Coexistence of protected avian predators: does a recovering population of White-tailed Eagle threaten to exclude other avian predators?

The processes of competition and predation determine the degree to which species can coexist; the importance of competition in particular has been emphasized at high trophic levels. Competition exclusion will occur when habitat overlap between sympatric species is high. In this study, we investigated nesting habitat overlap between internationally protected diurnal tree-nesting avian predators of central Europe, namely, White-tailed Eagle (Haliaeetus albicilla), Lesser Spotted Eagle (Aquila pomarina), Black Stork (Ciconia nigra), and Osprey (Pandion haliaetus). We found significantly different nesting habitats among the study species and suggest that this could be a consequence of the resource-based segregation, but not a consequence of asymmetrical interspecific competition. The results also show that habitat of the recovering populations of White-tailed Eagle overlapped with the habitat used by the Lesser Spotted Eagle, Black Stork, and Osprey to varying extents with a niche overlap values being below the competition exclusion threshold. Nevertheless, we suggest that competition by White-tailed Eagle at a population level may limit Osprey, though not Lesser Spotted Eagle or Black Stork.     

Madagascar Fish Eagle productivity in the Tsimembo-Manambolomaty Protected Area and surrounding habitat of western Madagascar

We monitored the productivity of the critically endangered Madagascar Fish Eagle Haliaeetus vociferoides inside and outside of the Tsimembo-Manambolomaty Protected Area (T-M PA), western Madagascar from 2010 to 2015. We recorded 14 breeding pairs inside and 13 outside T-M PA. The T-M PA and surrounding habitat hosted respectively 10 and six breeding polyandrous pairs, composed of one adult female and two adult males. During the six-year study period, 101 eggs were laid in nests in T-M PA of which 60 hatched and 58 young fledged. We recorded 62 eggs laid in nests outside the T-M PA of which 39 hatched and 36 young fledged. Productivity was similar at both sites, inside and outside T-M PA, with 0.84 (58/69) and 0.76 (36/47) fledgling per nesting attempt and 0.69 (58/84) and 0.5 (36/72) fledglings per territorial pair, respectively. Polyandrous pairs have higher productivity compared with normal pairs. Threats to Madagascar Fish Eagles and their habitat were low due to the existence of a community-based resource management system called the Local Management Secured System (GELOSE) inside and outside the T-M PA. This system is based on strengthening local traditional customs and rules, and involving local people in managing their natural resources sustainably along with biodiversity conservation.     

SOME NOTES ON THE BIRDS OF THE KAOKOVELD

Whitfield, A. K. &; Blaber, S. J. M. 1978. Feeding ecology of piscivorous birds at Lake St Lucia, Part 1: Diving birds. Ostrich 49:185-198.

The diets of three species of diving piscivorous birds at Lake St Lucia, Natal, South Africa during 1975 and 1976 are described and related to availability and abundance of different prey species. Mugilidae and Clarias gariepinus were the chief prey of the Fish Eagle Haliaeetus vocifer, while Johnius belengcrii and Sarotherodon mossambicus were, respectively, the main food items of Caspian Terns Hydroprogne tschegruva, and Pied Kingfishers Ceryle rudis. The methods of prey capture used by the three species of birds are described.

Predation on particular species of fish was related primarily to their swimming depth, abundance, size and distribution in littoral areas. Fishing habits, densities and foraging periodicity of the birds are discussed with regard to physical parameters such as thermals, wind speeds and water turbidity. The timing of breeding seasons at Lake St Lucia is related to fish densities, lake levels, turbidity of the water and air temperatures.     

Development and multiplex PCR amplification of novel microsatellite markers in the White‐tailed Sea Eagle,Haliaeetus albicilla (Aves: Falconiformes,Accipitridae)

We report the development of 14 novel polymorphic microsatellite markers cloned from the White‐tailed Sea Eagle, Haliaeetus albicilla, a formerly threatened raptor that has received much conservation attention throughout Eurasia. We also present a protocol for multiplex polymerase chain reaction (PCR) amplification of the loci. Among 40 unrelated H. albicilla individuals from southern Sweden, the markers produced two to eight alleles per locus, and average observed and expected heterozygosities were 0.463 and 0.468, respectively. We further present five microsatellite markers that appeared monomorphic in H. albicilla, but which may be of interest for use in other raptor species.     

