Article

Motion capacity, geography and ecological features explain the present distribution of a migratory top predator

Authors:
  • Independent Researcher
  • Independent Researcher
  • Agenzia Regionale per la Protezione dell'Ambiente Ligure
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Abstract and Figures

Presence and distribution of ecological barriers shapes the distribution of migratory birds as well as any other living organism. In Italy, short-toed snake eagles (Circaetus gallicus) breed in the northern and western areas of the peninsula but the species is rare in the south or the islands. The Italian population of this species migrates across the Mediterranean at the Strait of Gibraltar rather than crossing the large stretch of sea between Sicily and Tunisia. This suggests that, in Italy, fall migration is oriented south–north and spring migration north–south. In this paper we test the hypothesis that the accessibility of the suitable habitat area along the Italian Peninsula is in relation to the geographical migration pattern of the studied species. We integrated information from the movement ecology, the geography and the traditional ecological features in order to provide an ecological explanation of the current biogeographical pattern of our model species. We compared statistical models with and without latitude as a predictor. Each model was based on ecological and geographical variables, including land use, prey availability, spatial distribution of environmental elements (patch analysis), geomorphology, and geography. These models predict two patterns of suitability for short-toed snake eagles in Italy. Our results suggest that the abundance of this species increases with latitude despite the existence of large areas of suitable habitat in southern Italy. We suggest that the actual distribution of the short-toed snake eagle in Italy is influenced by the particular migration path used by this population, supporting the hypothesis that this species is still colonizing the Italian Peninsula through an unexpected colonization direction from north to south.
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... As a result, we focused the analysis on this species. Since this eagle does not breed in the study area (Panuccio et al. 2015), our observations involved migrating birds. The median date of the passage of this species was on 26 September, and among aged eagles (N = 40) 33 (82.5%) were first calendar year (cy) birds. ...
... This would have led to the evolution of the extremely detoured path in birds breeding both in Italy and Greece, favored by the partial overlap in the migration periods of individuals belonging to different age classes (Agostini et al. 2002;Panuccio et al. 2012). Notably, nearly all short-toed snake eagles breeding in Italy cross the Mediterranean Sea at the Strait of Gibraltar during both spring and autumn, probably retracing the colonization process (Agostini & Mellone 2008;Panuccio et al. 2015). Most juveniles learn this flyway by following adults (see also Mellone et al. 2016), while some, migrating later in the season than older (experienced) birds, head southward passing along southern continental Italy and concentrating over the island of Marettimo (western Sicily; Figure 1) located 130 km NE of the Cap Bon promontory (Tunisia), and rarely also over Malta (Agostini et al. 2002;Sammut & Bonavia 2004;Mellone et al. 2016). ...
... In reference to birds passing over Malta, their reluctance to continue migrating over the water surface makes them more vulnerable to poaching, since they probably remain on the island longer than other migrating birds do (Del-Hoyo et al. 1994). In a recent paper, Panuccio et al. (2015) suggested that the Italian population of the shorttoed snake eagle should be considered part of a metapopulation comprising those in Western Europe (France, Spain). Small and peripheral populations of southern Italy could be considered small patches of this metapopulation system cut off from the bulk of the population of Western Europe (Panuccio et al. 2015). ...
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We investigated the directions of migration (reversed vs. expected) of raptors approaching a geographical strait in relation to local wind conditions, time of day, flock size and location of the observation post (coastal zone vs. inland zone). Fieldwork was conducted during autumn migration in 2011, 2012 and 2013 at a migratory bottle neck located in the southernmost part of the Italian Peninsula (Calabrian Apennines), using four watch points. In this area, migrating birds face the narrowest water surface between continental Italy and Sicily, the Strait of Messina. The only species showing substantial reverse migration was the short-toed snake eagle (Circaetus gallicus). In particular, eagles, mostly first cy birds, showed this behavior when passing closer to the coast (5 km inland of the Strait of Messina). Our results could reflect the reluctance of these birds to head south when approaching this relatively short stretch of sea, even before reaching the coastline. This behavior could be evidence of the strong selective pressure, which would have led to the evolution of the extremely detoured flight path of birds breeding in Italy.
... At the 25 km 2 scale, slope and elevation were highly correlated (r = -0.72); we chose to enter elevation in the models since it has been found to predict the presence of the short-toed snake eagle at a larger scale (Panuccio et al. 2015). The best models (ΔAIC < 2) for both spatial scales are shown in Table 1. ...
... The highlight of this study is the strong relationship between snake species richness and the occurrence of breeding short-toed snake eagles at both spatial scales (1 and 25 km 2 ). The same link was found in other Mediterranean environments, such as southeastern Spain and across Italy, but at a much larger scale (100 km 2 ; Moreno-Rueda & Pizarro 2007;Panuccio et al. 2015). The result that prey species richness can explain nest-site selection at a very fine scale (1 km 2 ) suggests that optimal breeding sites are placed as close as possible to hunting areas, as has previously been shown for another raptor species, the red kite (Milvus milvus, Pfeiffer & Meyburg 2015). ...
