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Within the diurnal raptors (hawks and allies, Order Falconiformes), familial relationships are among the most problematic of all avian orders (Feduccia 1999) and there are no less than six recent conflicting classifications based on morphology, molecular evidence, or both (Sibley and Ahlquist 1990, Griffiths 1994, Holdaway 1994, Mindell et al. 1997, Wink et al. 1998, Mayr et al. 2003). The owls (Order Strigiformes), on the other hand, are currently classified separately (e.g., del Hoyo et al. 1999) from the diurnal raptors, but their possible relationships have been debated since the 19th century (Garrod 1874, Cracraft 1981, Mayr et al. 2003). Here we will show that the Elanus kites present unique ecological and phenotypic traits, most overlooked so far, suggesting evolutionary convergence with the owls. Recent phylogenetic studies have determined, nonetheless, that the Elanus kites belong with the Accipitridae as an ancestral and basal group which may be recognized as a subfamily.

Elanus is composed of four species having comparable plumage patterns and sizes (about 300 g adult weight). Three of the species, the Black-shouldered Kite E. caeruleus from southern Asia and Africa, White-tailed Kite E. leucurus from the Americas and Australian Black-shouldered Kite E. axillaris from Australasia, are so similar in plumage characteristics and behavior that, until recently, they have been considered a cosmopolitan super-species with geographically-replacing forms (Parkes 1958, Husain 1959, Mendelsohn and Jaksic 1989). Slight differences in size, proportions, plumage and behavior led Clark and Banks (1992) to propose separate species recognition for the White-tailed Kite. The fourth species, the Letter-winged Kite E. scriptus, is endemic to Australia, breeds colonially, and hunts mainly at night (Ferguson-Lees and Christie 2001).

The Elanus kites inhabit savannah-like habitats in temperate and arid zones (Brown and Amadon 1968) and often prey on small rodents out-breaking at annual (Dunk 1995) or irregular intervals (Mendelsohn 1982, Mendelsohn and Jaksic 1989, Ferguson-Lees and Christie 2001, Jaksic and Lima 2003). They are able to disperse over long distances (up to many hundreds of kilometers) between their birth sites and first breeding sites, and even between successive breeding sites (Mendelsohn 1983, Scott 1994, Ferguson-Lees and Christie 2001). A few studies have indicated the osteological (Holdaway 1994), genetic (Wink et al. 1998), and karyotypic (Bed'Hom et al. 2003) distinctiveness of Elanus, but without challenging their inclusion within the Accipitridae.

In the southern hemisphere, Elanus appears to fill the niche of the nomadic owls of northern latitudes, which prey on multi-annually cyclic populations of small mammals (Korpimäki 1992) and are able to disperse over long distances to find local density peaks of small rodents (Korpimäki et al. 1987, Korpimäki 1993). As with some owls (e.g., Barn Owls Tyto alba; Taylor 1994), it is one of the few diurnal raptors able to raise more than one brood in a year, and can reproduce practically at any time (Mendelsohn 1984). In temperate regions, such as California or the Argentine Pampas, the White-tailed Kite has a long breeding season (mid-winter to late summer, see Dunk 1995). In Spain, the Black-shouldered Kite has the longest breeding period for any raptor; even though there is a peak of clutches in early spring, breeding attempts have been recorded at all seasons (authors unpubl. data).

Recent studies based on Cytochrome-b sequences (Roulin and Wink 2004) and Cyt-b plus RAG nuclear genes (Lerner and Mindell 2005) have provided genetic evidence that Elanus evolved from a group of raptors basal to the largest falconiform family (i.e., Accipitridae) and only distantly related to the Falconidae. The lack of a fossil record clearly attributable to Elanus may explain why the importance of this ancient group for inferring evolutionary relationships in raptors has been overlooked.

