Journal of Ornithology

, Volume 149, Issue 2, pp 151–159

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

Authors

    • Dirección General para la BiodiversidadMinisterio de Medio Ambiente
Original Article

DOI: 10.1007/s10336-007-0252-z

Cite this article as:
González, L.M. J Ornithol (2008) 149: 151. doi:10.1007/s10336-007-0252-z

Abstract

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.

Keywords

Aquila heliacaAquila adalbertiFossil recordsOriginSpeciation

Introduction

Speciation is, with few exceptions, a historical phenomenon that is difficult to detect in the evolutionary time scale as it requires an interpretation of past events (Newton 2003). Natural selection is one of the potential mechanisms by which a species separates itself in two, and because of this it is considered to be one of the main causes of speciation (Mayr 1963). It is also the cause of adaptive divergences of populations that live in different environments and of the reduction in its genetic exchange (Mayr 1963; Rice and Hostert 1993; Schluter 2000). Changes in morphological, ecological and behavioural aspects in birds that result from selection pressure, more than from neutral mutations, are those that lead to divergence and speciation (Newton 2003). With respect to birds, in general, new species mostly arise by means of allopatric divergence of groups geographically isolated from an ancestral species (Newton 2003). In contrast, sympatric speciation, through which a new species arises without any kind of geographic isolation, is regarded as an unusual occurrence (Cracraft 1983).

The Spanish Imperial Eagle (Aquila adalberti), which is considered worldwide to be an endangered bird of prey, is currently only found in the southwest of the Iberian Peninsula (González and Oria 2004). This species has been traditionally considered to be a subspecies of the Eastern Imperial Eagle (Aquila heliaca), which is widely spread throughout the Palearctic (Cramp and Simmons 1980; Ferguson-Lees and Christie 2001); however, it has also been viewed as an independent species (Hiraldo et al. 1976; González et al. 1989a; Del Hoyo et al. 1994; Sangster et al. 2002), as a semispecies (Voous 1960) and even as an allospecies inside a superspecies (Sibley and Monroe 1990). Its origin and formation are controversial; some researchers have described these in terms of allopatry (Tyrberg 1991; Ferrer and Negro 2004), while others refer to sympatry or parapatry (González et al. 1989a). The age of divergence between both species of eagles was originally estimated as being up to 1 million years, based on genetic analysis of the cytochrome b gene of mitochondrial (mt)DNA and assuming a non-calibrated rate of mitochondrial evolution of 2% My−1 (Seibold et al. 1996). However, this age has been recently brought back to a few thousand years based on analyses of nuclear DNA microsatellites and a fragment of the control region of the mtDNA (Martínez-Cruz and Godoy 2007).

The origin and formation of a species is of great relevance to an understanding of some aspects of their ecology, taxonomy and biogeography (Grant and Grant 1997; Newton 2003). A study of archeozoological records is one approach used for elucidating the distribution range of birds (see Tyrberg 1991; Zachos and Schmölcke 2006). In this context, paleontological and biogeographical information on both species could contribute towards clarifying some aspects of the taxonomy, origin and formation of A. adalberti. With this aim, in this article I review the information available in the paleontological records of A. heliaca and A. adalberti and analyse this information in terms of paleoecological environment, geographic distribution and biology. I then propose an explanation on the origin and shaping of A. adalberti.

Methods

Information published on fossil records of both taxons, both worldwide (see Tyrberg 1998, 2005; Mlikovský 2002) and in the Iberian Peninsula (Hernández 1993; Sánchez 1996; Arribas 2004), was reviewed. Based on this information, their distribution range was determined. Identification criteria between A. heliaca and A. adalberti remains as stated in the publications quoted, even though identification was not carried out by osteological differentiation but according to the current distribution of both taxons (Hernández 1993; Arribas 2004). Identification not considered to be certain—i.e. given as cf. (conformis) and af. (affinis) in the bibliography—was not included in this study.

The paleontological information was analysed in relation to the paleoenvironment in Eurasia during the Pleistocene and Holocene ages using the paleovegetation maps published by QEN “Quaternary Environment Network” (see Adams and Faure 1997) and from studies on paleobiocenosis in Iberia (see Sánchez 1996; Arribas 2004).