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Birds of prey, as top predators, play a key role in ecosystem functioning by regulating prey populations and, by means of cascade effects, promoting biodiversity. This makes them adequate sentinels of ecosystem health. Here we analyse the relationship between the occurrence of breeding short-toed snake eagle (Circaetus gallicus) and both the richness of potential prey species and landscape characteristics by taking into account two different spatial scales (i.e. nest-site scale and landscape scale). The short-toed snake eagle offers an interesting case study for investigating the relationships between top predators, prey diversity, and habitats, because it is an extremely specialised raptor that feeds on mesopredators, mostly snakes. Additionally, short-toed snake eagles are mainly threatened by changes in agriculture and land use in Europe, which have reduced the extent of suitable hunting habitats, and by the decrease in snake populations. Our study was conducted in the Latium Region (central Italy) in 2007, where most of the Italian breeding population is concentrated. By means of habitat selection analyses using generalised linear models, our results showed that the species selected breeding areas characterised by low elevations, rugged slopes, and high snake species richness at the nest-site scale (1 km²). At the landscape scale (25 km²), the best model showed that birds selected areas characterised by lower elevations for nesting, with a tendency towards intermediate values of wood cover and high snake species richness. Our study highlights the strong relationship between snake species richness and the occurrence of breeding eagles at both spatial scales, with optimal breeding sites located closer to hunting areas than expected by chance. This study provides further support for the role of short-toed snake eagles as sentinel species for Mediterranean habitats, and highlights the link between the location of nesting sites and the occurrence of human-modified landscapes characterised by high prey richness.
... This comparison among the three species highlights how the response to a major obstacle like the sea crossing is species-specific and depends mainly upon wing morphology (Agostini, Panuccio, & Pasquaretta, 2015). Such differences in motion capacity have profound effects also on the colonization patterns of these raptors: while the osprey is virtually cosmopolitan, the short-toed eagle is absent from large Mediterranean islands, perhaps because its colonization pathways are much more constrained by the distribution of land masses (Panuccio, Lucia, Agostini, Ottonello, & Bogliani, 2015). ...
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In Focus Santos, C. D., Silva, J. P., Muñoz, A.‐R., Onrubia, A., & Wikelski, M. (2020). The gateway to Africa: What determines sea crossing performance of a migratory soaring bird at the strait of Gibraltar? Journal of Animal Ecology, 89, 1317–1328. Migrating birds undertake long journeys which pose several challenges. Water bodies are the most demanding ecological barriers for soaring birds, due to the increase in energy consumption and mortality risk. Through high‐resolution GPS, Santos et al. (2020), analysed how the flight performance of 73 black kites crossing the Strait of Gibraltar was affected by external (e.g. weather conditions) and internal factors (individual experience). Kites waited for weaker crosswinds to start the crossing to minimize energy consumption, drift and altitude loss. Moreover, adults were quicker and lost less altitude than juveniles. These processes are likely to occur in all soaring species and have consequences also at a much wider spatial scale. In the Mediterranean region, species‐ and population‐specific migration strategies appear to be influenced by interactions between species' morphology and the distribution of the land masses they traverse.
... In other words, coexistence requires some form of niche difference or partitioning where species' niche consists of four major axes: resources, natural enemies, space and time (Chesson 2000). It has been recently pointed out that, in order to understand mechanisms of species distribution patterns, it is essential to account not only for classical ecological features but also for the movement and dispersal ability of an organism, defined as a change in spatial location in time (Nathan et al. 2008, Cumming et al. 2012, Panuccio et al. 2015. All of these processes interact in shaping the structure and dynamics of populations, communities and ecosystems. ...
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A growing number of ecological studies suggest that animal distributions are not only influenced by classical ecological features such as habitat availability, but also by the motion capacity of the studied animal. Here we analyse the diversity and density of two wintering raptor communities from Crete and Sicily, two large Mediterranean islands located along migratory flyways. We performed 611 and 1030 km of transects in Crete and Sicily respectively, examining the spatial distribution of raptors in relation to land use, topography, raptor species diversity and abundance. Our results show that community diversity and specific abundance are strictly related in accordance with the ‘More Individuals Hypothesis’. Comparing the two most common raptors, the density of the Eurasian kestrel was the highest in Sicily and that of the common buzzard in Crete. An overall positive effect of Eurasian kestrel density on that of the common buzzard was found in both islands, but higher in Crete. Our findings suggest that the distribution and density of the Eurasian kestrel, because of its higher movement ability, are less influenced by the presence of ecological barriers along potentially migratory flyways. We cannot exclude that higher inter specific competition with common buzzards in Crete might have pushed the smaller species to cross the Mediterranean Sea in order to overwinter in Africa.
... Only 34 species schemes were run on an international geographical scale. These involved two programmes monitoring only non-breeding populations: 'Mission Migration' from France including 32 species schemes; Short-toed Snake Eagle Circaetus gallicus satellite tracking from Italy (Panuccio et al. 2015); and one programme monitoring breeding populations of Redfooted Falcon Falco vespertinus based in Hungary (Solt et al. 2010 ...
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Capsule: Territory monitoring using playback of calls is a reliable approach for assessing population trends of Tawny Owls Strix aluco, particularly when human resources are limited or survey areas are difficult to access. Aims: To explore whether response calls of Tawny Owls towards broadcast conspecific and heterospecific male owl playback calls would provide similar estimates of population size and trends over time as nest-box monitoring. Methods: Between 1998 and 2017, Tawny Owls were monitored in a predominantly forested area of central Slovenia. Throughout the year, territories were monitored using a playback protocol comprising silent listening during five minutes before and after ten minutes of broadcasting male Tawny Owl. Seasonal variation in response rate was examined and results from the playback method were compared to data on occupancy rate of nest-boxes. Results: Territory monitoring using playback calls showed a similar direction of population trends as nest-box monitoring but a different population dynamics pattern. The overall response rate in occupied territories to conspecific playback calls at first visits was 70%. This was significantly higher than for heterospecific playback calls and the frequency of spontaneous vocalizations. The response rate to conspecific playback calls when including two visits rose to nearly 90%. There was no difference in response rate between seasons. The average time to respond to conspecific playback calls was five minutes. Conclusions: Compared to nest-box monitoring of breeding pairs, territory monitoring of breeding and non-breeding Tawny Owls using playback provides a robust and cost-effective method for monitoring. We recommend conducting territory monitoring between January and May during the breeding season, with two visits to each site using conspecific playback of territorial male hoot calls using the 5 + 10 + 5 minutes protocol.