Spatial and temporal variations in the White-tailed Eagle Haliaeetus albicilla breeding diet revealed by prey remains

ABSTRACT

Capsule: Despite its population increase, the White-tailed Eagle Haliaeetus albicilla has not become food-limited, and does not pose a serious threat to birds of conservation concern in the region east of the Baltic Sea.

Aims: We aimed to test variations in the main prey groups and dietary niche breadth of White-tailed Eagles through the years and along an environmental gradient as well as to evaluate the extent of predation on bird species of conservation concern.

Methods: Prey remains were collected during nestling ringing from successful nests located in Lithuania.

Results: The frequency of the prey groups varied along an environmental gradient, with a general pattern of increasing avian prey consumed by pairs breeding in areas with less abundant aquatic habitats. The frequency of alternative prey (i.e. terrestrial birds, mammals) and dietary niche breadth did not increase between 2005 and 2018 and during which time the White-tailed Eagle population nearly doubled. Instead, the occurrence of Northern Pike Esox lucius remains, which was the most frequent single prey species, increased throughout the study period. Bird species of conservation concern constituted only a small fraction of all the prey identified.

Conclusions: The White-tailed Eagle has not become more dependent on alternative prey despite its population increase.     

The Orange River avifauna: abundance,richness and comparisons

Like many other southern African rivers, the avifaunal richness and abundance of the Orange River, southern Africa's longest, remains virtually unknown. Given that the Orange River runs through hyper-arid regions in its lower reaches, its linear oasis effect and its role as a reservoir for Red Data bird species is likely but has not been quantified. Two surveys either side of the border town of Noordoewer, in the rainy and dry seasons of 1996 and 1997 deter-mined that species richness was higher in riverine habitat (71 species) than in the surrounding Nama Karoo (46 species) confirming the linear oasis effect. Surveys revealed five Namibian Red Data species. Peregrine Falcons, Falco peregrinus, and African Fish Eagles, Haliaeetus vocifer, were relatively common while the endangered Cape Eagle Owl, Bubo capensis, occurred sporadically. Among wetland Red Data bird species, Black Storks, Ciconia nigra, occurred in less disturbed areas, and White-backed Night Herons, Gorsachius leuconotus, were an unexpected species at this locality. The overall species richness of 103 birds and a density of riparian birds of 31 -3 4 birds/10 km, makes the lower Orange River avifauna similar to the Cunene River but tenfold less abundant than Namibia's other perennial rivers. Despite the low diversity, the arid habitats of the lower Orange River support greater diversity than more easterly regions when all current surveys of richness and abundance from Lesotho to the mouth on the Namib coast are combined. This east-west trend may be explained by the slower flow and warmer water of the lower Orange River but natural turbidity and regulation may explain the generally low diversity and abundance indices. Major rivers in southern Africa appear to have been over-looked as wetland habitats, and further avifaunal surveys of all of them are recommended.     

Predicting Bald Eagle Collision at Wind Energy Facilities

Bald eagles (Haliaeetus leucocephalus) are currently protected in the United States under the Bald and Golden Eagle Protection Act of 1940 and Migratory Bird Treaty Act of 1918. Given these protections and the increasing development of wind energy throughout the United States, it is important for regulators and the wind industry to understand the risk of bald eagle collisions with wind turbines. Prior probability distributions for eagle exposure rates and collision rates have been developed for golden eagles (Aquila chrysaetos) by the United States Fish and Wildlife Service (USFWS). Given similar information has not been available for bald eagles, the current recommendation by the USFWS is to use the prior probability distributions developed using data collected on golden eagles to predict take for bald eagles. But some evidence suggests that bald and golden eagles may be at different risk for collision with wind turbines and the prior probability distributions developed for golden eagles may not be appropriate for bald eagles. We developed prior probability distributions using data collected at MidAmerican Energy Company's operating wind energy facilities in Iowa, USA, from December 2014 to March 2017 for bald eagle exposure rates and collision rates. The prior probability distribution for collision rate developed for bald eagles has a lower mean collision rate and less variability relative to that developed for golden eagles. We determined that the prior probability distributions specific to bald eagles from these operating facilities are a better starting point for predicting take for bald eagles at operating wind energy facilities in an agricultural landscape than those developed for golden eagles. © 2021 The Wildlife Society.     

