Migratory Raptor Research Articles

Using satellite telemetry and environmental niche modelling to inform conservation targets for a long-distance migratory raptor in its wintering grounds

AbstractUnderstanding the ecology of migratory birds during the non-breeding season is necessary for ensuring their conservation. Using satellite telemetry data we describe winter ranging behaviour and movements of pallid harriersCircus macrourusthat bred in Kazakhstan. We developed an ecological niche model for the species in Africa, to identify the most suitable wintering areas for pallid harriers and the importance of habitat in determining the location of those areas. We also assessed how well represented suitable areas are in the network of protected areas. Individual harriers showed relatively high fidelity to wintering areas but with potential for interannual changes. The ecological niche model highlighted the importance of open habitats with natural vegetation. Most suitable areas for the species were located in eastern Africa. Suitable areas had a patchy distribution but were relatively well included in the network of protected areas. The preferential use of habitats with natural vegetation by wintering pallid harriers and the patchiness of the most suitable areas highlight the harrier's vulnerability to land-use changes and the associated loss of natural vegetation in Africa. Conservation of harriers could be enhanced by preserving natural grasslands within protected areas and improving habitat management in the human-influenced portions of the species’ core wintering areas.

Summer pre-breeding movements of Eleonora’s Falcon Falco eleonorae revealed by satellite telemetry: implications for conservation

SummaryRecent advances in bird tracking technologies are revealing that migratory birds use temporal staging sites other than breeding and wintering areas, and these areas deserve conservation efforts. Eleonora’s Falcon Falco eleonorae is a long-distance migratory raptor that breeds colonially on islands and is considered a priority species for conservation. Anecdotal observations indicate that during the pre-breeding period, Eleonora’s Falcons stay in inland areas far away from the colonies, but, to date there are no detailed data concerning the connectivity between these areas and breeding colonies. Using satellite telemetry, we analysed data from four summering events belonging to three individuals breeding in two colonies in the Western Mediterranean (Spain). All of them made inland movements in areas up to c.400 km distant from the respective breeding colonies, visiting several habitats, from forests to arable lands, probably taking advantage of high densities of insects. Perturbations occurring in these areas could threaten Eleonora’s Falcons with serious consequences at the population level. We suggest that conservation measures implemented at breeding and wintering grounds may not suffice and that temporary staging areas should be identified at a larger scale and deserve protection as well.

Different Location Sampling Frequencies by Satellite Tags Yield Different Estimates of Migration Performance: Pooling Data Requires a Common Protocol

BackgroundMigration research is in rapid expansion and increasingly based on sophisticated satellite-tracking devices subject to constant technological refinement, but is still ripe with descriptive studies and in need of meta-analyses looking for emergent generalisations. In particular, coexistence of studies and devices with different frequency of location sampling and spatial accuracy generates doubts of data compatibility, potentially preventing meta-analyses. We used satellite-tracking data on a migratory raptor to: (1) test whether data based on different location sampling frequencies and on different position subsampling approaches are compatible, and (2) seek potential solutions that enhance compatibility and enable eventual meta-analyses.Methodology/Principal FindingsWe used linear mixed models to analyse the differences in the speed and route length of the migration tracks of 36 Black kites (Milvus migrans) satellite-tagged with two different types of devices (Argos vs GPS tags), entailing different regimes of position sampling frequency. We show that different location sampling frequencies and data subsampling approaches generate large (up to 33%) differences in the estimates of route length and migration speed of this migratory bird.Conclusions/SignificanceOur results show that the abundance of locations available for analysis affects the tortuosity and realism of the estimated migration path. To avoid flaws in future meta-analyses or unnecessary loss of data, we urge researchers to reach an agreement on a common protocol of data presentation, and to recognize that all transmitter-based studies are likely to underestimate the actual distance traveled by the marked animal. As ecological research becomes increasingly technological, new technologies should be matched with improvements in analytical capacity that guarantee data compatibility.