... Only 34 species schemes were run on an international geographical scale. These involved two programmes monitoring only non-breeding populations: 'Mission Migration' from France including 32 species schemes; Short-toed Snake Eagle Circaetus gallicus satellite tracking from Italy (Panuccio et al. 2015); and one programme monitoring breeding populations of Redfooted Falcon Falco vespertinus based in Hungary (Solt et al. 2010 ...
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Capsule: A questionnaire identified 1196 raptor monitoring species schemes within 236 monitoring programmes across 37 countries. Aims: To assess the level of monitoring of status/trends of raptors across Europe, to produce a web-based inventory of activities. Methods: A questionnaire promoted by voluntary national coordinators assessed monitoring coverage, focusing on breeding populations. Results: One thousand one hundred and ninety-six species schemes (236 monitoring programmes; 90% active in 2012) were reported from 37 countries. Sixty per cent of schemes were of over 10 years duration and nine countries ran schemes of over 40 years duration. Nineteen species had at least one scheme in 10 or more countries, and 15 species had schemes that ran for over 10 years. Thirteen species had breeding monitoring schemes in over 50% of countries where they breed, including widespread species (e.g. Peregrine Falcon Falco peregrinus) and localized species (e.g. Rough-legged Buzzard Buteo lagopus). Lanner Falcon Falco biarmicus, Levant Sparrowhawk Accipiter brevipes and Booted Eagle Hieraaetus pennatus had the least representative coverage, and four rare species had no coverage. Coverage was more representative in north and west Europe than further south and east. Coverage was more representative for widespread species and those with more favourable conservation status. Conclusions: Large potential exists to enhance reporting on status/trends, ecotoxicology analyses and volunteer-based monitoring at the pan-European scale. National coordinators provide an ideal network to develop and disseminate best practice guidance across Europe.
... L'areale dell'Italia centromeridionale del Nibbio bruno è frammentato e localizzato principalmente lungo la fascia tirrenica dell'Italia centrale e in alcune province dell'Italia meridionale (Basilicata), altrove, la specie è estremamente localizzata e nidificante con pochissime coppie (Nardelli et al. 2015). Probabilmente, analogamente ad altre specie di rapaci migratori (Panuccio et al. 2015), le coppie di Nibbio bruno nidificanti nell'Italia peninsulare rappresentano una sub-popolazione di un sistema di meta-popolazione che hai il suo centro nell'Europa (Whitcomb et al. 1996, Hanski 1998. Nel complesso quindi la necessità di monitorare ed operare interventi per la conservazione di questa specie non deriva unicamente dal fatto che il Nibbio bruno è specie inserita nella Direttiva "Uccelli" 79/409/CEE, ma anche perché si registra un forte declino di alcune colonie storiche del Lazio (Aradis et al. 2008, Calvario et al. 2008, De Giacomo et al. 2015) e perché si tratta di una popolazione vicina al margine dell'areale. ...
... High productivity rate, availability of broader hunting territories and new suitable nesting sites are probably not sufficient to explain the Short-toed Eagle population increase in Italy. Considering the circuitous migration of this species in Italy in spring (Premuda 2004b), the wintering areas revealed by satellite tracking (Mellone et al. 2011) and the studies carried out in Greece (Panuccio et al. 2012), the western origin of the Italian population is evident and it reflects a possible recent colonization (Agostini & Mellone 2008, Baghino 2013, Panuccio et al. 2014. ...
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This review discusses a hypothesis about the root causes of the increasing trend of the Short-toed Eagle Circaetus gallicus population in Italy, such as: the high productivity rate of the Italian population, the withdrawal of farming and the agricultural landscapes exploited as hunting areas for the species, the increasing maturity of woods exploited as new nesting sites, and the immigration from other areas due to the expansion of the species. In addition, this paper provides an evaluation of the potential population increase yearly rate (PPIYR = 9.5%) in Italy.
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Capsule: Juvenile Short-toed Snake Eagles Circaetus gallicus hatching in the peripheral populations of Greece and Italy have limited opportunities for social learning of migration routes compared to those hatched elsewhere. Aims: To test the prediction that there would be a higher degree of migration synchrony between adult and juvenile Short-toed Snake Eagles originating from peripheral populations and using an extremely detoured flyway, when compared to other populations using a direct overland flyway. Methods: We use linear regression models to compare seasonal changes in the age distribution of migrating Short-toed Snake Eagles counted at two migration watch-sites in Italy (Arenzano) and Georgia (Batumi), along a detoured and a direct flyway, respectively. Results: Juveniles migrated a fewdays later thanadults at both sites and the age ratios recorded at these two sites was similar. The daily proportion of juveniles increased along a similar slope during the migration season, thus showing a similar degree of synchrony between the age classes on both flyways. Conclusions: Contrary to our hypothesis, juvenile and adult migration is not more synchronized in peripheral populations using a detoured flyway compared to a core population using a direct migration flyway. Our results suggest that juveniles do not learn detours to complete transMediterranean migration from their parents, but from other elders.