Spatial,seasonal and individual variation in the diet of White‐tailed Eagles Haliaeetus albicilla assessed using stable isotope ratios

Many raptor species are considered to be generalists, taking a range of prey species. However, longitudinal dietary records are often scarce, although necessary for characterizing niche width of species at population and individual levels. Quantifying raptor diets at large spatio‐temporal scales is often necessary for refining conservation efforts, although it can be particularly difficult and may involve a great effort by conventional means. Therefore, we adopted the analysis of stable isotopes in tissues of predators and their potential food sources as a complementary methodology for assessing animals' diet. We examined the isotopic composition (δ¹³C and δ¹⁵N) of White‐tailed Eagles Haliaeetus albicilla from Germany, Finland and Greenland to detect patterns of dietary variation and quantify diet composition. The isotopic analysis included liver and muscle samples from Eagles of the three populations together with 16 potential food sources in the German population. Our results suggested dietary differences between German and Greenlandic Eagles, in accordance with the availability of freshwater and marine habitats in each population. Within the German population, we found seasonal shifts in isotopic ratios, suggesting the birds responded to temporal changes in food availabilities, and age‐related isotopic differences, indicating different diets in adults and juveniles. Isotopic values of liver and muscle tissues collected from the same animal showed intra‐individual short‐term changes in the German and Finnish but not Greenlandic population. This suggests that local feeding niches of this generalist predator may vary with local food supplies, which determines the niche width (from generalist to specialist) at the individual level. Our results also revealed that game mammal carcasses constitute an important food source (29.5% of diet) for the German Eagle population during the winter half‐year corresponding to the hunting season. This result is of relevance to management and conservation because the White‐tailed Eagle and other raptor species are affected by the ingestion of lead ammunition from shot mammalian carcasses.     

Modelled sensitivity of avian collision rate at wind turbines varies with number of hours of flight activity input data

Collision risk modelling of birds at wind turbines typically requires vantage point (VP) data to quantify bird flight activity. The number of VP observation hours required to provide such data, and the associated error in predicted collision rate, have not been formally assessed. Using the Band model and a randomization procedure, we examine the sensitivity of collision rate predictions for the White‐tailed Eagle Haliaeetus albicilla to varying hours of input data on flight activity. Variability in collision rate decreased with increasing number of observation hours. However, at the asymptote in variability (about 62 observation hours) there was still considerable variability in predicted collision rate. VP watches are likely to be inherently variable, and collision rate predictions should assess the potential error associated with such results.     

Flight strategies of migrating raptors; a comparative study of interspecific variation in flight characteristics

