid-winter survival was crucial as most mortality occurred then, and models showed that density-dependent reductions in population growth rates were partially due to reduced survival. Wintering eagles in British Columbia fed heavily on chum salmon (Oncorhynchus keta) runs, and then switched to birds in late winter, when mortality was highest. Eagles tended to arrive after the peak in salmon availability at streams in BC as part of a migration associated with salmon streams from Alaska to northern Washington. Eagles were most abundant in southern BC during cold Alaskan winters and in years of high chum salmon availability. We suggest that eagle populations in the Pacific Northwest are currently partially limited by density on the breeding grounds and partially by adult mortality in late winter, likely due to reduced late winter salmon stocks forcing eagles to exploit more marginal prey supplies. Larger eagle populations have affected some local prey populations. © 2011 The Wildlife Society.     

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.     

A non-invasive genetic evaluation of population size,natal philopatry,and roosting behavior of non-breeding eastern imperial eagles (<Emphasis Type="Italic">Aquila heliaca</Emphasis>) in central Asia

Roughly one-third of all eagle species are considered to be threatened or endangered, but the ecology of most eagles remains poorly understood. While the pronounced territoriality of breeding adults facilitates behavioral studies, the demography of non-breeding individuals (pre-adults and non-territorial floaters) is almost completely unknown. Traditionally, limited data on pre-adult and floater movement come from wing-tagging and/or telemetry studies. As an alternative to these methods we used genetic analyses of non-invasively collected feathers to investigate the population biology of non-breeding eastern imperial eagles (Aquila heliaca) in Kazakhstan. Microsatellite profiles of shed feathers indicate that eastern imperial eagles roost communally with other raptors. Furthermore, roosts are large and dynamic: 287 non-breeding eastern imperial eagles were detected in our sample, and a mark-recapture analysis estimated the total number to be 308 ± 8. The natal origins of these individuals were investigated by comparing their microsatellite profiles to those available for >90% of the eastern imperial eagle chicks hatched at the study site over the six previous breeding seasons. Only 4% of the individuals genetically matched a chick, suggesting that the reserve may serve as a critical refugium for pre-adults and itinerant floaters. Feathers have long been recognized as a suitable source of DNA, but few studies have used wide-scale, non-invasive collections of feathers (>1,000 samples) to investigate fundamental aspects of avian biology. Our research demonstrates that non-invasive genetic analyses of feathers can be used to evaluate population size, natal philopatry, and local movements of birds that are difficult to study using traditional means.     

Avian top predator and the landscape of fear: responses of mammalian mesopredators to risk imposed by the golden eagle

Top predators may induce extensive cascading effects on lower trophic levels, for example, through intraguild predation (IGP). The impacts of both mammalian and avian top predators on species of the same class have been extensively studied, but the effects of the latter upon mammalian mesopredators are not yet as well known. We examined the impact of the predation risk imposed by a large avian predator, the golden eagle (Aquila chrysaetos, L.), on its potential mammalian mesopredator prey, the red fox (Vulpes vulpes, L.), and the pine marten (Martes martes, L.). The study combined 23 years of countrywide data from nesting records of eagles and wildlife track counts of mesopredators in Finland, northern Europe. The predation risk of the golden eagle was modeled as a function of territory density, density of fledglings produced, and distance to nearest active eagle territory, with the expectation that a high predation risk would reduce the abundances of smaller sized pine martens in particular. Red foxes appeared not to suffer from eagle predation, being in fact most numerous close to eagle nests and in areas with more eagle territories. This is likely due to similar prey preferences of the two predators and the larger size of foxes enabling them to escape eagle predation risk. Somewhat contrary to our prediction, the abundance of pine martens increased from low to intermediate territory density and at close proximity to eagle nests, possibly because of similar habitat preferences of martens and eagles. We found a slightly decreasing trend of marten abundance at high territory density, which could indicate that the response in marten populations is dependent on eagle density. However, more research is needed to better establish whether mesopredators are intimidated or predated by golden eagles, and whether such effects could in turn cascade to lower trophic levels, benefitting herbivorous species.     

