In the Old World Sparrows family, Passeridae, the ubiquitous House Sparrow Passer domesticus serves as an excellent subject in a study of hybridization. This species is one of the most numerous birds on the planet, and it has been recorded cross-breeding with several other sparrow species in its native home range, but also all over the world in the areas where it has been introduced, or which it has invaded. Reportedly, it hybridizes ‘quite frequently’ with the Eurasian Tree Sparrow Passer montanus (Summers-Smith 2009b in del Hoyo et al., p. 804), both in the native and introduced ranges of the two species. It also hybridizes ‘extensively’ with the Spanish Sparrow Passer hispaniolensis (Summers-Smith 2009b in del Hoyo et al., p. 794), with the Italian Sparrow Passer italiae (ibid.), with the Somali Sparrow Passer castanopterus in Somalia (Ash and Colston, 1981; Summers-Smith 2009b in del Hoyo et al., p. 793) and with the Sudan Golden Sparrow Passer lutetus (Avibase).
Interesting but mostly unconfirmed reports suggest that the House Sparrow’s list of sex partners may include several species outside of the sparrow family. Somewhat dubious reports claim that it may hybridize with finches and canaries (Fringillidae) such as the Chaffinch Fringilla coelebs, Domestic Canary Serinus canaria (var. domesticus) and European Greenfinch Chloris chloris. Furthermore, hybrids with the Estrildidae, like the Java Sparrow Lonchura oryzivora, are suspected; ditto for the buntings and New World sparrows (Emberizidae) like Chipping Sparrow Spizella passerina. Such unusual pairings are highly unlikely in nature, and probably happen only in artificial situations in aviculture (Avibase; McCarthy 2006).
Hybrids of the common Cape Sparrow are documented: with the House Sparrow (Summers-Smith 2009a in del Hoyo et al., p. 800), and in captivity with the Grey-headed Sparrow Passer griseus and the Sudan Golden Sparrow (McCarthy 2006, p. 269-270). A literature search and scouring of the Internet revealed only one documented record (Fraser et al. 1992) and a few photos of an apparent hybrid (LINK HERE). However, in the non-scientific literature I found the statement that ‘the two species freely and naturally hybridize in many areas of South Africa and Namibia’ (Carnaby 2008, p. 193, no further references given).
Is this just the tip of the iceberg?
Some of the sparrow species are very similiar by systematical and phenomenological criteria and hybrids with their variation of mixed features might be undetected, overlooked or regarded as subspecies. See the example of the Sind Sparrow Passer pyrrhonotus (in Summers-Smith 2009f:795, and in the EOL database), for example. Truth be told, we simply do not know how many sparrow hybrids live unperceived. To the best of my knowledge this is the only documented occurrence of a Passer melanurus x Passer motitensis hybrid.
I thank Dane Paijmans for the fruitful discussion and for editing the text, and Les Underhill for his support.