The species studied

Aquila heliaca is distributed throughout most of the Palearctic (Fig. 1). In general, the birds of this species behave as migrant birds in most of their distributional area, migrating to mid-eastern Africa and southern Asia (Ueta and Ryabtsev 2001; Bird Life International 2005). However, some populations, such as those of Eastern Europe (Carpathians and Balkans), are partially migrant: they move southward only in severe winters (Meyburg et al. 1995; Ferguson-Lees and Christie 2001). Other populations, such as those of Cyprus, Turkey and northern Iran, and the already extinct populations of India and Pakistan, are sedentary (Swan and Wetmore 1945; Glutz von Blotzheim et al. 1971; Ferguson-Lees and Christie 2001). Individuals from the densest known populations, those of Central Asia, are monogamous and territorial (Katzner et al. 2005). Aquila heliaca inhabits wooded plains, humid areas with rivers, trees and steppes where the highest density rates are reached (Katzner et al. 2003; Bird Life International 2005), and it specializes in capturing steppe rodents and lagomorpha, such as hamsters (Cricetus cricetus), land squirrels or susliks (Spermophilus fulvus, S. major, S. pygmaeus), steppe marmots (Marmota bobak) and hares (Lepus europaeus, L. timidus, L. capensis) (Dementiev et al. 1966; Katzner et al. 2003, 2005). Egg-laying dates vary with latitude; for example, laying take place in February–March in India and Pakistan, (Swan and Wetmore 1945; Bird Life International 2005), late March to early April in Eastern Europe (Swan and Wetmore 1945; Svehlik and Meyburg 1979), early April to early May in Russia and Siberia (Brown and Amadon 1968) and in the second half of April to early May in Eastern Russia, north of Central Asia and China (Dementiev et al. 1966; Bird Life International 2005).
https://static-content.springer.com/image/art%3A10.1007%2Fs10336-007-0252-z/MediaObjects/10336_2007_252_Fig1_HTML.gif
Fig. 1

Present breeding distribution area (dark grey) and wintering area (light grey) of Aquila adalberti (Iberian Peninsula) and Aquilaheliaca (from Ferguson-Lees and Christie 2001). Star dots indicate the localities of the fossil records cited in the text

Aquila adalberti is sedentary—although some individuals in dispersion can occasionally reach tropical areas of western Africa (González and Oria 2004)—territorial and monogamous (but see González et al. 2006). Their highest densities are found in plains occupied by “dehesas”, a kind of cleared Mediterranean forest of Quercus of anthropic origin (González et al. 1990). The Spanish Imperial Eagle nests in trees, although it occasionally found nesting in electric towers (González 1991). The main component of its diet is rabbit (Oryctolagus cuniculus), but waterfowl (Anas spp., Anser anser) and wood pigeons (Columba palumbus) complement its diet in winter (Delibes 1978; González 1991). Most egg-laying occurs in March, with 9 February being the earliest reported date (González 1991; Margalida et al. 2007).

Results

Aquila heliaca

The earliest records of A. heliaca come from the mid-Pleistocene in China and Azerbaijan (Table 1). Records from the Upper Pleistocene have been reported in the Caucasian region (Georgia), Eastern Europe (Hungary and Romania) and Central Europe (Austria, Switzerland and Italy), and records from the end of the Pleistocene have been reported in southern Italy. The earliest records (within the Pleistocene epoch) are from the most eastern places, while the most recent ones refer to the most western ones. No other fossil remains from that period have been found anywhere else despite the large number of sites that have been examined both in Europe and Asia (see review in Mlikovský 2002; Sánchez 2004; Tyrberg 2005). During the nineteenth and twentieth centuries their known distributional area (Fig. 1) reached the Pyrenees and southern France to the west (González et al. 1989b; Arribas 2004) and up to Manchuria in the east (Glutz von Blotzheim et al. 1971; Bird Life International 2005). Such information indicates that A. heliaca spread throughout a wide strip of the Palearctic, from eastern Asia to Central Europe (Fig. 1).
Table 1

Fossil records of Aquila heliaca

Location

Date

Reference

Middle Pleistocene

 Zhoukoudian (Loc2; Hopei), China

Early Middle Pleistocene

Tyrberg (1998)

Upper Pleistocene

 Binagady, Azerbaijan AZ 2

MP/LP approximately 95–120KA bp?