APalaearctic migratory raptor species tracks shifting prey availability within its wintering range in theSahel

Mid-winter movements of up to several hundreds of kilometres are typical for many migratory bird species wintering in Africa. Unpredictable temporary food concentrations are thought to result in random movements of such birds, whereas resightings and recoveries of marked birds suggest some degree of site fidelity. Only detailed (e.g. satellite) tracking of individual migrants can reveal the relative importance and the causes of site choice flexibility and fidelity. The present study investigates how mid-winter movements of a Palaearctic-African migratory raptor, Montagu's harrier Circus pygargus, in the Sahel of West Africa are related to the availability of food resources. Thirty harriers breeding or hatched in northern Europe were satellite tracked (2005-2009). On average, four home ranges, each separated by c. 200 km, were visited during one overwinter stay in the Sahel. Wintering home ranges were similar in size to breeding season home ranges (average over wintering and breeding home range size c. 200 km(2) ), and harriers showed high site fidelity between years. Most preferred habitat types in the Sahel were mosaics of grass- and cropland, indicating similar habitat preferences in both the breeding- and wintering seasons. The main prey of Montagu's harriers in the Sahel were grasshoppers Acrididae. Highest grasshopper numbers in the field occurred at relatively low vegetation greenness [normalized difference vegetation index (NDVI) values 0.17-0.27]. We used NDVI as a proxy of food availability for harriers. During their overwinter stay, Montagu's harriers moved in a South-South-western direction between consecutive home ranges. The birds selected areas within the range of NDVI values associated with high grasshopper numbers, thus tracking a 'green belt' of predictable changes in highest grasshopper availability. Contrary to earlier hypotheses of random movements in the Sahelian-wintering quarters, the present study shows that Montagu's harriers visited distinct home ranges, they were site-faithful and tracked seasonal changes in food availability related to previous rainfall patterns, caused by the shifting Intertropical Convergence Zone. Itinerancy may be the rule rather than an exception among insectivorous birds wintering in African savannahs.

Breeding biology and nestling development of the Grasshopper Buzzard

Research into the effect of environmental variables on reproductive success of tropical raptors is often constrained by the lack of information on breeding biology. We provide the first detailed information of the breeding biology and nestling development of the Grasshopper Buzzard Butastur rufipennis, an Afrotropical migratory raptor threatened by extensive land transformation in its breeding range. Breeding coincided with the transition from the dry to the wet season. The mean incubation period was 30 d and mean fledging period 36 d. The breeding period was characterised by rapid establishment of territories and short post-fledging dependency periods related to the onset of migration shortly after breeding. Growth rate of body mass, tarsus, wing and primary feathers were similar between sexes, which showed significant but moderate body mass dimorphism at fledging (♂85–90% of ♀). Second hatchlings were smaller in body mass and structural dimensions compared to similarly-aged first hatchlings and singles of the same sex. Survival of second hatchlings was related to body mass ratio with first hatchlings, whereas brood reduction occurred through food competition and siblicide. We provide ageing formulae and photographic records to facilitate further studies of Grasshopper Buzzard nestling development and reproductive success in the region.

Multi‐temporal distribution modelling with satellite tracking data: predicting responses of a long‐distance migrant to changing environmental conditions

Summary1. Despite the wealth of data available from satellite tracking (ST) studies, such data have rarely been used to model species distributions. Using a novel method, we show how to exploit satellite data to analyse whether and how a migratory species responds to fluctuating environmental conditions in its wintering area. This is particularly crucial for establishing comprehensive conservation measures for rare species in areas that are threatened by increasing land use and climate change.2. We use ST data of Eleonora’s falcon Falco eleonorae, a long‐distance migratory raptor that winters in Madagascar, and assess the performance of static species distribution models (SDM) as well as multi‐temporal models. ST data were derived from seven falcons tracked during three consecutive wintering periods and for a total of 2410 bearings, of which 512 locations were used in SDMs. We employed environmental predictors (climate, topography and land cover) with a spatial resolution of 30 arc seconds (c. 1 km2) to match rigorously filtered ST data with an accuracy of ≤1 km.3. We first created a model with low temporal but high spatial resolution (half‐year). To predict suitable habitat for each month of the wintering season, we took advantage of the high temporal resolution inherent in ST data and employed temporally corresponding remote sensing data [Normalized Difference Vegetation Index (NDVI) 10‐day composites] together with other variables to create monthly models.4. We show that ST data are suited to build robust and transferable SDMs despite a low number of tracked individuals. Multi‐temporal SMDs further revealed seasonal responses of the study species to changing environmental conditions in its wintering area.5. Synthesis and applications. We present a transferable approach to predict the potential distribution of organisms as well as their dynamic response to changing environmental conditions. Future conservation management plans could include the prediction of a species’ reaction to changing land‐use practices or climate change based on the methodology proposed here. This would provide an early warning system for the decline of populations wintering in remote areas that underlie strong climatic fluctuations.

