The ranges of Greater Spotted Eagle Aquila clanga and Lesser Spotted Eagle Aquila pomarina overlap in Eastern Europe, where interspecific hybridisation has been recorded repeatedly (Bergmanis et al. 1997, Lõhmus & Väli 2001, Bergmanis & Strazds 2001, Väli & Lõhmus 2004, Dombrovski 2005, Meyburg et al. 2005b, Treinys 2005, Schwanbeck 2008). Although field observations and genetic analyses suggest that at least some hybrids are fertile (Väli & Lõhmus 2004, Dombrovski 2005, Helbig et al. 2005, Väli et al. 2010b), verified cases with detailed descriptions are lacking. Knowledge on fertility of hybrids is important for both theoretical (e.g. taxonomical) and conservation reasons.

In the current study, I describe a male hybrid spotted eagle, breeding with a female A. pomarina in Estonia, as well as his two offspring, backcrosses between hybrid and A. pomarina, using a number of species-specific morphological characters proposed by Bergmanis (1996), Forsman (1999), Väli & Lõhmus (2004) and Dombrovski (2006). Measurements were taken following Bergmanis (1989) and Baker (1993). As comparisons of natal and breeding sites of eagles are scarce, I also give detailed descriptions of the two nest sites using the same parameters as in previous papers (Treinys 2004, Väli et al. 2004, Lõhmus & Väli 2004, 2005, Mirski 2009).

Results

On 20 July 1999, a spotted eagle nestling was studied and ringed (Estonia Matsalu R7400, hereafter R7400) at its nest in east-central Estonia. According to the DNA-analysis (see for method: Griffiths et al. 1998 and Väli 2004), the nestling was a male. In general, its plumage was typical of A. pomarina: a large ochre nape patch, brown body colour, few yellowish spots on medium upperwing coverts and contrasting bars on rectrices. However, typical of A. clanga, he had rather narrow dark bands on the secondaries (though not as narrow as in most A. clanga) and a large bill (Table 1, Figs 1 A, B). The DNA-analysis (for details, see Väli et al. 2010a) confirmed that the bird is an F1 hybrid between the two species. Field observations indicated that his father had been A. pomarina, his mother A. clanga. The latter was also confirmed by DNA-analysis of feathers collected at the nest. Two years later, a moulted feather of the male A. pomarina was found at the same nest site and sharing of alleles at all studied loci suggested that this bird had sired nestling R7400 in 1999.

Figure 1.

Hybrid spotted eagle R7400 as a nestling on 20 July 1999 (A–B), and as an adult on 28 July 2009 (C–F).

Ten years later, on 28 July 2009, hybrid R7400 was caught close to his nest, 9.4 km away from his natal site, where he was breeding with a female A. pomarina. Some of his morphological characters, such as overall dark colour, single prominent carpal crescent and large bill resembled those of A. clanga. Other characters, such as rather short and narrow wings, and banded rectrices, would have suggested A. pomarina. Some characters, such as a yellow-brown iris (which, however, looked dark from a distance) and medium length of the notch on the 4th primary, were intermediate (Table 1, Figs 1 C–H). Surprisingly, several immature characters (see Forsman 1999) had been retained: some fresh medium upperwing-coverts had whitish tips, there were yellowish spots on the back and light edges to some of the feathers on the breast and on the belly (forming spots from a distance), the undertail coverts had whitish tips and bars which together formed a pale area, the secondaries were contrastingly barred and the fresh nape feathers had rufous tips. The bird also showed white edges on the greater underwing coverts, a character pronounced in juveniles and immatures (forming spots), but also present in adult spotted eagles (Forsman 1999, Ü. Väli & A. Nurmla, unpubl. data). At the same time, the bird had several simultaneously ongoing moult waves of remiges (primaries P and secondaries S, numbered respectively descendantly and ascendantly): fresh P2, P4, P5, P7, P8, S2, S3, S6, S7, S8, S10, S12 on the left wing, and fresh P2, P4, P5, P7, P9, S2, S3, S6, S7, S9, S11 on the right wing), suggesting an advanced age of the bird.

Table 1.

Morphological characteristics of the spotted eagle R7400 (F1 hybrid between A. clanga and A. pomarina) in 1999 (nestling, 20 July) and in 2009 (adult, 28 July), and his two offspring (backcrosses between F1 hybrid and A. pomarina) in 2006 (25 July) and 2009 (15 July). All birds were described by the author.