The comparison of flight styles and flight parameters of migrating raptors in Israel revealed the following. (1) Climbing rate in thermal circling did not differ between species, indicating that chiefly the strength of thermal updrafts determined the climbing rate and that morphological features were less relevant. (2) In interthermal gliding, air speed was positively and gliding angle negatively related to the species' average body mass. Heavier species glided faster and had smaller gliding angles. (3) In soaring and gliding flight, cross-country speed relative to the air was positively related to the species' body mass; it was obviously the result of the gliding ability increasing with body mass. (4) Eagles and buzzards used soaring and gliding flight for more than 95% of the observation time. Additional soaring in a straight line whilst gliding was extensively used by the Steppe Eagle Aquila nipalensis, Lesser Spotted Eagle Aquila pomarina and Booted Eagle Hieraætus pennatus and even more frequently by the resident species, the Griffon Vulture Gyps fulvus and Shorttoed Eagle Circaetus gallicus. Smaller species, such as the Levant Sparrowhawk Accipiter brevipes, harriers (Circus sp.) and small falcons (Falco sp.). showed the highest proportion of flapping and gliding flight (9–33%). (5) In a comparison of the flight parameters and proportions of flight styles, a cluster analysis distinguished two main groups: The first consisted of Montagu's Harrier Circus pygargus, Pallid Harrier Circus macrourus, Levant Sparrowhawk and small falcons; their flight behaviour was characterized by both the high proportion of flapping and the low gliding performance. The second group comprised the typical soaring migrants: Steppe Eagle, Lesser Spotted Eagle, Booted Eagle, Steppe Buzzard Buteo buteo vulpinus, Honey Buzzard Pernis apivorus and Egyptian Vulture Neophron percnopterus, and they had very similar flight behaviour and were closely clustered. The Black Kite Milvus migrans and Marsh Harrier Circus aeruginosus were intermediate between typical soarers and flappers. The two resident species, Griffon Vulture and Short-toed Eagle, were grouped separately from the soaring migrants.     

Isolation of 22 new Haliaeetus microsatellite loci and their characterization in the critically endangered Madagascar fish‐eagle (Haliaeetus vociferoides) and three other Haliaeetus eagle species

We isolated a total of 22 microsatellite loci from two Haliaeetus species: the Madagascar fish‐eagle (Haliaeetus vociferoides) and the bald eagle (Haliaeetus leucocephalus). Five loci were monomorphic in both the Madagascar fish‐eagle (n = 24–43) and the bald eagle (n = 2–8) but were found to be polymorphic in other Haliaeetus species. Haliaeetus loci have proved useful for investigating gene flow in Haliaeetus and Aquila eagles. Ten loci were polymorphic in the critically endangered Madagascar fish‐eagle and will be used to investigate the genetic population structure and mating system in this species.     

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     

On the diet of the Pharaoh Eagle Owl,Bubo ascalaphus (Savigny, 1809), in Qatar,with an overview of its feeding habits

The diet of Bubo ascalaphus in Qatar was assessed based on pellets collected from the first known nesting site of the species in the country. The pellets contained a total of 68 prey items, representing 9 different species: 4 mammals, 1 bird, 1 reptile, and at least 3 scorpions. Mammals clearly comprised the major food source (89.7% and 97.7% in frequency and biomass respectively). Our data suggest that Pharaoh Eagle Owls are opportunistic predators that feed on a variety of prey depending on their temporal/spatial availability, which is consistent with previous studies. A literature review clearly suggests that Eagle Owls in arid to semi-arid environments are opportunistic predators with small mammals being their main prey. Predation on migrating Blue-cheeked Bee-eaters Merops persicus supports this hypothesis.     

Community reorganization revealed by exploring shifts in the diet of an apex predator,the Golden Eagle Aquila chrysaetos,with stable isotopes and prey remains

Golden Eagles Aquila chrysaetos colonized the northern California Channel Islands, an archipelago located off the coast of southern California, USA, in the early 1990s, owing in part to the presence of Feral Pigs Sus scrofa. Eagles preyed on piglets and on the endemic Island Fox Urocyon littoralis, driving three endemic subspecies of the Island Fox (U. l. santacruzae, U. l. santarosae and U. l. littoralis) toward extirpation. Using stable isotopes (δ¹³C and δ¹⁵N) measured from feathers and blood of Golden Eagles and from meat and bone of their prey, along with the proportion of prey consumed estimated from nest excavations as informative priors, we show that shifts in Eagle isotopic signatures tracked community reorganization: when pigs and foxes were abundant, Eagles consumed them in higher proportions than other prey; when foxes became scarce owing to Eagle predation, Eagle diet shifted toward pigs, but also diversified to include Common Ravens Corvus corax and seabirds (Larus spp. and Phalacrocorax spp.); and finally, after pigs were eradicated and 35 Golden Eagles had been captured and removed from Santa Cruz Island, a lone remaining pair of Eagles reversed their diet to include more foxes. Our findings reveal that shifts in stable isotope signatures from tissues of an apex predator can be used as a general proxy to monitor changes in the prey community; however, if sample sizes are small, informative priors are both essential and overwhelmingly influential, and changes in abundance of preferred prey may mask or fail to reveal changes in less preferred prey. Given that apex predators are at the top of trophic hierarchies, that they are often important drivers of community structure and that they sample prey continuously from their community, isotopic signatures of their tissues may be useful for monitoring community change.     