Tyrberg (2005)

 Rîpa, Bihor, Romania

“End Riss/Würm”, Würm I”, Mousterian (70,000–60,000 bp)

Tyrberg (1998)

 Grotte de Cotencher Neuenburg, Switzerland

Mousterian, Early Würm; prob. IS5a; (70,000–40,000 bp)

Tyrberg (1998)

 Kudaro 3; South Ossetia, Georgia

Mousterian; (70,000–35,000 bp)

Tyrberg (1998)

 Saccopastore, Rome, Italy

Early Wurm (115,000–60,000 bp)

Tyrberg (1991)

 Kalman Lambrecht Cave, Hungary

Early Wurm (115,000–60,000 bp)

Tyrberg (1991)

 Grotta Romanelli, Puglia, Italy IT 40

Layer A1 LP LG Late Dryas 3 14C(A1 + 2) 10,320 ± 130 bp, 9,880 ± 100 bp, 11,800 ± 600 bp, 9,050 ± 100 bp

Tyrberg (2005)

 Grotta Romanelli, Puglia, Italy IT 40

Layer B LP LG Dryas late Middle Dryas 3 14C = 11,930 ± 520 bp

Tyrberg (2005)

 Grotta Romanelli, Puglia, Italy IT 40

Layer C sup. Late LG Middle Dryas 3 14C (C sup + inf) = 10,390 ± 80 bp, 9,790 ± 80 bp

Tyrberg (2005)

 Grotta Romanelli, Puglia, Italy IT 40

Layer C inf. LP LG Middle Dryas 3 (13,000–10,700 bp)

Tyrberg (2005)

 Grotta Romanelli, Puglia, Italy IT 40

Layer D, LP LG Early Dryas 3 14C = 10,640 ± 100 bp

Tyrberg (2005)

 Grotta Romanelli, Puglia, Italy IT 40

Layer E, LP LG Late Alleröd (11,000–11,800 bp)

Tyrberg (2005)

During the Mid and Upper Pleistocene, most of Europe was occupied by steppe or dry wooded steppe, with steppic rodents and lagomorpha, such as Marmota, Spermophilus, Cricetus and Lepus, all potential prey of A. heliaca. This steppe extended as far as Western Europe and the Iberian Peninsula during several periods of maximum expansion (between 150,000 and 13,000 bp), and at the same time, southern Europe was occupied by a semi-desert steppe that resembled the cold and arid mountainous steppes of Central Asia (Adams and Faure 1997; Arribas 2004). With the last Pleistocenic glaciation in the Late-glacial (less than 13,000 bp) and Early Holocene periods (10,000 bp), the steppe disappeared from Central and Western Europe, where it was replaced by open deciduous forests, remaining up to the present only in Eastern Europe (Adams and Faure 1997; García Antón et al. 2002).

Aquila adalberti

The first records of A. adalberti come from the end of the Pleistocene period and only in localities from the southeastern Iberian Peninsula. Records indicated that during the recent prehistoric and historic times up to the eighth century the species may have been confined to the eastern half of the Iberian Peninsula (Table 2). However, in the nineteenth and twentieth centuries, its distributional area extended to the midwestern Iberian Peninsula and northern Morocco (Fig. 1), showing that it must have expanded throughout the Iberian Peninsula and reaching Morocco. It is at the present time no longer a breeding species in Morocco and in most of western and northern areas of the Iberian Peninsula (Portugal and north of Duero river) (González and Oria 2004) of the Iberian Peninsula. No record of the Imperial Eagle exists in Iberia previous to Upper Pleistocene, although the number and geographic extension of the sites that have been checked is very large (see Hernández 1993; Sánchez 1996; Arribas 2004). In some widely studied sites (e.g. Atapuerca, Burgos province) similar birds of prey have been found relatively often, e.g., Aquila chrysaetos (Arribas 2004). The fossil records also indicate that, unlike the current situation, Imperial Eagles were formerly spread throughout the eastern half of the Iberian Peninsula. However, this distribution may be an artefact, as the remains were found in caves, which abound in calcareous southeastern Iberia, whereas in the siliceous Iberia (north and west), caves are unusual and bones decompose much more easily.
Table 2