Sources of variation for nutritional condition indices of the plasma of migratory lesser kestrels in the breeding grounds

Although published information on reference values for biochemical parameters in birds of prey has increased during the last years, little is known on their sources of variation. We used an insectivorous and small migratory raptor species, the lesser kestrel Falco naumanni, as a model. We looked for sources of variation of nutritional biochemical parameters (i.e. triglycerides, cholesterol, uric acid and urea) of both nestlings and adults. Reference values indicated that, as a rule, lesser kestrel showed more elevated triglycerides, urea and uric acid levels than other raptors. All analyzed factors except gender (i.e. year, colony, sampling time, presence/absence of a geolocator, body mass, laying date and capture date) reached significance for at least one biochemical parameter. In the morning, we found an important postprandial increase in the concentration of all biochemical parameters in nestlings, and uric acid and urea levels in adults. A positive relationship was detected between triglycerides and body mass in nestlings and adults. Although we did not find differences between blood biochemical parameters of the oldest and youngest chick of each brood, we found that cholesterol levels were lower in nestlings from larger broods. Coloration of tarsi (measured as brightness) was related to triglycerides and urea levels of nestlings and adults, respectively. The feeding habits of lesser kestrel probably explain the different levels and patterns of variation of metabolites in comparison to more carnivorous raptors eating mammals or birds.

Seasonal Shifts in Foraging Site and Prey of Grey-Faced Buzzards (Butastur indicus), Breeding in Satoyama Habitat of Central Japan

Abstract The Grey-faced Buzzard (Butastur indicus) is a migratory raptor breeding in Japan. It has a high breeding density in what is known as “Satoyama”, a traditional mosaic landscape of paddy fields, woodland, grassland, and streams. We studied the foraging behavior of five buzzard pairs in Satoyama of Tochigi Prefecture, central Japan, for two years. Seasonal changes in foraging sites and differences in the amount of prey captured at each foraging site were analyzed by Bayesian inference using a Markov Chain Monte Carlo (MCMC) algorithm. The main vegetation types which characterized the foraging areas of the buzzards varied over the course of the breeding season from paddy fields to levees and grass-arable fields, and eventually to wooded areas. Along with this shift, the main prey of the buzzards changed from frogs to insects. In paddy fields, frogs and small mammals were frequently captured. A variety of prey including frogs, small mammals, lizards, snakes, and insects were taken at levees and grass...

Avian Pox Infection in a Free-Living Crested Serpent Eagle (Spilornis cheela) in Southern Taiwan

Avian pox viruses (APVs) have been reported to cause infection in diverse avian species worldwide. Herein we report the first case of APV infection in a free-living bird, a subadult crested serpent eagle (Spilornis cheela), in Taiwan. In addition to the typical wart-like lesions distributed on the cere, eyelid, and face, there were also yellowish nodules below the tongue and on the hard palate. Phylogenetic analysis of the 4b core protein gene showed that the APV is very close to that found in white-tailed sea eagle (Haliaeetus albicilla) in Japan recently. Because both cases are located on the same major flyway for migratory birds, the impact of this virus with regard to the wild and migratory raptor species along the East Asian-Australasian Flyway and West Pacific Flyway requires immediate investigation.

