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OSTRICH 2013, 84(1): xxx–xxx Printed in South Africa — All rights reserved
OSTRICH ISSN 0030–6525 EISSN 1727-947X http://dx.doi.org/10.2989/00306525.2013.772922
Hybridisation between the Common Buzzard Buteo buteo buteo and the North African race of Long-legged Buzzard Buteo rufinus cirtensis in the Strait of Gibraltar: prelude or preclude to colonisation? Javier Elorriaga1 and Antonio-Román Muñoz1,2* 1
Fundación Migres, Ctra. N-340, Km 96.7, Huerta Grande, Pelayo, Algeciras, Cádiz E-11390, Spain Animal Biology, University of Malaga, Campus de Teatinos s/n, Malaga 29071, Spain * Corresponding author, e-mail: [email protected]
Natural hybridisation in Old World buzzards (Buteo) is an uncommon phenomenon with important ecological implications. This genus constitutes an intricate radiation of genetically poorly differentiated raptors whose taxonomic classification is a frequent subject of debate. We report the first case of successful hybridisation between the African subspecies of the Long-legged Buzzard Buteo rufinus cirtensis and Common Buzzard Buteo buteo buteo in a new contact zone in the Strait of Gibraltar (southern Spain). The hybrid offspring showed mixed characters from parental species indicating that, among others, hybridisation could explain the increasing presence of phenotypically odd reddish buzzards in southern Spain and northern Morocco. Given their close phylogenetic relation and their recently reduced allopatry, an increase in the hybridisation rate, fertile descendants and genetic introgression seem to be viable. We identify the potential contact zones where genetic monitoring is needed to gain insight on the real extent of this hybridisation and its possible effects on the current climate change scenario. Keywords: Buteo rufinus cirtensis, Buteo buteo buteo, buzzard, hybridisation, Strait of Gibraltar
Introduction Hybridisation is of ecological and conservation importance, as it might play a role as a source of variation, speciation, despeciation and evolution (Arnold 1997, Newton 2003, Price 2008, Hermansen et al. 2011), might present a threat to the population dynamics of many species and subspecies (i.e. swamping of one species by another; Rhymer and Simberloff 1996, Sangster 2000, Allendorf et al. 2001, Kelly et al. 2010, Väli et al. 2010) and might complicate taxonomic classification of certain (sub)populations affecting conservation programs (Sangster 2000). The occurrence of hybrids is common in some avian orders (e.g. Anseriformes, Galliformes and Psittaciformes), whereas it is rare or absent in others (e.g. in the order Apodiformes) (McCarthy 2006). Instances of hybridisation in birds of prey are uncommon except in captivity (Newton 1979). However, Buteo hawks seem to be prone to natural intrageneric hybridisation both in the New World (Clark et al. 2005, Clark and Witt 2006, Hull et al. 2007) and the Old World (McCarthy 2006). Among these latter, which number at least nine species (Ferguson-Lees and Christie 2005), several cases have been reported, e.g. between Common Buzzard Buteo buteo and Rough-legged Buzzard B. lagopus in Norway (Gjershaug et al. 2006) and Finland (Forsman and Lämsä 2007). Regarding Long-legged Buzzard B. rufinus, hybridisation has been documented concerning its nominate subspecies and Common Buzzard in Hungary (Dudás et al. 1999), with Upland Buzzard B. hemilasius in central Asia (Pfänder and Schmigalew 2001) and with Steppe Buzzard B. b. vulpinus in India
(McCarthy 2006). Hybridisation of the less well-known North African subspecies B. rufinus cirtensis has recently been reported for the first time. Its mixed breeding with Common Buzzard was found in 2008 and 2009 in Pantelleria Island (Italy), 70 km north of Tunisia (Corso 2009). In recent years, several studies have been conducted in an attempt to clarify the genus Buteo taxonomy in Afro-Eurasia, some of them dealing with difficult-to-classify populations and/or individuals showing mixed characters of different species (Clark and Davies 2000, Clouet and Wink 2000, Screiber et al. 2001, Londei 2003, Riesing et al. 2003, Kruckenhauser et al. 2004, Clark 2007, Griffiths et al. 2007, Lerner et al. 2008, Amaral et al. 2009, Clark 2009, Porter and Kirwan 2010). Indeed, the problems posed by Old World Buteo are amongst the thorniest in avian taxonomy (Porter and Kirwan 2010), as it constitutes a relatively young and intricate taxonomic radiation of genetically poorly differentiated raptors (Kruckenhauser et al. 2004, Griffiths et al. 2007). In spite of an insufficient understanding of the phenotypic variation of certain subspecies and the poor genetic differentiation, genetic introgression by means of hybridisation could be an explanation for some of these cases (e.g. Kruckenhauser et al. 2004). However, difficulty in identifying hybrids in the field may result in underestimation of the extent of hybridisation in the wild (McCarthy 2006). In northern Morocco and southern Spain, particularly around the Strait of Gibraltar (Figure 1), there are an increasing number of controversial observations involving
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Elorriaga and Muñoz