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Migrating juvenile birds rely on endogenous information in choosing the direction in which to fly, but such input may be over-ridden by social interactions with experienced individuals. We tagged seven juvenile Short-toed Eagles Circaetus gallicus with GPS transmitters in southern Italy. This trans-Saharan migrant flies mainly by soaring and is therefore not well adapted to performing long water crossings. Five of the seven tagged juveniles used the longer but apparently safer route towards the Strait of Gibraltar, while two migrated along a southerly trajectory and subsequently spent the winter in Sicily, apparently forced to do so by the 150-km wide Sicily Channel. One of these individuals took the longer route the following autumn. These results, combined with long-term (15 years) visual field observations involving thousands of individuals, suggest that inexperienced Short-toed Eagles may learn their migratory routes from experienced adults, while some of them migrate south in response to an innate orientation instinct. Transport costs, inherited information and geography apparently interact, forcing some Short-toed Eagles to winter 3000 km to the north of the majority of their conspecifics. This article is protected by copyright. All rights reserved.
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Migratory behavior of raptors is affected by several factors, including weather, geography, and topographical features. Here we provide information on how these factors may affect the behavior and detectability of the Short-toed Snake-Eagle (Circaetus gallicus). We observed passage of Short-toed Snake- Eagles at two watchsites along mountain chains in northeastern Greece (Mount Olympus) and northwestern Italy (Arenzano) during the peak periods of migration in autumn 2009 and spring 2010. More Short toed Snake-Eagles were observed in spring than in autumn; this difference was more evident at Arenzano. Temperatures influenced the number of migrants observed. In particular, the number of individuals observed decreased drastically when temperatures were higher than 24uC during post-reproductive movements. At both sites, daily patterns showed a lower proportion of raptors observed during midday and early afternoon in autumn than in spring. These results suggest that, during autumn, individuals may pass undetected by flying at higher altitudes during midday and in early afternoon at both sites. The lack of difference in number of eagles observed during westerly (lateral) winds compared to other wind directions at Mount Olympus suggests that these birds may be able to compensate for drift effect toward the Aegean Sea. Wind strength and lateral northerly wind negatively affected the number of migrants observed at Arenzano during spring movements.
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The short-toed eagle Circaetus gallicus is a summer resident in Europe, wintering in tropical Africa (Cramp and Simmons 1980). In Italy, a breeding population of about 400 pairs has been estimated, mostly distributed in the Alps, pre-Alps, Ligurian Apennines and along the western slope of central Italy (Cattaneo and Petretti 1992, Cattaneo 1998). During migration, this species uses mostly soaring flight over land, avoiding long sea crossings (Kerlinger 1989, Meyburg et al. 1998). In the Mediterranean basin, the greatest concentration of migrating individuals is observed at the Strait of Gibraltar, both during autumn and spring movements (Finlayson 1992). On the other hand, spring movements appear to be virtually non-existent over the central Mediterranean area (Beaman and Galea 1974, Agostini and Malara 1997, Agostini and Logozzo 1998, Agostini 2001). Individuals breeding in central Italy use a circuitous route both during autumn and spring migration, cross-ing the Mediterranean at the Strait of Gibraltar and migrating along the Ligurian Apennines (northwest-ern Italy), where the greatest concentration of migrat-ing short-toed eagles occurs (Baghino and Leugio 1989, 1990, Baghino 1996, Agostini and Malara 1997, Agostini et al. 2002a, b, Premuda 2002, Baghino 2003). They limit the flight over water (only 14 km across the Strait of Gibraltar), probably favouring a safe migration, as a result of a conservative strategy (Agostini et al. 2002b). During autumn, in southern continental Italy the migratory flow of this species is very limited, simi-larly to that observed during spring at the Strait of Messina (between southern continental Italy and Sicily; Giordano 1991, Agostini and Logozzo 1995a, 1995b, 1997); some birds are also observed over the islands of Malta during the second half of September and early October (Beaman and Galea 1974, Sultana and Gauci 1982, Agostini et al. 2002a), and occasion-ally in late autumn (late October-November) (Coleiro 1999 and C. Coleiro, pers. comm.). The aim of this study was to provide information on the autumn migration of the short-toed eagle along the western slopes of central Italy, through systematic observations on the Apuane Alps (Tuscany, central
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Negli anni 2004-2007 è stata realizzata la terza versione dell’Atlante degli uccelli nidificanti nelle province di Forlì-Cesena e Ravenna, dopo quelle “storiche” degli anni ’80 (Foschi e Gellini 1987; Provincia di Ravenna 1987) e della metà degli anni ’90 (Gellini e Ceccarelli 2000). Il primo atlante aveva un approccio puramente qualitativo, e si basava su di un reticolo geografico a maglia larga. Il secondo atlante costituì un importante miglioramento, sia in termini di definizione geografica della distribuzione delle specie, sia in termini di qualità delle metodologie di censimento: per la prima volta fu adottato un approccio quantitativo, che consentì di valutare le differenze di abbondanza delle specie nelle maglie del reticolo geografico utilizzato. Le ricerche su campo degli anni 2004-2007 finanziate, come il precedente atlante, dalle amministrazioni provinciali di Forlì-Cesena e Ravenna, sono state realizzate utilizzando la stessa struttura di lavoro, lo stesso reticolo geografico, la stessa metodologia di rilevamento, ed in larga misura anche gli stessi rilevatori su campo, delle ricerche utilizzate per l’atlante degli anni ’90. Si tratta dunque di una vera e propria replica dell’atlante semi-quantitativo realizzato oltre 10 anni fa, e pertanto fornisce una grande opportunità di “mettere a confronto” i risultati conseguiti e di analizzare le contrazioni ed espansioni di areale delle specie di uccelli, le variazioni quantitative, le estinzioni e colonizzazioni verificatesi nel corso di un decennio, sfruttando il vero potenziale degli atlanti di distribuzione, che è quello di consentire un monitoraggio dei popolamenti faunistici, della biodiversità e con essi delle condizioni ambientali “sottostanti”. Allo scopo di rendere particolarmente leggibili i risultati delle ricerche, in questo volume vengono presentate le carte di distribuzione delle specie rilevate ed anche, a fianco, le carte di distribuzione del vecchio atlante degli anni ’90. In questo modo, per ogni specie, i mutamenti intercorsi sono visibili “ a colpo d’occhio”. Per tutte le specie sono inoltre fornite tabelle e diagrammi relativi alla presenza ed abbondanza, ed un testo di commento alla distribuzione, con l’analisi delle eventuali differenze riscontrate rispetto all’atlante degli anni ’90. Questo volume è il risultato degli sforzi congiunti di un gran numero di collaboratori, che si sono dedicati al miglioramento delle conoscenze dell’avifauna dei territori romagnoli sin dagli anni ’80, e dell’impegno profuso dalla Amministrazione provinciale di Ravenna e dalla Amministrazione provinciale di Forlì-Cesena, che ha reso possibile la grande mole di ricerche su campo necessaria alla realizzazione dell’atlante.