Evidence of connectivity between continental and differentiated insular populations in a highly mobile species

Aim Genetically differentiated insular populations are candidates for independent units for conservation. However, occasional immigration to reduced island populations may occur and potentially have important consequences in their future viability and evolutionary potential. In this study, we investigate the conservation implications of population structure and connectivity of insular and continental populations of a migratory raptor as determined using genetic tools and satellite tracking. Location Western European populations in the Iberian Peninsula and two insular populations in the Mediterranean Sea (Balearic Islands) and Atlantic Ocean (Canary Islands). Methods We genotyped 22 microsatellite loci in 96 Egyptian vultures (Neophron percnopterus) from the Iberian Peninsula, 36 from Menorca (Balearic archipelago) and 242 (85% of the current population) from Fuerteventura (Canary Islands). We analysed genetic variation to estimate structure, gene flow, genetic diversity, effective size and recent demographic history of the populations. Additionally, 19 vultures were marked with satellite transmitters to track their migration routes. Results Insular populations were genetically differentiated from those of the mainland. We detected immigration in the insular populations and within the continental counterpart. We found similar levels of genetic variability between the continent and the islands, and a bottleneck analysis indicated recent sharp population declines in both archipelagos but not on the continent. Main conclusions Our study provides evidence that, in spite of significant differentiation, insular populations of highly mobile species may remain connected with the mainland. Conservation programmes should take into account population connectivity and integrate differentiated units of management within complex units of conservation that can best maintain processes and potential for evolutionary change.

Individual variation in migratory movements and winter behaviour of Iberian Lesser Kestrels Falco naumanni revealed by geolocators

The population decline of the Lesser Kestrel Falco naumanni has been the subject of studies across its Western Palaearctic breeding range, but little is known about its use of pre‐migratory areas or African wintering quarters. We used geolocators to describe the temporal and spatial patterns of Portuguese Lesser Kestrel migration and wintering behaviour. Data on the complete migration were obtained from four individuals and another three provided further information. Prior to southward migration, Lesser Kestrels showed two different behaviours: northward‐orientated movements to Spain and movements in the proximity of the breeding area. Autumn migration took place mostly in late September; spring departures occurred mainly in the first half of February. Wintering grounds included Senegal, Mauritania and Mali, with individuals overlapping considerably in Senegal. Movements registered within the wintering grounds suggest itinerant behaviour in relation to local flushes of prey. During spring migration, birds crossed the Sahara Desert through Mauritania, Western Sahara and Morocco before passing over the Mediterranean to reach Portugal. Autumn migration lasted 4.8 ± 1.1 days, and spring migration lasted 4.1 ± 0.3 days. The mean daily flight range varied between approximately 300 and 850 km for an entire journey of around 2500 km. Effective protection of roosting sites in both pre‐migratory and wintering areas and maintaining grasshopper populations in Sahelian wintering quarters appear crucial in preserving this threatened migratory raptor across its African–Eurasian flyway. There was no evidence of any deleterious effects of fitting birds with loggers.

Weather conditions promote route flexibility during open ocean crossing in a long-distance migratory raptor

Weather conditions are paramount in shaping birds' migratory routes, promoting the evolution of behavioural plasticity and allowing for adaptive decisions on when to depart or stop during migration. Here, we describe and analyze the influence of weather conditions in shaping the sea-crossing stage of the pre-breeding journey made by a long-distance migratory bird, the Eleonora's falcon (Falco eleonorae), tracked by satellite telemetry from the wintering grounds in the Southern Hemisphere to the breeding sites in the Northern Hemisphere. As far as we know, the data presented here are the first report of repeated oceanic journeys of the same individuals in consecutive years. Our results show inter-annual variability in the routes followed by Eleonora's falcons when crossing the Strait of Mozambique, between Madagascar and eastern continental Africa. Interestingly, our observations illustrate that individuals show high behavioural plasticity and are able to change their migration route from one year to another in response to weather conditions, thus minimising the risk of long ocean crossing by selecting winds blowing towards Africa for departure and changing the routes to avoid low pressure areas en route. Our results suggest that weather conditions can really act as obstacles during migration, and thus, besides ecological barriers, the migratory behaviour of birds could also be shaped by "meteorological barriers". We briefly discuss orientation mechanisms used for navigation. Since environmental conditions during migration could cause carry-over effects, we consider that forecasting how global changes of weather patterns will shape the behaviour of migratory birds is of the utmost importance.