difficult-to-classify reddish buzzards attributable to the buteo–vulpinus–cirtensis complex (i.e. Corso 2006, JE and A-RM unpublished data), some of whose official identification and acceptance has been postponed, awaiting further information (Dies et al. 2007). At this point, it is important to highlight that the North African Long-legged Buzzard B. r. cirtensis is one of the least-studied raptor taxa occurring in the Western Palaearctic, both in terms of ecology and phylogeny. In this paper we report, for the first time, the successful mixed breeding of an African Long-legged Buzzard and a Common Buzzard in continental Europe, in a recently established contact zone, as a result of the northwards shift in the African species’ range (see Elorriaga and Muñoz 2010). Methods Study area The Strait of Gibraltar (35º45’ to 36º10′ N, 5º10′ to 6º00′ W, hereafter the Strait; Figure 1), where the Mediterranean Sea and the Atlantic Ocean met, separates the African and European continents by as little as 14 km at the shortest distance. It is an important biogeographic barrier that separates North African and European fauna and flora, delimitating the northern and southern breeding ranges of many African and European taxa, respectively (Busack 1986, Castella et al. 2000, Gantenbein and Largiadèr 2003, Rodríguez-Sánchez et al. 2008). At the same time, it is a natural corridor for millions of migratory and dispersive birds travelling between both continents (Bernis 1980, Tellería 1981, Newton 2008). During recent decades the Strait has played an important role as a focal point for the colonisation of Europe by African avifauna, being on its European shore
the place of first sighting for those African species that have successfully expanded their breeding range northward, e.g. Black-shouldered Kite Elanus caeruleus (Ferrero and Onrubia 2003), White-rumped Swift Apus caffer (Barrios 2003), Little Swift Apus affinis (Molina 2003) and Rüppell’s Vulture Gyps rueppellii (Ramírez et al. 2011). The species The Long-legged Buzzard is a medium-sized raptor, for which two allopatric subspecies are recognised (Cramp and Simmons 1980). The nominate rufinus is a partial migrant distributed from Balkans and Minor Asia to Mongolia and north-western India. The smaller cirtensis inhabits North Africa, from Mauritania and Morocco to Egypt (del Hoyo et al. 1994), and is regarded as sedentary and dispersive, with some individuals reaching Senegal, the Iberian Peninsula and possibly the south of Israel (Bergier 1987, Forsman 1999, de Juana 2006). Both subspecies of Long-legged Buzzard are listed as rarities in Spain (Dies et al. 2008), where the increasing number of records shows a marked southern bias towards the Strait of Gibraltar, pointing to the prevailing presence of African origin individuals (i.e. B. r. cirtensis). In northern Morocco it is locally common (Thévenot et al. 2003). The species was not confirmed breeding in peninsular Spain until 2009, when a pair of putative pure cirtensis successfully reared chicks on the European side of the Straits (Irby 1895, Muñoz 2003, Elorriaga and Muñoz 2010, Dies et al. 2011). After that first event, further breeding signs (as territorial adults in suitable nesting habitat) have been found over a broader area and repeated overwintering has been confirmed as well (JE and A-RM unpublished data). In turn, the Common Buzzard is a widespread species throughout Spain and Europe
Po rtu ga l
Spain MEDITERRANEAN SEA
Strait of Gibraltar
AFRICA 0 10° W
Figure 1: Common Buzzard Buteo buteo buteo and Long-legged Buzzard Buteo rufinus cirtensis distributions, contact zones (adapted from Ferguson-Lees and Christie 2005) and confirmed hybridization sites (arrows)
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(Hagemeijer et al. 1997) and an uncommon winter visitor in north-western Africa (Thévenot et al. 2003). Monitoring the breeding buzzards The breeding territory where the previously mentioned pair of Long-legged Buzzards successfully bred in 2009 was monitored intensively during 2010. In March a single territorial adult female Long-legged Buzzard was frequently observed. Phenotypically, it showed all the typical characteristics attributable to B. r. cirtensis, including: rather short and broad winged silhouette, rufous tail becoming whitish towards the base lacking a dark subterminal band, buff-tinged pale breast with dark flanks and trousers, pale rufous underwing coverts, pale flash on the upper hand, and a rather pale head with a dark line through the eye (see Forsman 1999). On the basis of its physical characteristics and behaviour, it was concluded to be the same individual occupying that territory the previous year. In May, an adult male Common Buzzard settled in the breeding territory. It was a homogeneous dark-brown individual, with densely barred dark tail and a contrasting broad pale breast-band (Figure 2). Results Both birds performed courtship behaviour and copulated repeatedly. In direct comparison, the female was notably bigger than the male, showing a much more conspicuous size dimorphism than that observed on pure pairs of these species. Both birds actively attended the nest, placed on a Black Alder Alnus glutinosa within a densely forested stream valley. A brood yielding three nestlings was successfully reared. The chicks were metal ringed (Madrid-Icona Bird Ringing Scheme) and blood sampled at the age of 20–25 days old. No other Long-legged Buzzards were recorded in the breeding territory throughout the breeding season. The fledglings stayed near the nest site until mid August, when they abandoned the area. The three of them were dark-brown phased, one being slightly paler in the breast, all showing a variable extent of orange-buff fringes in mantle