Phenotypical Affinities Between Elanus and the Owls

Elanus may resemble true kites (i.e., Milvinae) in general appearance and behavior, showing for instance, and unlike any owl, carotenoid-pigmented tarsi and ceres, and nest-building abilities (Brown and Amadon 1968). However, while handling wild kites (>50 adult and 200 nestling E. caeruleus), examining museum specimens (N  =  20 E. caeruleus and E. leucurus), and consulting the literature, we noticed that Elanus have species-specific traits, the majority unreported, that are absent in other members of the order Falconiformes but present in owls. First, the upper surface of the primary and secondary feathers has a velvety comb structure (Fig. 1), an adaptation for silent flight also exhibited by nightjars (Caprimulgiforms). Second, unlike all other Falconiformes except the fish-eating Osprey Pandion haliaetus, the Elanus kites are zygodactilous, with a reversible outer toe. Third, they have long vibrissae around the beak and disproportionately large, frontally-placed eyes (Ferguson-Lees and Christie 2001). Fourth, the pellets regurgitated by Elanus are owl-like in shape and compactness (Brown and Amadon 1968), and contain some undigested bones indicating low stomach acidity (Duncan 2003). Owls' stomach pH ranges from 2–4, whereas hawks' pH ranges from 1–2. The acidity of one E. caeruleus stomach that we measured was pH 3.4. Fifth, Elanus often ingest their small-mammal prey whole, a task facilitated by a large gape (larger prey are, however, pulled apart; J. Dunk pers. comm.). Sixth, E. scriptus, the most nocturnal species in the genus, has asymmetrical placement of the ear openings (Burton 1989), like owls of the genera Aegolius and Tyto (Norberg 1978, Taylor 1994), that may help them to locate prey by sound. In this respect, it is worth mentioning that other Accipitridae, the harriers of the genus Circus, which also tend to prey on rodents and are often crepuscular, seem to have converged with the owls in having facial disks and a very slight asymmetry in the ears (J. Lazell pers. comm.).

Figure 1

Primary feathers of Elanus caeruleus showing owl-like velvety barbules in the inner vane.


Phylogenetic analyses (Roulin and Wink 2004, Lerner and Mindell 2005) suggest that Elanus and the other diurnal raptors share a common ancestor, and that the ecological and morpho-physiological similarities to the owls likely resulted from evolutionary convergence. Although most Elanus kites are diurnal and crepuscular hunters (Mendelsohn and Jaksic 1989), at least Letter-winged Kites are nocturnal hunters (Brown and Amadon 1968, Burton 1989, Pettigrew 1991). Nonetheless, the evolution of nocturnal activity may not face significant physiological barriers (Mrosovsky 2003), and it has recently been suggested that the dark-activity phenotype characteristic of the strigiform and caprimulgiform orders is also an example of convergent evolution (Fidler 2004). All Elanus kites are specialized to feed on small rodents, the populations of which, particularly in arid zones, may show irregular density outbreaks, which differ from the multi-annually cyclic small mammals of the northern hemisphere (Lima et al. 2002, Korpimäki et al. 2004). The ability to disperse over long distances and to produce many broods annually likely helps Elanus kites to take full benefit from local rodent outbreaks. These traits, unusual for the diurnal raptors in the family Accipitridae, may have driven the independent evolution of the characteristics that give Elanus the appearance of a hybrid between a hawk and an owl.


This study was supported by Dirección General de Medio Ambiente de la Consejería de Agricultura y Medio Ambiente (Junta de Extremadura) through an Interreg Project. Drs. J. Dunk, J. Lazell and an anonymous reviewer provided helpful comments and references that improved the manuscript.