Metropolitan garbage dumps: possible winter migratory raptor monitoring stations in peninsular India

Winter raptor migration and movement is poorly documented for peninsular India, mainly due to the lack of geographical bottlenecks. We describe, for the first time, the use of a garbage dump in a metropolitan city as an alternative visual winter raptor monitoring station. The daily count, adult to juvenile ratios and species composition of three migratory raptor species, Steppe Eagle Aquila nipalensis, Black-eared Kite Milvus migrans lineatus and Tawny Eagle Aquila rapax are presented. Ground temperatures at the garbage dump site and surrounding area, and the wing beat rate of migratory raptors before and after arrival in the early morning were measured. A total of 355 raptors migrating over a period of six observation days with 250 adults and 105 juveniles were recorded. The temperature of the garbage dump was significantly higher than the surrounding area, while the wing flapping rate was significantly lower over the garbage dump area. It is possible that migrating raptors use garbage dump thermals in the early morning to save energy with soaring and gliding flight (versus flapping flight). We propose that such sites may be used as visual winter migration monitoring stations in metropolitan cities in peninsular India.

Spring Migration of Chinese Goshawks (Accipiter soloensis) in Taiwan

Little is known about the abundance and distribution of Chinese Goshawks (Accipiter soloensis) during spring migration along the Western Pacific flyway around Taiwan. This species is the most numerous migratory raptor in East Asia. In April 2004, we employed weather radar at Kenting and Chiku to study the spatial and temporal distribution and flying behavior of migrants in the Taiwan area. Goshawk flocks migrating across Luzon Strait toward Taiwan (n = 193) averaged 3.1 km in length (SD = 3.41, range 1.0–27.5 km) and 0.7 km in width (SD = 0.30, range 0.3–2.5 km). Using a regression model relating bird numbers to the reflectivity of radar images, we estimated that at least 225 935 Chinese Goshawks passed through the study area during 5 d in the month of April. Flocks crossing the Luzon Strait approached the Hengchun Peninsula, at the southern tip of Taiwan, along several inland and offshore routes, making it difficult to obtain accurate and complete observations from the ground. Farther north at Chiku, we used radar imaging to estimate 150 619 northbound migrants, 67% of the number detected by the Kenting radar. Of these, 24% bypassed Taiwan and flew nonstop to mainland China via the Pescadores, located off the west coast of Taiwan. Another 55% flew over the southern end of Taiwan and then across the Taiwan Strait to the mainland. The remaining 21% continued north over Taiwan after spending up to 2 d in the central mountains of Taiwan. This likely explains why relatively small numbers of goshawks are observed in central and northern Taiwan. Chinese Goshawks flew more slowly and higher over land than over water, and their ground speed increased with favorable winds.

From the Mediterranean Sea to Madagascar: Are there ecological barriers for the long-distance migrant Eleonora’s falcon?

We examined the connection between landscape characteristics and behaviour of a long-distance migratory raptor. Our main goal was to test whether long-distance migratory birds adjust their migration programme according to the different characteristics of the habitats crossed during the journey with special emphasis in the so-called “ecological barriers”, inhospitable environments such as deserts, ice fields, seas and mountain ranges, where the opportunities to fulfil energy requirements are low or absent and environmental factors could be extremely severe. To this end, 11 Eleonora’s falcons were tracked by satellite telemetry in their ca. 9000 km autumn migration route from colonies located in Western Mediterranean to their wintering grounds in Madagascar during 2007 and 2008. Our results show that Eleonora’s falcons migrated during day and night-time, adjusting migration speed and daily distance in relation to the crossed region. Unlike other migrant species, Eleonora’s falcons did not avoid ecological barriers by making unnecessary detours around them or converging on narrow corridors. Nocturnal migration and higher daily distances were observed when flying across the Sahara Desert and the Mozambique Channel. The circadian pattern of activity budget shows that Eleonora’s falcon relies on an internal navigation mechanism that works during both day and night. Finally, our results suggest that the Sahara is an ecological barrier not only for passerines but also for raptors migrating within the Palaearctic-African flyway.