Biomass-dependent effects of common carp on water quality in shallow ponds

We examined the biomass-dependent effects of common carp (Cyprinus carpio) on water quality in 10 ponds at the Eagle Mountain Fish Hatchery, Fort Worth, Texas, USA. Ponds contained 0–465 kg ha⁻¹ of common carp. We measured limnological variables at weekly intervals for four weeks in early summer, after which ponds were drained and the biomass of fish and macrophytes was determined. Common carp biomass was significantly positively correlated with chlorophyll a, total phosphorus, total nitrogen, and Keratella spp. density and negatively correlated to bushy pondweed (Najas guadalupensis) biomass. In addition, we combined our data with data from comparable studies to develop more robust regression models that predict the biomass-dependent effects of common carp on water quality variables across a wide range of systems.     

Origin and formation of the Spanish Imperial Eagle ( Aquila adalberti )

A review of the paleontological records of the Eastern Imperial Eagle (Aquila heliaca) and the Spanish Imperial Eagle (Aquila adalberti) in the context of the paleoecological environment in Eurasia during the Pleistocene and Holocene epochs is presented. The Eastern Imperial Eagle expanded its range in Eurasia during the last Pleistocenic glaciation favoured by the expansion of the steppes. The first records of Spanish Imperial Eagles are from the Late Pleistocene and Early Holocene in the eastern Iberian Peninsula, and their distribution seems to have been limited to the distribution areas of Mediterranean vegetation and the European rabbit. Individuals of migrant A. heliaca could have reached the Iberian Peninsula at the end of the Pleistocene–beginning of the Holocene. These individuals could have adapted to the Mediterranean ecosystem, subsequently specializing in a diet of rabbit, a prey which is in abundance all year round. Due to the availability of such prey, A. heliaca would become more sedentary. These individuals may have overcome their breeding phenology and paired up assortatively, becoming genetically separate from the migrant populations and initiating the speciation mechanisms for sympatry or parapatry that resulted in A. adalberti. This is one possible mechanism. These findings reported here support the recent age of divergence between both taxons, and the incipient speciation supports its taxonomical considerations as a semi-species. The study reported here complies with the current laws of Spain.     

Long‐term variation in the distribution of the White‐bellied Sea‐Eagle (Haliaeetus leucogaster) across Australia

Abstract The White‐bellied Sea‐Eagle Haliaeetus leucogaster (Accipitridae) is widespread within Australia. However, in a number of states it is thought to be declining locally in response to human induced disturbance. Here we analyse the Australian Bird Atlas data to identify the extent and pattern of change in range and density of the species between three Atlas Periods (1901–1976, 1977–1981 and 1998–2001) using a new standardized frequency measure, the Occupancy Index (OI) for 1° blocks (approx. 100 km²) across the continent. At the continental scale, there was no significant difference in the spatial extent of occupancy between Atlas Periods. However, there were considerable changes in frequency and range extent between defined regions, and there were distinct differences in the pattern of change in OI between coastal and inland blocks over time. Coastal blocks showed much more change than inland blocks, with a clear increase in the use of coastal blocks, accompanied by a decrease in inland blocks, during the 1977–1981 Atlas Period, relative to both other Atlas Periods. While there were slight (and not statistically significant) trends for OI to increase in areas containing dams, and to decrease in urbanized coastal areas, the over‐riding factor associated with distributional shifts and frequency changes was apparently climatic fluctuation (the 1977–1981 period showing the influence of El Niño associated drought). Within this study, the impression of abundance was strongly dependent on both the temporal and spatial scale of analysis. This highlights the importance of large‐scale analysis in interpreting change in distribution and abundance of widespread species.     