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The Short-toed Eagle (Circaetus gallicus) uses mainly soaring flight during migration and avoids long water crossings between Italy and Africa by crossing at the Strait of Gibraltar. Observations were made 15-26 September 2000 at four sites in the central Mediterranean area: Arenzano (Ligurian Apennines, northwest Italy), Circeo promontory (central Italy), Marettimo (southern Italy) and Malta. In addition, 68 hr of observations were made 18-24 September 1998, 1999, and 2000 over the Apuane Alps along the western slope of central Italy. At Arenzano, 476 Short-toed Eagles were counted (5.4/ hr) consisting of 368 adults, 6 immatures, and 102 juveniles, with an overlap in the migration periods of age classes. The Short-toed Eagles migrated in flocks averaging 4.3 _+ 0.9 (SE) birds. Over the Apuane Alps, 289 Short-toed Eagles, all migrating northwest, were counted (4.3/hr). Few birds were seen at the other three sites, with a maximum of eight individuals recorded at Marettimo. These results confirm the circuitous autumn migration around the Mediterranean of Short-toed Eagles breeding in central Italy and suggest that at least some juveniles learn this route by following the adults.
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Migration patterns are rather flexible systems, consider-ing that the last glaciation occurred just a few thousand years ago and that postglacial changes in travel patterns indicate a high degree of evolutionary plasticity in migra-tion traits (Alerstam 2006, Science 313:791–794). Many mi-grating birds do not use the shortest flyways to reach their destinations, but avoid the crossing of ecological barriers such as water surfaces, and follow detours where barrier passages are reduced (Alerstam 2001, J. Theor. Biol. 209:319– 331). This is particularly evident in soaring birds, for which the extra costs (in terms of energy and safety) of flapping flight over water are higher than for other birds (Kerlinger 1989, Flight strategies of migrating hawks, Univ. of Chicago Press, Chicago, IL U.S.A.). In addition, migration routes are shaped by the historic processes of geographic range expan-sion and colonization (Sutherland 1998, J. Avian Biol. 29:441–446). In this letter, we detail a migratory route ap-parently reflecting the colonization process. The Short-toed Snake-Eagle (Circaetus gallicus) is a sum-mer breeder in Europe, wintering in the savannah zones south of the Sahara desert (Ferguson-Lees and Christie 2001, Raptors of the world, Helm Edition, London, U.K.). Adults breeding in central Italy cross the Mediter-ranean Sea at the Strait of Gibraltar, using a route through northwestern Italy during both autumn and spring migra-tion (Fig. 1; Agostini et al. 2002, J. Raptor Res. 36:111–114; Premuda 2004, Riv. Ital. Ornitol. 74:119–124). Thus, while migrating along the western slope of central Italy, these eagles fly in the direction opposite to that taken by other species migrating in the same season. Several circumstances suggest that the Short-toed Snake-Eagle is apparently still colonizing Italy from the western part of its European breeding range and that its popula-tion has not yet reached the carrying capacity. In particu-lar, the bulk of the breeding pairs in Italy are located along its migration route (Agostini et al. 2002) and, despite ap-Figure 1. Approximate flyway used during both autumn and spring by adult Short-toed Snake-Eagles breeding in central Italy (solid arrow), and the alternative route (not used) across the central Mediterranean (A 5 Arenzano).
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The short-toed snake eagle (Circaetus gallicus) uses mostly soaring flight over land during migration to avoid long sea crossings. In particular, birds breeding in central Italy cross the Mediterranean Sea at the Strait of Gibraltar, using a route through northwestern Italy during both autumn and spring migration. Birds breeding in Greece, such as those breeding in Italy, are expected to use the same strategy passing through northeastern Greece and avoiding the longer sea crossing between southern Greece and Libya. In order to verify this hypothesis, contemporaneous observations were made at two watchsites, in northwestern Italy (Apuane Alps) and northeastern Greece (Mount Olympus), during autumn 2009 and spring 2010. During autumn migration 376 birds were seen migrating at Mount Olympus, nearly all heading NNE. Most birds were seen migrating in flocks, and at least 23 flocks contained both adults and juveniles. Over the Apuane Alps a total of 1042 short-toed snake eagles, all migrating NNW, was counted. At this watchsite the proportion of juveniles was lower than that reported at Mount Olympus. During spring migration, 606 birds were seen at Mount Olympus, 602 heading south. At the Apuane Alps 1307 birds were counted, all heading SSE. The orientation behaviour of short-toed snake eagles confirms that those breeding in Greece, like those breeding in central Italy, use a circuitous route during both spring and autumn. In particular those breeding in Greece are expected to cross the sea at the Dardanelles and/or at the Bosphorus. In addition, the higher proportion of juveniles reported at Mount Olympus during autumn migration would suggest that social learning could have been much favoured by natural selection in the case of birds breeding in Greece rather than in Italy, highlighting a relationship between the length of the barrier and the tendency of juveniles to follow the adults.