Fossil records of Aquila adalberti

Location

Date

Reference

Nerja Cave, Nerja, Málaga

Epipaleolithic NT79; 14C = 8,770 ± 140 a 13,330 ± 270 bp

Hernández (1993)

Cueva del Moro, Olvena, Huesca

Neolithic (7,000–2,000)

Hernández (1993)

Los Castillejos de Montefrío, Peñas de los Gitanos, Granada

End of Neolithic (3,000–2,000)

Hernández (1993)

Fuente Alamo, Los Campos, Almería

Bronze (2,200–700)

Hernández (1993)

Los Millares, Santa Fé de Mondujar, Almería

Bronze (2,200–700)

Hernández (1993)

Terrera del reloj, Dehesas de Guadix, Granada

Mid Bronze-Argaric period (1,800–1,100)

Arribas (2004)

Castillo de Doña Blanca, Puerto de Santa María, Cádiz

Eighth–seventh century

Hernández, (1993)

Castillo de Doña Blanca, Puerto de Santa María, Cádiz

Fifth–fourth century

Hernández (1993)

Castillo de Doña Blanca, Puerto de Santa María, Cádiz

Fourth century

Hernández (1993)

Castillo de Doña Blanca, Puerto de Santa María, Cádiz

Third century

Hernández (1993)

During the Upper Pleistocene, forests and bushes co-existed with steppes in Iberia (Sánchez 1996). In central and northern Iberia, fauna consisted of typical glacial European species, known as “tundra-steppe paleofauna association of the mammoth” (Arribas 2004). Some potential prey of the A. heliaca, as Marmota marmota, spread throughout the Iberian mountains and, unlike the present situation, inhabited steppes and meadows in locations of low or moderate altitude; Lepus spp., Cricetus sp. and the Quaternary Suslik (Spermophilus superciliosus), appear here from 17,320 bp to the Post-Solutrense period, matching with the periods of expanding steppe (Arribas 2004). In the south and east parts of the Iberian Peninsula, the fauna of this Age was known as “paleofauna association of open forest of the Mediterranean mountain”, and consisted of Mediterranean and endemic species (Arribas 2004). The most abundant species, the European rabbit, was one of the Imperial Eagle’s potential prey, and it emerged as a species in the Iberian Peninsula and southern France during the Mid-Pleistocene (Lopez-Martínez 1989; Callou 1995). The rabbit survived the latest Pleistocenic glaciations in two Iberian glacial shelters, one in the coastal Mediterranean region and another one in the southern area (Branco et al. 2000). Its habits are similar to those of the steppic rodents but, unlike those previously mentioned, it may not have hibernated—a supposition partially supported by the fact that no current population is known to hibernate despite its wide distributional area and the large climatic range to which it is exposed including conditions similar to those of the Pleistocene (Thompson and King 1994). Apparently, rabbits must have been common, at least locally, as records are often found in the Iberian sites of this period (Arribas 2004), and the levels of genetic variability and regional differentiation of populations also provide proof of this (Branco et al. 2002).

In the Late Dryas (9,900 bp), an arid and pyrophytic period begins in Iberia, and Mediterranean vegetation expands its distributional area from the south and east to the Central Plateau and high-altitude areas (Carrión et al. 2002; Arribas 2004). In the last 4,500 years, fire and shepherding increased, and favoured the formation of evergreen Quercus sp. and open pinewoods with an abundance of gramineous plants and thorny species spread along the Iberian Peninsula (Carrión et al. 2000; Carrión 2002). Marmota, Spermophilus and have Cricetus disappeared, but Lepus has remained (Arribas 2004), and other species, such as the rabbit, have expanded their distributional area to most of the Iberian Peninsula (Branco et al. 2000, 2002).