Literature Cited


B. Bed'Hom, P. Coullin, Z. Guillier-Gencik, S. Moulin, A. Bernheim, and V. Bolobouev . 2003. Characterization of the atypical karyotype of the Black-winged Kite Elanus caeruleus (Falconiformes: Accipitridae) by means of classical and molecular cytogenetic techniques. Chromosome Res 11:335–343. Google Scholar


L. Brown and D. Amadon . 1968. Eagles, hawks and falcons of the world. Country Life Books. London, U.K. Google Scholar


P. Burton 1989. Birds of prey. Gallery Books. New York, NY U.S.A. Google Scholar


W. S. Clark and R. C. Banks . 1992. The taxonomic status of the White-tailed Kite. Wilson Bull 104:571–579. Google Scholar


J. Cracraft 1981. Toward a phylogenetic classification of the recent birds of the world (Class Aves). Auk 98:681–714. Google Scholar


J. del Hoyo, A. Elliott, and J. Sargatal . 1994. Handbook of the birds of the world. Vol. II: New world vultures to guineafowls. Lynx Editions. Barcelona, Spain. Google Scholar


J. del Hoyo, A. Elliot, and J. Sargatal . 1999. Handbook of the birds of the world. Vol. V: Barn Owls to Hummingbirds. Lynx Editions. Barcelona, Spain. Google Scholar


J. Duncan 2003. Owls of the world. Key Porter Books. Toronto, Canada. Google Scholar


J. R. Dunk 1995. White-tailed Kite (Elanus leucurus). In A. Poole and F. Gill , editors. eds. The birds of North America, No. 178. and The American Ornithologists' Union. Philadelphia, PA The Academy of Natural Sciences. Washington DC U.S.A. Google Scholar


A. Feduccia 1999. The origin and evolution of birds. 2nd Ed. Yale University Press. New Haven, CT U.S.A. Google Scholar


J. Ferguson-Lees and D. A. Christie . 2001. Raptors of the world. Christopher Helm. London, U.K. Google Scholar


A. E. Fidler 2004. Convergent evolution of strigiform and caprimulgiform dark-activity is supported by phylogenetic analysis using the arylalkylamine N-acetyltransferase (Aanat) gene. Mol. Phylogenet. Evol 33:908–921. Google Scholar


A. H. Garrod 1874. On certain muscles of the thigh of birds and on their value in classification. Part II. Proc. Zool. Soc. Lond 1874:111–123. Google Scholar


C. S. Griffiths 1994. Monophyly of the Falconiformes based on syringeal morphology. Auk 111:787–805. Google Scholar


R. N. Holdaway 1994. An exploratory phylogenetic analysis of the genera of the Accipitridae, with notes on the biogeography of the family. 601–637. in B-U. Meyburg and R. D. Chancellor , editors. eds. Raptor Conservation Today. World Working Group on Birds of Prey, The Pica Press. Berlin, Germany. Google Scholar


K. Z. Husain 1959. Notes on the taxonomy and zoogeography of the genus Elanus. Condor 61:153–154. Google Scholar


F. M. Jaksic and M. Lima . 2003. Myths and facts of ratadas: bamboo blooms, rainfall peaks and rodent outbreaks in South America. Austral Ecol 28:237–251. Google Scholar


E. Korpimäki 1992. Population dynamics of Fennoscandian owls in relation to wintering conditions and between-year fluctuations of food. 1–10. in C. A. Galbraith, I. R. Taylor, and S. Percival , editors. eds. The ecology and conservation of European owls. UK Nature Conservation, No. 5. Joint Nature Conservation Committee. Peterborough, U.K. Google Scholar


E. Korpimäki 1993. Does nest-hole quality, poor breeding success or food depletion drive the breeding dispersal of Tengmalm's Owls? J. Anim. Ecol 62:606–613. Google Scholar


E. Korpimäki, P. R. Brown, J. Jacob, and R. P. Pech . 2004. The puzzles of population cycles and outbreaks of small mammals solved? BioScience 54:1071–1079. Google Scholar


E. Korpimäki, M. Lagerströ, and P. Saurola . 1987. Field evidence for nomadism in Tengmalm's Owl Aegolius funereus. Ornis. Scand 18:1–4. Google Scholar


H. L. Lerner and D. P. Mindell . 2005. Phylogeny of eagles, Old World vultures and other Accipitridae based on nuclear and mitochondrial DNA. Mol. Phylogenet. Evol 37:327–346. Google Scholar