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Capsule: There is some evidence of susceptibility to stochastic or human factors. Aims: To describe the phenology and breeding success of one of the densest populations of Short-toed Eagle in Europe. Methods: All nests in the Dadia-Lefkimi-Soufli forest in northeast Greece were located and visited regularly throughout the 1996-98 breeding seasons. Data on every stage of the breeding cycle were collected and related to among-year variation in the weather conditions during March to June. Results: A total of 58 pairs were located during the three-year study spread across 22 territories (the same territories are usually occupied each year). The nests were evenly spaced (mean of 2.7 km between nests). Adults arrived between mid-March and mid-April. Only one egg per nest was laid. Nestlings fledged on average after 68.9 days. Eagles departed between 8 September and 2 October. Conclusions: Arrival date determines laying date. The population size appears to be stable but the species has a relatively low reproductive rate and takes three to four years to mature, consequently it may be susceptible to stochastic or human-mediated factors.
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The area surrounding and including Dadia Forest, north‐eastern Greece, is well known for its diversity of breeding raptors, including many species of conservation concern. Dadia Forest has been exploited by humans for many centuries, but more recent social and economic changes have stimulated proposals that the forest should be subject to habitat management to protect the fauna of the region. We examined the distribution of reptiles over nine different habitat types with a view to assessing the importance of these habitats for foraging by short‐toed eagles Circaetus gallicus. In addition, data on the diet of the species were collected from direct observations at nests. The short‐toed eagle relies heavily on snakes for food. The most important prey species was the grass snake Nutrix natrix, although the Montpellier snake Malpolon monspessulunus and large whip snake Coluber jugularis also featured prominently as prey items at certain nests. Montpellier snakes and large whip snakes were distributed across all habitat types, but grass snakes were concentrated in areas of mainly intensive, but also non‐intensive, cultivation. Analysis showed that short‐toed eagles concentrated their foraging efforts in three habitat types: intensive and non‐intensive cultivation and grasslands. Grass snakes were abundant on cultivated land but relatively scarce on grassland. Forested areas were largely avoided by foraging eagles. The data show that for the short‐toed eagle the distribution and abundance of prey items on the ground does not reflect food availability. The possible effect of changes in habitat management on the short‐toed eagle population in Dadia is discussed, in particular the establishment of exclusion zones that could result in progressive canopy closure.
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“Optimization is the process of minimizing costs or maximizing benefits, or obtaining the best possible compromise between the two. Evolution by natural selection is a process of optimization” (R. McNeill Alexander 1982).
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Metapopulation biology is concerned with the dynamic consequences of migration among local populations and the conditions of regional persistence of species with unstable local populations. Well established effects of habitat patch area and isolation on migration, colonization and population extinction have now become integrated with classic metapopulation dynamics. This has led to models that can be used to predict the movement patterns of individuals, the dynamics of species, and the distributional patterns in multispecies communities in real fragmented landscapes.
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The short-toed eagle (Circaetus gallicus) is a trophic specialist with a diet based almost exclusively on ophidians. In this work, the distribution of this eagle in southeastern Spain taken from national atlases, is analysed in relation to environmental variables. The results show that the short-toed eagle is distributed primarily in shrublands, probably because there it can easily locate and capture its prey. It also prefers an intermediate cover of forest, because it needs trees for nesting. Lastly, there was a correlation, after statistically controlling for other variables, between snake species richness and eagle presence. It is possible that snake species richness favours the presence of this raptor, as it preys on different species. Alternatively, because this raptor preys preferentially on dominant ophidians in the study area, it is also possible that the presence of this eagle favours snake diversity by a top-down regulation on the ophidian community.
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The marsh fritillary butterfly Euphydryas aurinia is an endangered species in most of northern Europe. We describe the metapopulation structure of E. aurinia in Finland, where the species has declined drastically in the past decades. We found two types of habitat patches suitable for the species: semi-permanent meadows and transient clearcuts in the forest. Patch area was the most significant variable predicting the occurrence of E. aurinia in a habitat patch. The species tended to be found in young rather than old clearcuts, apparently because the vegetation became too high in the latter. We used the incidence function model to simulate the metapopulation dynamics of E. aurinia in its dynamic landscape and discovered that the continued presence of the semi-permanent meadows is essential for the survival of the species in the study area in southeast Finland.
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We surveyed 18 habitat patches (black spruce (Picea marinana) – tamarack (Larix larcina) wetlands) for spruce grouse (Dendragapus canadensis canadensis) on Mount Desert Island, Maine, during April–May in 1992 and 1993 to determine patch occupancy relative to patch area. We also equipped nine juvenile grouse with radio transmitters to determine movement and habitat use outside of patches during autumn dispersal. The 2 large patches (77 and 269 ha), 5 of 6 medium-sized (11–26 ha) patches and 1 of 10 small (4–8 ha) patches were occupied. Spruce grouse occupied smaller habitat patches than previously reported and occupied patches were closer (P < 0.05) to the nearest occupied patch ( = 1.2 km) than were unoccupied patches ( = 2.5 km). Eight of nine juvenile grouse left their natal habitat patch during autumn dispersal and net dispersal distance ( = 2.3 km) was greater than that reported for grouse in areas with more contiguous habitat. Dispersing juveniles used all major forest types and 33% of relocations were in deciduous forest. Thus, deciduous forest was not an absolute dispersal barrier.