Discussion

Origin

Based on the paleontological records examined, A. heliaca spread from East–Central Asia westward during the Upper Pleistocene, reaching mid-western and southern Europe in the Late Pleistocene and the Iberian Peninsula in the early Holocene. Fossil remains of A. heliaca, which might indicate whether the Imperial Eagle arrived in the Iberian Peninsula via Western Europe (Tyrberg 1991) or via northern Africa, are not available beyond the most western Pleistocenic locations (Switzerland and Italy). Fossil records of the birds in the Iberian Peninsula indicates an invasion of bird species from Northern Europe that specialized in capturing rodents from the steppe and the tundra, such as the Snowy Owl (Nyctea scandiaca), as well as an invasion by European species, typically from the Eastern steppes, such as the Demoiselle crane (Anthropoides virgo), Long-legged Buzzard (Buteo rufinus), Pin-tailed Sandgrouse (Pterocles alchata) and Black-bellied Sandgrouse (Pterocles orientalis) (Tyrberg 1991; Sánchez 1996; Hernández and Tyrberg 1999; Arribas 2004). However, conditions in Northern Africa during the last glaciation were not too arid, since species such as Megaloceros algiricus and Bos primigenius ssp. were present; the north of Africa belonged to the so-called Levantian corridor, where some of the species that can be found there nowadays (in Morocco, for example), seem to be more closely related to those from the Eastern Mediterranean than to Iberian ones (Arribas 2004). Northern Africa could consequently meet A. heliaca’s ecological preferences and could be used as a route for expansion. The distributional area of A. heliaca, like that of other species studied (Newton 2003), seems to have varied following changes in habitat distribution as a reply to environmental changes on a worldwide scale.

The fossil record in Iberia also indicates that A. adalberti appeared relatively recent (Late Pleistocene–Early Holocene), which matches with the divergence age of this taxa proposed recently by Martínez-Cruz and Godoy (2007), and that their distributional area in the Holocene matches with that of Mediterranean vegetation (Carrión et al. 2000) and that of the rabbit (Branco et al. 2000, 2002). Consequently, Imperial Eagles would feed on rabbits as well as steppic rodents in southern and eastern Mediterranean Iberia. The process of expansion of A. adalberti within Iberia may have been linked to the expansion of the rabbit and their preferred habitats, the “dehesas” of Quercus sp., which are also the present preferred habitat of A. adalberti (González et al. 1990). According to anthracological and pollen data, the formation of dehesas began in southern and eastern Iberia in the Neolithic Age (about 4,000 bp), initially expanding towards western Iberia in the first and second centuries (Stevenson and Harrison 1992; Joffre et al. 1999; Carrión et al. 2000) and more extensively in the ninth and tenth centuries (San Miguel 1994; Olea et al. 2005); this favoured the expansion of the rabbit in the same direction (Soriguer 1980; Rodgers 1981; Callou 1995; Branco et al. 2000) and would result in the expansion of A. adalberti in the Iberian Peninsula an northwestern Morocco.

Formation

The formation of A. adalberti, as in most cases of bird speciation, must have followed a process of isolation, divergence and genetic differentiation (Grant and Grant 1997; Newton 2003). Such a process may have started with the speciation of the Eastern Imperial Eagle that colonized Iberia and thrived with the capture of rabbits. Such an abundance of easy prey may have reduced its migratory activity and increased its degree of sedentarization. The present-day Eastern Imperial Eagle behaves as a migrant in locations where food is seasonal and as sedentary in areas where food is circumannual. A change from migratory to sedentary behaviour has been reported in birds (Newton 2003). For example, the Spotless Starling (Sturnus unicolor) has altered its migratory behaviour in the Iberian Peninsula, turning towards a more sedentary life-style than that shown by the Common Starling (S. vulgaris), the species from which it originally segregated (Stewart 2002). Migratory behaviour is facultative in birds, and it results from an interaction with the physical and social environment (Terrill 1991; Pulido et al. 1996; Berthold 2001). It has been suggested that all birds are partially migrant and that even species which are totally migrant or totally sedentary have individuals showing both behaviours, although at an undetectable frequency (Pulido et al. 1996; Berthold 2001). Changes in these behaviours are found to occur without the introduction of new alleles (Pulido et al. 1996), and they may occur within a few generations (Able and Belthoff 1998; Berthold 2001; Zink 2002) and be linked to an increase in environmental temperature (Berthold 2001; Coppack and Both 2002) and/or seasonal availability of trophic resources (Gill 1995; Newton 2003).