Territorial occupancy and breeding performance in a migratory raptor do not follow ideal despotic distribution patterns

AbstractIt has commonly been argued that many territorial species select their breeding sites following an ideal despotic distribution model, in which the most productive, high‐quality territories are more frequently occupied. Theoretical and empirical studies have shown that this occupancy pattern may have population regulatory consequences, leading to density dependence in heterogeneous habitats. During a 9‐year research project in a forested area of south‐eastern Spain, we tested some of the predictions of the ideal despotic distribution model and the site‐dependent population regulation theory in a migratory raptor species, the booted eagle Hieraaetus pennatus. Contrary to the predictions of the despotic model, our results showed that the temporal pattern of territorial occupation did not differ from randomness, and that the territory occupancy rate was not significantly related to the reproductive parameters considered. At population level, the breeding variables were density independent, suggesting the absence of site‐dependent regulation. In addition, we were unable to find significant differences in the habitat characteristics between high‐quality and low‐quality territories, classified according to the criteria of both occupancy frequency and average productivity. Overall, our results suggest that booted eagles select their territories at random, probably due to the lack of strong environmental heterogeneity, and that occupancy rate is not a good measure of territory quality for the population studied.

Predictors of floater status in a long‐lived bird: a cross‐sectional and longitudinal test of hypotheses

1. Few studies have been capable of monitoring the nonterritorial sector of a population because of the typically secretive behaviour of floating individuals, despite the existing consensus over the demographic importance of floating. Furthermore, there is almost no information on floating behaviour for migratory species. 2. The factors that determine whether an individual will be a floater or a territory owner have been framed into five, non-mutually exclusive hypotheses: (i) territory holders are morphologically superior to floaters (resource-holding potential hypothesis); (ii) age confers skills and fighting motivation which lead to social dominance and territory ownership (age hypothesis); (iii) occupancy time of a site determines asymmetries in its knowledge, familiarity and value for potential contenders (site-dominance hypothesis); (iv) contenders use an arbitrary rule to settle contests leading to pre-defined cut-off points for a biologically meaningful trait (e.g. age, body size) separating floaters from territory holders (arbitrary convention hypothesis); and (v) floaters set up a 'war of attrition' at arbitrarily chosen territories (arbitrary attrition hypothesis). 3. We tested these hypotheses using long-term data on a long-lived, migratory raptor, the black kite Milvus migrans Boddaert. 4. Floating status was best explained by the concerted action of mechanisms consistent with the age and site-dominance hypotheses. 5. In both cross-sectional and longitudinal analyses, acquisition of a territory was determined by a complex interaction between age and early arrival from migration, suggesting: (i) a progressive incorporation of early arriving individuals in the territorial contingent of the population, and (ii) the existence of an alternative restraint strategy of delayed territoriality mediated by long-term acquisition of social dominance. 6. Such results suggested that territory acquisition was mediated by the establishment of site dominance through pre-emption and, secondarily, despotism. In this population, age and arrival date aligned individuals along a demographic continuum ranging from successful breeders monopolizing high-quality resources to floaters with no resources, consistent with the notion of floating as an extreme form of breeding failure.