Article
The diet of the short-toed eagle (Circaetus gallicus) was studied during the breeding season by analysing pellets and remains generated by adults and nestlings. The raptor proved to be a specialist feeder, as snake prey comprised almost 95% of the diet, in both frequency and biomass. We gathered information on prey availability and prey size availability (1499 specimens of nine different species) by searching for snakes in the study area. Regressions of vertebra centrum length and dorsal-scale length on snout–vent length (SVL) of the snakes were used to calculate prey size. The taxonomic diet composition differed depending on the sample analysed – remains or pellets – but we failed to find between-year diet differences. Most of the snakes identified (140 out of 141) belonged to only three species, Malpolon monspessulanus, Elaphe scalaris, and Coluber hippocrepis. Other prey included Lacerta lepida, Natrix maura, Bufo bufo, and Alectoris rufa, and many secondary prey (prey from stomach of the prey) were also detected in the pellets. The three main prey species were consumed according to their availability in the study area, but the eagles selected on the basis of prey size. Large snakes within 700–1000 mm SVL were positively selected, whereas snakes under 600 mm SVL were negatively selected. Adult eagles consumed the same prey species as those carried to the nest to feed their single nestling, although prey given to nestlings proved larger in size and biomass, suggesting that adults consumed the smallest prey, reserving the largest for nestlings.
Article
The major threat to Short-toed Eagles (Circaetus gallicus) is the reduction of suitable foraging habitats, but no quantitative studies have been conducted to understand this process. Here, the spatial distribution of foraging Short-toed Eagles was studied in relation to nine habitat types in Dadia-Lefkimi-Soufli National Park, Greece, during 1996-1998. Compared to the observed ccurrence of foraging individuals over a particular habitat type with the expected utilization of that same habitat type, Short-toed Eagles concentrated their foraging efforts on three types of open habitat: intensive and non-intensive cultivation, and grasslands. Forested areas (pine forests, oak forests and mixed oak-pine forests) were largely avoided by foraging individuals. The density of prey items on the ground may not necessarily be a good indicator as to where an eagle individual will forage, as vegetation structure is also highly influential. The results highlight the importance of open habitat types which provide foraging opportunities for the Short-toed Eagle population. Management guidelines that maintain the region as a patchy network of open and wooded habitats are discussed in order to conserve a viable population of Short-toed Eagles, and possibly certain other raptor species that forage over open areas.
Chapter
Publisher Summary This chapter presents a study on butterfly metapopulations. Butterfly populations are often structured in space in a manner that is broadly consistent with the metapopulation concept. The ecology of butterflies is well known in most countries in Europe and elsewhere. The chapter describes the general features of spatial population structure in butterflies, and the effects of landscape structure on butterfly populations. The two critical elements in the metapopulation framework are the effects of habitat patch area and isolation on the distribution of species. The chapter also discusses two key metapopulation processes, extinction and colonization, which are better studied for butterflies than perhaps for any other comparable taxon. The chapter further reviews empirical tests of theoretical predictions, another area where butterfly studies have played a critical role in recent years. The chapter discusses about various particulars that are now missing from metapopulation models, but should probably be included in the next generation of models. Examples used in this chapter exclusively concern butterfly metapopulations; however, many of the patterns and processes described may be applied to a much wider range of organisms.
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After a review of the first steps in marine palynology, we show that the understanding of the northwest African setting is crucial to evaluate the potential of marine palynological studies elsewhere. We studied distribution patterns of pollen grains in recent marine sediments off NW Africa and were able to clearly relate patterns to modern pollen source areas (vegetation belts) and operating transport systems (wind belts and ocean currents). In particular patterns of Quercus, Artemisia, Chenopodiaceae–Amaranthaceae, Ephedra, Gramineae, and wet forest trees are very indicative of the position of the vegetation belts on the adjacent continent. Aeolian pollen transport is carried out by the northeast trade winds and the African Easterly Jet (AEJ). In the rain forest belt transport of pollen and fern spores also occurs by rivers. A detailed comparison between recent pollen rain samples from terrestrial and marine sites between 21 and 121N showed that the latitudinal range of vegetation belts is clearly reflected in the pollen samples of both environments. A migration of the southern border of the Sahara is reflected by the changing ratio between Chenopodiaceae–Amaranthaceae pollen from the desert and Gramineae pollen from the savannah belt. Distribution patterns of pollen for 9000 and 18,000 14 C yr BP (last glacial maximum) time-slices, based on pollen records from eleven marine cores between Portugal and the Gulf of Guinea show significant latitudinal migrations of vegetation belts, but a stable position of the main wind trajectories. The AEJ had a stable position around 211N. The belt with trade winds had a stable position from Morocco southwards. Changing vigour of the trade winds is clearly reflected by the patterns of isopollen contours and by changes in pollen influx records.