Imperial Eagles may have originally behaved as migrants in areas with summer prey (steppic and hibernating rodents) as well as in areas with an abundance of prey year-round (rabbit), and they may have reduced their migratory activity progressively, thus evolving towards sedentarization. This sedentarization process would have caused the next step—the advancement of the reproductive cycle. We have observed that current sedentary Imperial Eagles, both A. heliaca and A. adalberti, have early reproductive cycles; these can be up to 4 weeks earlier than those of migrant populations. A significant number of advancements of reproductive cycles have been recorded among birds following an increase in temperature and available food (see Coppack and Both 2002; Dunn 2004). Thus, Imperial Eagles in Iberia, after becoming sedentary, may have adjusted their reproductive cycle to new conditions of trophic availability and brought their reproductive cycle forward. Therefore, populations of sedentary eagles (with an earlier reproductive cycle) may have co-existed with migrant eagles (with a later one).

Based on these suggested life-cycle changes, it is possible that sedentary individuals paired up with one another selectively. This would mean that when migrant eagles returned to Iberia, sedentary eagles had already mated. Assortative mating has been observed in a certain number of species (Connors 1983; Monteiro and Furness 1998; Bearhop et al. 2005), and because they involve a time restriction in genetic interchange, this may lead to time adaptations in phenetic aspects (Hendry and Day 2005). Assortatively mating, which is regarded as one of the mechanisms of reproductive isolation preceding speciations and it frequently occurs within a few generations (Hendry 2004) and is based on relevant ecological adaptations leading to the shaping of new species, especially in recently colonized habitats (Dieckmann and Doebeli 1999). This could explain the high levels of homozygosity currently shown by A. adalberti (Negro and Hiraldo 1994), similar to that proposed in other research (Crow and Kimura 1970). This mechanism could have resulted in sedentary and migrant Imperial Eagles being reproductively isolated and led to a genetic divergence, thus boosting the speciation mechanisms which would eventually lead to A. adalberti.

Both eagles may have evolved in different habitats through habitat specialization, as predicted in the sympatric speciation model (Rice 1987) and as found in other species (Lande 1982; Orr and Smith 1998). Therefore, their reproductive isolation would be a “by-product” of ecological adaptation. As the steppes and the steppic rodents disappeared from Iberia, migrant Imperial Eagles, specialized in such habitats, would move backwards together with their habitat, towards Central and Eastern Europe and would adapt to survive in the deciduous plains of these regions, where they still survive, although in small numbers (Bird Life International 2005). Meanwhile, the eagles adapted to Mediterranean vegetation and prey species may have survived in Iberia; this would therefore be a case of food shelter (Sánchez 1996) rather than a climatic one, as has been suggested (Ferrer and Negro 2004). As proposed, the formation of A. adalberti may have occurred under conditions of sympatric (or parapatric) speciation, as previously suggested (González et al. 1989a).