Prey abundance and reproductive success of the golden eagle Aquila chrysaetos in Sweden

Reproductive output of the golden eagle Aquila chrysaetos (L.), was studied in two areas within the species distribution in northern Sweden during 1975–1980. Reproductive success was compared with the abundance (based on hunting bag statistics) of small game prey species and with the microtine cycles. The proportion of golden eagle pairs with successful breeding and the number of young produced per occupied territory varied greatly between years (21–85% and 0.27–1.24, respectively). In the northern study area I found a significant correlation between the proportion of pairs with successful breeding versus total hunting bag of small game species. There were also significant correlations between vole density and breeding success one year later. This was not the case in the southern study area, mainly due to a good reproductive year for golden eagles in 1977 when small game species were scarce. The good reproductive output in 1977 may be explained by favourable weather conditions that spring. Whether breeding occurs or not is probably determined by prey abundance early in spring just before the golden eagle female lays her eggs.     

A conservation paradox for the 21st century: the European wild rabbit Oryctolagus cuniculus,an invasive alien and an endangered native species

• 1 This review highlights the status of the European wild rabbit Oryctolagus cuniculus, which is threatened within its native range and yet is a highly successful colonizer across its worldwide, introduced range.
• 2 The European wild rabbit is a keystone species in Iberia, and the survival of a range of threatened predator species, including the Iberian lynx Lynx pardinus and Spanish imperial eagle Aquila aldabertii, is dependent upon the restoration of rabbit populations. Although not native to the UK, the rabbit also performs significant ecosystem services for nationally rare UK species, by maintaining short sward heights in heathland and grassland ecosystems, and serving as a prey item for populations of predators.
• 3 We identify the European wild rabbit as an excellent model to demonstrate the wide range of complex effects that an introduced mammalian species may exert on ecosystems to which it has been introduced. These effects include habitat degradation following overgrazing, competition with native mammals and facilitating meso‐predator release and hyperpredation.
• 4 We also show that rabbit eradication from some sites may generate more problems than are solved because of the impacts of trophic cascades stemming from dependence on rabbits by native predator assemblages.

     

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.     

Eighteen polymorphic microsatellite markers for the highly endangered Spanish imperial eagle (Aquila adalberti) and related species

Here we describe the development of 18 polymorphic microsatellite markers for the endangered Spanish imperial eagle (Aquila adalberti). Microsatellites were tested in five other raptor species. These markers were revealed as good molecular tools for genetic population studies, individual identification and parentage assessment in Spanish imperial eagle and closely related species.     

Induced changes in island fox (Urocyon littoralis) activity do not mitigate the extinction threat posed by a novel predator

Prey response to novel predators influences the impacts on prey populations of introduced predators, bio-control efforts, and predator range expansion. Predicting the impacts of novel predators on native prey requires an understanding of both predator avoidance strategies and their potential to reduce predation risk. We examine the response of island foxes (Urocyon littoralis) to invasion by golden eagles (Aquila chrysaetos). Foxes reduced daytime activity and increased night time activity relative to eagle-na?ve foxes. Individual foxes reverted toward diurnal tendencies following eagle removal efforts. We quantified the potential population impact of reduced diurnality by modeling island fox population dynamics. Our model predicted an annual population decline similar to what was observed following golden eagle invasion and predicted that the observed 11% reduction in daytime activity would not reduce predation risk sufficiently to reduce extinction risk. The limited effect of this behaviorally plastic predator avoidance strategy highlights the importance of linking behavioral change to population dynamics for predicting the impact of novel predators on resident prey populations.     