Sequential settlement and site dependence in a migratory raptor

Habitat selection models, such as the ideal free, ideal despotic, site-dependent, and conspecific cuing models, are of great importance to behavioral ecologists given their capability to predict habitat distributions and to link individual behavior to population processes. However, there have been relatively few field tests of their predictions. We tested the 4 models by studying the process of sequential settlement on territory in 2 distant populations of a migratory raptor, the black kite Milvus migrans. Results were mainly consistent with the site-dependent model: on arrival, kites settled on progressively lower quality territories, and earlier arriving individuals were older, larger, and in better body condition than later arriving ones, leading to a state-dependent arrival sequence also predicted by a previous theoretical model of settlement pattern. Occupation of superior territories by superior phenotypes resulted in cascading advantages for earlier arriving individuals in terms of subsequent reproductive performance. At the population level, the populations expanded/retracted from lower quality sites during population increases/declines. The above scenario was consistent across the 2 populations, and a review of the literature uncovered a remarkably consistent picture of state-dependent arrival, progressive monopolization of best quality sites, and cascading effects on subsequent breeding performance. We propose as a general paradigm of sequential settlement the following process: 1) arrival date is a reliable surrogate of phenotype quality, 2) early-arriving individuals have preferential access to the best quality sites and partners, 3) the above conditions cascade into a number of benefits ultimately related to higher fitness for earlier arriving individuals.

What landscape elements are correlated with the distribution of wintering Grey-faced Buzzards Butastur indicus in the Sakishima Islands, southwestern Japan?

The Grey-faced Buzzard (Butastur indicus) is a typical migratory raptor species in East Asia, and has suffered a population decline recently. In southern Japan, the wintering buzzards are widely distributed to the Sakishima Islands. The mainly human activity on these islands is agriculture, and the natural environments are fragmented by such exploitative land-use that produces a characteristic landscape pattern. In this study, we describe the distribution pattern and habitat selection of the buzzards wintering in the Sakishima Islands, and focus on landscape elements in order to give a basic reference for further conservation study. Ten islands were investigated during January to March 2005. We used the generalized linear mixed model (GLMM) on a set of landscape elements to model buzzard distribution in the 1×1 km grid cell within each island, and studied the relationships between the numbers of buzzard and landscape elements among islands by using the generalized linear model (GLM). The results were similar on two spatial scales, and they both suggested that the area of farmlands (sugar cane, pasture, and rice paddy), and the perimeter of forests have significant correlation with the distribution and the numbers of the wintering buzzards. Different from breeding sites where distribution was limited by critical nest resource, distribution of Grey-faced Buzzards in wintering sites was simply related to foraging habitats. We suggest that further study should focus on measuring quantitatively the relationships of different farmland types with buzzard distributions in order to make more realistic predictions about distribution when habitat is lost or shifted from one category to another.

RESPONSES OF AN AVIAN PREDATOR TO VARIATIONS IN PREY DENSITY AT A TEMPERATE LATITUDE

Fluctuating prey populations and their effects on avian predator population dynamics have been studied particularly at high latitudes, where prey populations, especially microtines, are known to be cyclic; raptors show both numerical and functional responses to variations in their prey. In this paper, we investigate the response of a migratory raptor (Montagu's Harrier, Circus pygargus) to variations in abundance of its main prey (common vole, Microtus arvalis) in France. We document multiannual fluctuation in the abundance of common voles. The numerical response of Montagu's Harrier to this variable food supply was studied using breeding parameters (breeding density, breeding phenology, and various measures of breeding success). Breeding density and mean clutch size were strongly correlated with spring vole abundance, whereas mean brood size at fledging was correlated positively with summer vole abundance. The mechanism involved in the numerical response of Montagu's Harrier indicates that dispersal and colonization by yearlings may be more important than natality per se. Pellets collected between 1986 and 1997 were used to determine the functional response of Montagu's Harrier to variations in vole abundance. In terms of biomass, Montagu's Harrier diet consisted mainly of voles (33.7–86.6%, between 1986 and 1997). Vole biomass in pellets was closely related to vole abundance estimated by trapping. A type II functional response was detected, with satiation at high prey density, as the shapes of many relationships between breeding parameters and vole abundance were more curvilinear than linear. Our results fit the pattern of relationships between predators and cyclic prey found in Fennoscandia, in which specialist predators show a strong numerical response, although at such a temperate latitude, a more pronounced functional response might have been expected.