Genetics and taxonomic aspects

The differentiation percentage found between adalberti and heliaca (1.7–1.8%) for the cytochrome b gene of the mtDNA has been used as an argument to consider A. adalberti as a valid species (Seibold et al. 1996). However, the nuclear DNA analysis that places heliaca’s divergence age at only a few thousand years (Martínez-Cruz and Godoy 2007) largely matches the paleontological data reported here, suggesting A. adalberti, like other species of a similar age (see Newton 2003), may reflect a case of subspeciation. Nevertheless, it is not advisable to define a speciation event based only on the degree of genetic divergence, as the relationship between genetic and phenetic features is not completely clear, especially when mitochondrial genes, which are not influenced by reproductive events, unlike nuclear genes, are used (Ferguson 2002; Newton 2003). Therefore, it should be assumed that the Eastern Imperial Eagle which colonized Iberia, A. adalberti’s ancestor, might have had some kind of morphological differentiation and that this differentiation would explain the phenotypic difference between A. adalberti and A. heliaca with such a short period of divergence as that proposed. In support of this, several variations in the plumage of A. heliaca have been described—under the non-accepted sub-specific denomination of crassipes and ricketti—in individuals of the eastern (China) and southern (Pakistan) borders of its distributional area (Shaw 1936; Swan and Wetmore 1945). These consist of a certain phenetic variation linked to the borders of its distributional area. Accordingly, it is possible that (in our case also) the colonizing individuals could have had a certain phenetic variation in the western border of the distributional area during the Holocene—for example, incipient white feathers on their shoulders and front edge of the wing (in adult individuals) and non-striped feathers on young individuals’ breast. These variations could have become fixed in sedentary eagles, the ancestors of A. adalberti, when they became isolated, reproductively speaking, and would explain why A. adalberti has become significantly different in plumage in just a few thousand years of divergence, a relatively short time in terms of evolution.

Taking into consideration all of the paleontological information presented here as well as morphological and ecological differences between both eagles (Hiraldo et al. 1976), A. adalberti could be the result of incipient speciation according to the distinctive criteria between species and semispecies (Helbig et al. 2002; Newton 2003). Therefore, A. adalberti could be considered as a semispecies, as has been proposed in the past (Voous 1960). Given that the denomination of a semispecies must be expressed in brackets and trinomially (Amadon 1966), A. adalberti should be named as Aquila [heliaca] adalberti.

Zusammenfassung

Herkunft und Entwicklung des Spanischen Kaiseradlers Aquila adalberti

Wir gingen die paläontologischen Befunde des Östlichen Kaiseradlers (Aquila heliaca) und des Spanischen Kaiseradlers (Aquila adalberti) durch, die mit der paläontologischen Umwelt Eurasiens sowohl während des Pleistozäns als auch des Holozäns zusammenhängen. Der Östliche Kaiseradler breitete sich in Eurasien während der letzten Vereisung im Pleistozän aus, begünstigt durch die Ausbreitung des Steppen. Die ersten Funde des Spanischen Kaiseradlers stammen aus dem späten Pleistozän und dem frühen Holozän auf der östlichen Iberischen Halbinsel, und ihre Verbeitung scheint beschränkt zu sein auf das Verbreitungsgebiet von mediterraner Vegetation und europäischem Kaninchen. Einzelne Individuen ziehender A. heliaca könnten die Iberische Halbinsel zwischen Ende des Pleistozän und Anfang des Holozän von Europa kommend erreicht haben. Diese Individuen könnten sich an das mediterrane Ökosystem angepasst haben, indem sie sich auf die Kaninchen spezialisierten, einer Beute, die das ganze Jahr über reichlich zur Verfügung steht. In Folge könnten sie ihre Zugtätigkeit reduziert haben. Sie könnten dann eine eigene Brutphänologie ausgebildet und sich selektiv verpaart haben, wodurch sie sich von den ziehenden Populationen genetisch immer weiter entfernten und sich so sym- oder parapatrisch die neue Art Aquila adalberti entwickelte. Diese Ergebnisse unterstützen die Befunde des aktuellen Auseinanderentwickelns der beiden Taxa. Außerdem könnte die beginnende Speziation von Aquila adalberti taxonomische Überlegungen über seinen Status als Semi-Spezies unterstützen.

Acknowledgments

I am indebted to Roberto Sánchez, Javier Oria and Antoni Margalida for their help and assistance. I would particularly like to thank to Oscar Arribas for his comments and suggestions to the manuscript and to Tom Tyberg for providing me with useful information. This work is dedicated to the memory of Francisco Hernández Carrasquilla.

Copyright information

© Dt. Ornithologen-Gesellschaft e.V. 2007