Prey diversity and selectivity of the African fish eagle: data from a roost in northern Kenya

A total of 1872 bones from a fish eagle roost at Lake Turkana, Kenya was identified and analysed to provide data on fish eagle prey and diet. The approach of this study complements field observation studies in that it provides a long‐term analysis and quantification of the diet, rather than short qualitative observations. The study found that, at least where fish is relatively abundant, fish eagles are both selective and opportunistic: selective in taking preferred fish in much greater proportions than natural availability, and also in avoiding unwanted fish regardless of abundance, but opportunistic in taking certain other species in proportion to their availability. They also show considerable selection in size and habitat preferences of prey. Non‐fish prey forms a considerable proportion of the diet, and probably represents both opportunistic hunting, possibly when conditions for fishing are poor, and killing of other predators robbing fish eagle nests.
As well as describing and quantifying the fish eagle diet at Lake Turkana, data from this study suggest that further study is needed to quantify the impact of fish eagle predation on Clarias lazera , a rare, but valuable species to the Lake Turkana fishery. The study of bird roost prey remains also provides baseline data for studies of fossil microfaunal accumulations. Scatters of bones under fish eagle roosts are common in eastern Africa, and undoubtedly contribute to many of the fossil bone accumulations recovered. One of us (DM) has noted that the taxonomic and skeletal composition of the Koobi Fora roost bears a great resemblance to some of the clusters she is analysing from Olduvai Gorge. Further similar studies will provide additional data on bird roost accumulations.     

The link between feeding ecology and lead poisoning in white-tailed eagles

Lead poisoning affects numerous threatened raptors and is a major cause of death in white-tailed eagles (Haliaeetus albicilla). A major reason for intoxication is assumed to be lead fragments ingested while feeding on game animals killed by lead-based projectiles. However, empirical evidence on the relevance of carrion in raptor diets remains scarce. We therefore investigated the link between raptor feeding ecology and lead poisoning, with white-tailed eagles as a model species for scavenging birds. We collected data on seasonal diet composition and food availability of 7 territorial white-tailed eagle pairs in northeastern Germany. We also analyzed stomach contents (SCs) of 126 eagles found dead from 1996 to 2008 throughout Germany. Multiple regression models revealed that fish were the primary prey for eagles, and waterfowl and carcasses of game mammals comprised a large portion of alternative diet components. Eagles used individual foraging tactics, adjusted to local food supply, to maximize profitability. They showed a type II functional response to fish availability. When fish availability sharply declined, eagles switched to waterfowl and carrion. The consumption of game mammal carrion increased over autumn and winter and was positively correlated with a concomitant seasonal increase in the incidence of lead poisoning in eagles throughout Germany. The stomachs of lead-poisoned eagles predominantly contained game ungulate remains. These results indicate that carcasses of game mammals were the major sources of lead fragments. The link between raptor feeding ecology and lead poisoning is the specific functional response of raptors to changing food availability or poor habitat quality, leading to scavenging on lead-contaminated carrion. This shows that carrion constitutes a considerable threat to white-tailed eagles and other birds with similar feeding habits as long as it contains lead bullet fragments. Conservation management of scavenging birds would be substantially improved if carrion was free of lead bullet fragments. One method to achieve this is the widespread introduction of lead-free ammunition. © 2012 The Wildlife Society.     

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.     

Kleptoparasites influence foraging behaviour of the spider <Emphasis Type="Italic">Stegodyphus lineatus</Emphasis> (Araneae,Eresidae)

Prey captured by a predator may attract kleptoparasites which could significantly reduce the amount of food consumed. Stegodyphus lineatus, a cribellate spider, builds an energetically costly web. Ants raid the webs of S. lineatus to steal prey and behave as kleptoparasites. We investigated ant raids in a natural population of S. lineatus and their influence on the spider’s foraging behaviour. Considering spiders that had captured a prey, 31.2% suffered an ant raid within 24 h after the prey capture. Experimental tests showed that the response to ant raid is to delay web rebuilding and this was independent of a spider’s previous foraging success. There was a tendency for spiders that were exposed to ants to build larger webs. Neither prey-handling duration nor prey consumption was modified after exposure to ants. These results suggest that Stegodyphus lineatus adapt its web-building behaviour in response to the risk of kleptoparasitism.