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Metapopulation structure of species in fragmented landscapes is ultimately the result of spatial variability in demographic processes. While specific information on demographic parameters is desirable, a more practical approach to studying metapopulations in fragmented landscapes may begin with analyses of species' occurrence in relation to large-scale habitat variability. Here, we analyzed occurrence of stream-living bull trout (Salvelinus confluentus) in relation to physical, biotic, and geometrical characteristics of habitats. Bull trout occurrence was analyzed at several spatial (10 x m) scales. Data were from nested sampling of 720 sites (10 m), 179 reaches (10 2 m), and 81 patches (10 3 m) of stream habitats within the Boise River basin of central Idaho. Based on previous findings, patches were defined as stream catchments with suitable conditions for spawning and rearing of bull trout (1600 m elevation). Patch-scale bull trout occurrence was significantly related to patch area and isolation (stream distance between occupied patches). Lack of spatial autocorrelation between patches indicated that isolation effects were more likely the result of limited interaction among habitats (such as dispersal), rather than of correlated envi-ronmental conditions. A third factor, human disturbance in the form of roads, was associated with reduced bull trout occurrence at the patch-scale. Analyses of occurrence among reaches within occupied patches showed bull trout may select larger (2 m width) stream habitats. Occurrence of bull trout was not associated with nonnative brook trout (Salvelinus fontinalis) at large (patch), intermediate (reach), or small (site) spatial scales. Definition of a meta-population structure for bull trout in the Boise River basin was complicated by uncertainties in the frequency and magnitude of dispersal. From the distribution of patch sizes and isolation among occupied patches, we suggest that the metapopulation is a complex mosaic of several elements found in conceptual models. This complexity poses a challenge to empirical and theoretical attempts to study stream-living bull trout. Future work to define the structure of bull trout metapopulations must relate temporal and spatial patterns of patch occupancy with complex patterns of dispersal that likely interact with habitat spatial struc-ture, life history variability, and the historical context of regional climate changes. Results of this work suggest that conservation of bull trout should involve protection of larger, less isolated, and less disturbed (as indexed by road densities) habitats that may serve as im-portant refugia or sources of recolonization. Bull trout populations in smaller, isolated, and more disturbed habitats may be at risk of extinction. Finally, metapopulation structure implies the existence of suitable, but presently unoccupied habitat, which should be managed carefully to facilitate potential natural recolonization or reintroductions of bull trout.
Article
Aim The abundance distribution of organisms at regional scales is commonly interpreted as the result of spatial variation in habitat suitability. However, the possibility that geography itself may affect patterns of distribution has received less attention. For example, the abundance of wintering bird populations might be influenced by the cost of reaching areas located far away from the main migratory pathways. We studied the abundance distribution of three common migratory passerines (meadow pipits, Anthus pratensis ; common chaffinches, Fringilla coelebs ; and European robins, Erithacus rubecula ) wintering in farmlands located in the 600‐km long Cantabrian coastal sector of northern Spain, roughly perpendicular to the west Pyrenean migratory pathway that drives European migrant birds into the Iberian Peninsula. Location The study area occupies a belt located between the Atlantic coast and the Cantabrian Mountains in northern Spain. Methods We counted wintering and breeding birds and measured the structure of vegetation and environmental variables (altitude, rainfall, temperature) in 68 farmlands distributed at different distances from the west Pyrenean migratory flyway. We also studied the distribution of birds ringed in central and northern Europe and recovered in the study area between October and February. Analyses were based on single univariate statistics (chi‐square tests), ordination by principal components analysis and multiple regression. Results Controlling for the effects of climate, vegetation structure and local abundance of breeding conspecifics, the winter abundance of all three species decreased with the distance from their main migratory route in the western Pyrenees. Such patterns fitted well to the observed distribution of ringing recoveries. Main conclusions Our results support a link between the movements of birds along the Pyrenean migratory pathway and their winter abundance in northern Spain. According to this view, the sectors located near the migratory pathway seem to be more easily occupied by migrants, supporting the idea that proximity to passage areas may explain the fine‐grain regional patterning of species abundance in wintering grounds.
Article
1. Breeding sites of raptors were studied in relation to land-use and edge habitat using two different scales in semi-arid Mediterranean landscapes in south-eastern Spain. Habitat relationships were analysed using Generalized Linear Models. 2. The proportion of forest cover at a small scale was the best predictor for all species. At a larger scale, the proportion of forest cover was also a good predictor, and the amount of edge habitat between forest and extensive agriculture was a very good predictor of booted and short-toed eagle densities. 3. Models for sedentary species of raptor were similar using both scales whereas trans-Saharan migrant raptors seemed to be more sensitive to larger landscape features that included longer edges between forest and extensive agriculture. 4. Habitat mosaics created by forestry and traditional farming were especially important for Mediterranean raptors. Strengthening of the Agri-environmental Regulation (2078/92) will be necessary to compensate for agricultural intensification proposals promoted under the Common Agricultural Policy (CAP).
Article
Aim To test the effectiveness of statistical models based on explanatory environmental variables vs. existing distribution information (maps and breeding atlas), for predicting the distribution of four species of raptors (family Accipitridae): common buzzard Buteo buteo (Linnaeus, 1758), short‐toed eagle Circaetus gallicus (Gmelin, 1788), booted eagle Hieraaetus pennatus (Gmelin, 1788) and black kite Milvus migrans (Boddaert, 1783). Location Andalusia, southern Spain. Methods Generalized linear models of 10 × 10 km squares surveyed for the presence/absence of the species by road census. Statistical models use as predictors variables derived from topography, vegetation and land‐use, and the geographical coordinates (to take account of possible spatial trends). Predictions from the models are compared with current distribution maps from the national breeding atlas and leading reference works. Results The maps derived from statistical models for all four species were more predictive than the previously published range maps and the recent national breeding atlas. The best models incorporated both topographic and vegetation and land‐use variables. Further, in three of the four species the inclusion of spatial coordinates to account for neighbourhood effects improved these models. Models for the common buzzard and black kite were highly predictive and easy to interpret from an ecological point of view, while models for short‐toed eagle and, particularly, booted eagle were not so easy to interpret, but still predicted better than previous distribution information. Main conclusions It is possible to build accurate predictive models for raptor distribution with a limited field survey using as predictors environmental variables derived from digital maps. These models integrated in a geographical information system produced distribution maps that were more accurate than previously published ones for the study species in the study area. Our study is an example of a methodology that could be used for many taxa and areas to improve unreliable distribution information.