and upperwing coverts. Tails were densely barred wherein lighter coloured bars presented the same orange-buff tinge as in the coverts. Their general appearance showed mixed phenotypic characters from both parental species and could objectively resemble Steppe Buzzard Buteo buteo vulpinus, a taxon with no accepted records in Spain at present. The blood samples collected were not genetically analysed because, to achieve conclusive genetic results, a larger sample size is needed. Discussion
This hybridisation event, together with the first confirmed breeding of a pure pair of African Long-legged Buzzards in the same site in 2009, and the more regular appearance of long-staying territorial Long-legged Buzzards in mainland Spain in recent years (Elorriaga and Muñoz 2010, JE and A-RM unpublished data), confirms the Strait as a new contact zone where further hybridisation events might happen, as already proposed by McCarthy (2006). This case is in accordance with the Hubbs principle (Hubbs 1955), which states that hybridisation seems to be more common in those areas where two closely related species meet and one of them is rare, mate choice therefore being restricted (Randler 2002, Newton 2003). Likewise, the recent first reproduction of pure B. r. cirtensis pairs in Italy (Pantelleria Island; Figure 1) was followed by its repeated hybridisation with the Common Buzzard (Corso 2009, A Corso pers. comm.). This provides further evidence about the settlement, still in very low numbers, of pioneering cirtensis in southern Europe and, at the same time, evidences the eventual hybridisation of this taxa in different parts of its distribution range. This is in concordance with the results from Kruckenhauser et al. (2004), who found in a comprehensive phylogenetic investigation of the Old World Buzzards that the haplotypes of three cirtensis collected in Algeria and Tunisia were randomly distributed, two within buteo and one within rufinus–oreophilus Western Palaearctic clusters. These results could be explained by means of hybridisation (L Kruckenhauser and A Gamauf pers. comm.). These
Figure 2: Adult male Buteo buteo buteo perched in the breeding territory (left; June 2010) and adult female Buteo rufinus cirtensis flying over the nest site (July 2009). Both individuals formed a successful mixed breeding pair in 2010.
sampled birds were collected in the early nineteenth and twentieth centuries showing that hybridisation in cirtensis might not be a recently emerged phenomenon and that it may occur unnoticed in other less-well-monitored regions including the African range of the Long-legged Buzzard. Moreover, recent studies predict a northern shift in the potential distributions of many bird populations within this century, as an adaptive response to the current humaninduced global warming process (Huntley et al. 2007, Araújo et al. 2011). These results should be interpreted cautiously but, if this is the case for the African Long-legged Buzzard, a greater presence in southern Europe, the reduction of its allopatry with the Common Buzzard, and their increased potential hybridisation are to be expected. Given their close phylogenetic relationship, fertile hybrid descendants, successive backcrosses and genetic introgression between B. b. buteo and B. r. cirtensis appears to be feasible. However, McCarthy (2006), in an extensive review of avian hybrids, stated that when hybrids occur only at low frequency, parental populations can overlap broadly without significant genetic consequence. Otherwise, if the offspring is infertile, hybridisation is a waste of reproductive effort for the rarer species (Väli et al. 2010) and hybridisation would be acting as an ecological barrier against the northward expansion of cirtensis into southern Europe. The real extent of this phenomenon, and its broadscale phylogenetic consequences, remains unknown and deserves further research. The most palpable immediate effect will probably be the increase in the number of difficult to classify reddish buzzards, which could explain the presence of some of the aforementioned phenotypically odd individuals in southern Spain and northern Morocco. That said, it does provide an excellent opportunity to gain insights into intrageneric hybridisation in the complicated genus Buteo. We highlight the interest to implement a genetic monitoring of the buzzards, by means of microsatellite nuclear markers, on both shores of the Straits, as well as in other potential contact zones of northern Africa (i.e. Algeria and Tunisia). If climate warming continues, further arrivals of potential colonist African bird species to the European shore of the Straits are to be expected. Having closely related species at both sides of the strait, it could be of great importance to determine how hybridisation could act on the increasingly global change-mediated northward expansion of African species. Acknowledgements — We would like to thank ornithologists from Fundación Migres who participated in the field work. KL Bildstein (Hawk Mountain Sanctuary), A Gamauf (Naturhistoriches Museum Wien) and D Forsman provided valuable comments that improved an earlier version of the manuscript. P Zufiaur, S Daly/Andalucian Guides and A Justo shared their observations. Junta de Andalucía provided permits for capturing and ringing raptors, and Ministerio de Medio Ambiente (Spain) facilitated ringing activity.
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Received February 2012, accepted January 2013 Editor: M Virani