Feeding behavior of the cichlid,Sarotherodon galilaeum: selective predation on Lake Kinneret zooplankton

In laboratory feeding trials, we analyzed the feeding behavior and selectivity of the cichlid, Sarotherodon galilaeum, for zooplankton prey from Lake Kinneret, Israel. The feeding behavior was dependent on fish size. Fish less than 20 mm SL fed on zooplankton as obligate particulate feeders. Fish from 20 to 42 mm SL fed either as particulate feeders or as filter feeders. Fish larger than 62 mm SL fed as obligate filter feeders. Particulate-feeding fish were size selective and had highest feeding electivities for large-sized zooplankton species. Filter-feeding fish had highest feeding electivities for zooplankton species with poor escape ability. In general, S. galilaeum predation pressure would be greatest on Ceriodaphnia reticulata, a large-bodied and easily captured species which is selected by both particulate-feeding and filter-feeding fish.     

BIRD POPULATION STUDIES.-V. FIVE CONSECUTIVE DAILY COUNTS OVER 21 ACRES IN NORTHERN RHODESIA

Williams, M.C. &; Nesbit, J.W. 1996. Fatal herpesvirus infection in a Cape Eagle Owl Bubo capensis and a Spotted Eagle Owl Bubo africanus. Ostrich 67:50-62

Fatal, systemic herpesvirus infection was reported in a Cape Eagle Owl Bubo capensis and a Spotted Eagle Owl Bubo africanus. This is believed to be the first report of systemic herpesvirus infections in owls in South Africa. Post mortem examinations revealed multifocal necrosis in the liver and spleen. Histologically, intranuclear inclusion bodies were present, particularly in cells at the periphery of necrotic foci. Typical herpesvirus virions, demonstrable by transmission electron microscopy, were present. Viral isolations were unfortunately not undertaken.     

Movements by juvenile and immature Steller's Sea Eagles Haliaeetus pelagicus tracked by satellite

Twenty‐four juvenile Steller's Sea Eagles Haliaeetus pelagicus were tracked via satellite from natal areas in Magadan, Kabarovsk, Amur, Sakhalin and Kamchatka. Nestling dispersal occurred between 9 September and 6 December (n = 24), mostly 14 September–21 October, and did not differ among regions or years. Most eagles made stopovers of 4–28 days during migration. Migration occurred 9 September–18 January, mostly along previously described routes, taking 4–116 days to complete (n = 18). Eagles averaged 47.8 km/day excluding stopovers; 22.9 km/day including stopovers. The mean degrees of latitude spanned during migration was: Kamchatka, 2.1; Magadan, 11.6; Amur, 7.3; and Sakhalin, 1.1. Eagle winter range sizes varied. Eagles concentrated in 1–3 subareas within overall winter ranges. The mean size of the first wintering subareas was 274 km², the second 529 km², and the third 1181 km². Second wintering areas were south of first wintering areas. Spring migration started between 2 February and 31 March. Two eagles from Magadan were tracked onto summering grounds, well south of their natal areas. Both had early and late summering areas. One bird was followed for 25 months. It initiated its second autumn migration in the first half of October and arrived on its wintering grounds on 26 December. The second autumn migration covered 1839 km (20.9–22.4 km/day). Unlike its first winter when it used two subareas, this bird used only one subarea in 1998–99, but this was located near wintering areas used in 1997–98. It left its wintering ground between 13 April and 13 May, and arrived on its summering grounds between 7 June and 8 July. Unlike most satellite radiotracking studies, data are presented from a relatively large number of birds from across their breeding range, including new information on eagle movements on the wintering grounds and during the second year.