The offspring of R7400 (a backcross F1 × A. pomarina; Table 1, Appendix 1) was ringed and studied on 15 July 2009. The nestling had several characters of A. pomarina, such as overall brown coloration in upper-parts, large ochre nape patch (albeit larger than in most A. pomarina nestlings), dark rectrices with light bands to the tip, as well as several characters of A. clanga, such as large bill (relative to wing length), narrow bands on secondaries and very large spots on medium coverts, which were much more prominent than shown by its father in his nestling stage. Its light-coloured breast was also more typical of A. clanga, although it may occur in some A. pomarina as well (Forsman 1999). Probably, R7400 had been present already in 2006, since a ringed adult male spotted eagle with identical field characters was then breeding at the same nest with a female A. pomarina and the genotype of the nestling in 2006 suggested R7400 having been the probable father. The 2006-nestling was very similar in appearance to the chick in 2009, although the rufous areas in its plumage (underbody nape, spots on coverts) were much larger, and its bill was smaller (Table 1, Appendix 1).

Discussion

Earlier records of birds with intermediate or mixed characters (Forsman 1999, Lõhmus & Väli 2001 and references therein, Gutiérrez & Villa 2002, Dombrovski 2005, Treinys 2005) all suggested the viability of hybrid spotted eagles. Moreover, breeding of such birds (Dombrovski 2005, Treinys 2005, Ü. Väli unpubl. data) indicates hybrids to be potentially fertile. Although intermediate appearences can sometimes be explained by morphological variation, which is very large in both species (Forsman 1999), recent genetic studies have confirmed that at least some hybrids are fertile, as backcrossing and introgression of genes from one species to another have been recorded (Väli & Lõhmus 2004, Helbig et al. 2005, Väli et al. 2010b). As far as I know, this is the first case when the hybrid origin of an adult has been confirmed by a ringing record.

Surprisingly, I found several immature characters in the ten-year-old adult bird. According to Forsman (1999: 325, 340), spotted eagles are supposed to attain adult plumage in their fourth or fifth calendar-year, although subadult plumages may then still persist. However, data backing up this opinion are scarce. In A. pomarina, breeding of a four-year-old male with retained juvenile plumage characters has been recorded, whereas — on the other hand — a four-year-old female and a five-year-old female had already attained full adult plumage (Meyburg et al. 2005a). Therefore, Meyburg et al. (2005a) suspected that males attain adult plumage one year later than females, in the fifth year. The data presented here indicate that spotted eagles could retain immature characters until much later, perhaps even close to the average age of an adult spotted eagle (c. 11 years; Meyburg et al. 2005a). Meyburg et al. (1997) reported the breeding of an A. clanga in partial juvenile plumage, but the actual age of the bird was not known. Hybrid origin has been suspected to affect the development of adult plumage characters in spotted eagles (Dombrovski 2005), but it does not necessarily have to since immature characters could be retained throughout life also in other long-lived species that do not hybridise (e.g. Common Gull Larus canus (Olsen & Larsson 2003: 73, K. Rattiste & L. Saks, unpubl. data). Whatever the reason, proper ageing of eagles in the field is difficult; hence, some records of birds breeding at an unusually young age might be erroneous if not confirmed with other data, such as ringing or repeated DNA-analysis.

The hatching and breeding site of R7400 were very close to one another. This is not surprising, as eagles are known for their philopatry (Ferrer 1993, Wood 2009). However, in A. pomarina previously recorded distances between hatching and nesting sites varied between 0.13 and 550 km (Danko & Maderic 2008). At the nest level, the hatching site can be considered as intermediate but the nesting site was typical of A. pomarina — a patch of old-growth spruce forest (Appendix 2). At the landscape level, both sites closely resembled the habitat of A. clanga in Estonia, with large water bodies surrounded by wetland nearby. This is in concordance with other nest sites of mixed pairs in Estonia and supports the idea that mixed pairs have settled in former territories of A. clanga (Lõhmus & Väli 2005), and that hybridisation is an intermediate step in the extinction process of A. clanga populations (Väli et al. 2010b). Indeed, this region of east-central Estonia has been known for formerly harbouring several pairs of A. clanga while only one mixed pair (mother of R7400 still breeding with a male A. pomarina) and the backcrossing pair described in this paper have remained today. Moreover, the current breeding territory of R7400 was earlier occupied by an A. clanga pair in 1997–2001 (nesting only 200 m, 340 m and 820 m from the R7400 nest) and by an A. clanga × A. pomarina pair in 1993–1994 (nest site 1510 m away) and in 2002–2004 (340 m away).

I thank Urmas Abel, Asko Lõhmus and Ain Nurmla for their help in the field, and Ugis Bergmanis, Valery Dombrovski and Rimgaudas Treinys for fruitful discussions on field characters of spotted eagles. Rob G. Bijlsma, Valery Dombrovski, Rimgaudas Treinys and two anonymous referees gave valuable comments on a first draft of the manuscript. Fieldwork and genetic analyses were financed by the Estonian Science Foundation projects ETF6050 and ETF7593.