M. Lima, N. C. Stenseth, and F. M. Jaksic . 2002. Population dynamics of a South American rodent: seasonal structure interacting with climate, density dependence and predator effects. Proc. R. Soc. Lond. B 269:2579–2586. Google Scholar


G. Mayr, A. Manegold, and U. S. Johansson . 2003. Monophyletic groups within “higher land birds” – comparison of morphological and molecular data. J. Zool. Syst. Evol. Res 41:233–242. Google Scholar


J. M. Mendelsohn 1982. The feeding ecology of the Black-shouldered Kite Elanus caeruleus (Aves: Accipitridae). Durban Mus. Novit 13:75–116. Google Scholar


J. M. Mendelsohn 1983. Social behaviour and dispersion of the Black-shouldered Kite. Ostrich 54:1–18. Google Scholar


J. M. Mendelsohn 1984. The timing of breeding in Black-shouldered Kites in southern Africa. 799–808. in J. Ledger , editor. ed. Proceedings of the fifth Pan-African Ornithological Congress. Southern African Ornithological Society. Johannesburg, South Africa. Google Scholar


J. M. Mendelsohn and F. M. Jaksic . 1989. Hunting behaviour of Black-shouldered Kites in the Americas, Europe, Africa, and Australia. Ostrich 60:1–12. Google Scholar


D. P. Mindell, M. D. Sorenson, C. J. Huddleston, H. C. Miranda, A. Knight, S. J. Sawchuk, and T. Yuri . 1997. Phylogenetic relationships among and within select avian orders based on mitochondrial DNA. 213–247. in D. P. Mindell , editor. ed. Avian Molecular Evolution and Systematics. Academic Press. Ann Arbor, MI U.S.A. Google Scholar


N. Mrosovsky 2003. Beyond the suprachiasmatic nucleus. Chronobiol. Int 20:1–8. Google Scholar


RÅ Norberg 1978. Skull asymmetry, ear structure and function, and auditory localization in Tengmalm's owl, Aegolius funereus (Linne). Philos. Trans. R. Soc. Lond. B. Biol. Sci 282:325–410. Google Scholar


K. Parkes 1958. Specific relationships in the genus Elanus. Condor 60:139–140. Google Scholar


J. D. Pettigrew 1991. Nocturnal adaptation in the visual system of the Letter-winged Kite, Elanus scriptus. Proc. Aust. Neurosci. Soc 2:25. Google Scholar


A. Roulin and M. Wink . 2004. Predator-prey polymorphism: relationships and the evolution of colour: a comparative analysis in diurnal raptors. Biol. J. Linn Soc 81:565–578. Google Scholar


T. A. Scott 1994. Irruptive dispersal of Black-shouldered Kites to a coastal island. Condor 96:197–200. Google Scholar


C. G. Sibley and J. Ahlquist . 1990. Phylogeny and classification of birds of the world: a study in molecular evolution. Yale University Press. New Haven, CT U.S.A. and London, U.K. Google Scholar


I. R. Taylor 1994. Barn Owls. Predator-prey relationships and conservation. Cambridge University Press. Cambridge, U.K. Google Scholar


M. Wink, I. Seibold, F. Lotfikhah, and W. Bednarek . 1998. Molecular systematics of Holarctic raptors (Order Falconiformes). 29–48. in R. D. Chancellor, B-U. Meyburg, and J. J. Ferrero , editors. eds. Holarctic birds of prey. ADENEX-World Working Group on Birds of Prey. Mérida, Spain. Google Scholar
Juan J. Negro, Cino Pertoldi, Ettore Randi, Juan J. Ferrero, José M. López-Caballero, Domingo Rivera, and Erkki Korpimäki "CONVERGENT EVOLUTION OF ELANUS KITES AND THE OWLS," Journal of Raptor Research 40(3), 222-225, (1 September 2006).[222:CEOEKA]2.0.CO;2
Received: 16 November 2005; Accepted: 4 June 2006; Published: 1 September 2006

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