Diverse Interactions, Including Hybridisation, Between Brown and Inland Thornbills in South Australia
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18 South Australian Ornithologist 41 (1) Diverse interactions, including hybridisation, between Brown and Inland Thornbills in South Australia ANDREW BLACK, PHILIPPA HORTON AND LEO JOSEPH Abstract relative) the Mountain Thornbill, A. katherina, of the Wet Tropics rainforests; the Inland Thornbill, A. apicalis, widespread across southern and Brown and Inland Thornbills have three zones of central Australian semi-arid and arid zones and contact in South Australia. Two of these involve the more mesic south-west of Western Australia; the Mount Lofty Ranges Brown Thornbill, which and the Tasmanian Thornbill, A. ewingi, endemic hybridises extensively with the Inland Thornbill in to Tasmania and its larger offshore islands. coastal shrublands and mangroves of eastern Gulf St Vincent, and which forms an apparently narrow Though the Brown and Inland Thornbills are not hybrid zone near Meningie with Inland Thornbills of sister species, the nature of interactions between the Upper South-East of South Australia. The third them where they approach each other in several zone involves Brown Thornbills of the South-East parts of their ranges has long been a contentious of South Australia and the Inland Thornbill, again area of research (e.g. Mayr and Serventy 1938, its Upper South-East population. This is evidently a Boles 1983, Schodde and Mason 1999). This paper broad zone of overlap without interbreeding. Closer further addresses the issue. study of the Gulf St Vincent hybrid zone revealed a variety of hybrid phenotypes but no parental forms. Across their full geographic ranges three major Most of the hybrid phenotypes resemble Inland zones of interaction between Brown and Inland Thornbills while some resemble Brown Thornbills of Thornbills have been recognised: 1) the Upper the Mount Lofty Ranges or of the South-East of South South-East of South Australia (SA) and north- Australia. Further study could address why Brown western Victoria; 2) the eastern coast of Gulf St and Inland Thornbills show such a diverse range of Vincent north of Adelaide, SA; and 3) the western interactions in South Australia. slopes of the Great Dividing Range in New South Wales (NSW). Hybridisation between the two INTRODUCTION species has been reported across all three zones in reviews of geographical variation in either or The Australo-Papuan thornbills (Acanthiza both species (Boles 1983; Schodde and Mason spp.) comprise 12 Australian species (Schodde 1999, Matthew 2002). and Mason 1999) and two now recognised from New Guinea (Nyari and Joseph 2012). In Boles (1983) concluded that populations in the Australia they occur almost continent-wide South-East of SA and north-western Victoria except across much of the monsoon tropical were intermediate between nominotypical A. north. Phylogenetic studies (Nicholls et al. pusilla pusilla of south-eastern Australia and a 2000; Nicholls 2001; Gardner et al. 2010) have subspecies of the Inland Thornbill, A. apicalis established that one clade of thornbills comprises albiventris, primarily of interior NSW. Schodde four species: the Brown Thornbill, A. pusilla, and Mason (1999) suggested an alternative view primarily of wetter south-eastern Australian involving a second subspecies of the Inland woodlands and forests; its sister species (closest Thornbill, A. a. apicalis, which superficially December 2015 19 resembles some populations of Brown including plantations (Black 2015; see map Thornbills. They argued that in the north of the therein, Figure 1). Upper South-East of SA these two subspecies of the Inland Thornbill intergrade whereas towards The third major zone of interaction is on the the Lower South-East there is a zone of overlap western slopes of the Great Dividing Range in between Brown and Inland Thornbills without northern, central and southern NSW. Norman hybridisation. (1987; not accessed - see Schodde and Mason 1999 and Matthew 2002), using allozymes as well as Possingham and Possingham (2000) reported phenotypic data, found evidence of hybridisation field observations from the Upper South-East across three east-west transects in this region. with some birds identified as Brown Thornbills Phenotypically intermediate specimens were and others as Inland Thornbills, again implying found within only a short segment of each that the two species are sympatric there; none transect. Genetic introgression was limited but were thought by them to be of hybrid descent. occurred over distances of between 30 km in the They noted Brown Thornbills in denser habitats, north and 200 km in the south. Norman (1987) particularly those that included Melaleuca shrubs found that the integrity of both species was in the understorey, Inland Thornbills being in maintained by habitat choice: eucalypt forest by more open mallee communities. the Brown Thornbill, and Callitris and mallee scrub by the Inland Thornbill. Black (2015) showed that this major zone involves two separate contact zones, one Matthew (2002) examined most Australian between Inland Thornbills and South-East material of the two species, including that Brown Thornbills, A. pusilla pusilla, and another available to Boles (1983), Norman (1987) and between the former and Mount Lofty Ranges Schodde and Mason (1999). He could not confirm Brown Thornbills, A. pusilla samueli. Norman’s finding of intermediate forms on her three transects but identified three putative The second major zone of interaction between hybrid specimens, one from each of the major the two species was defined by Schodde and zones of interaction just discussed here. Mason (1999) as ‘the coastal shrubberies of east coast Gulf St Vincent north of Adelaide’, Brown Clearly, then, there are alternative views Thornbills being on the lower Adelaide Plains on how Brown and Inland Thornbills interact north to the Gawler River and Inland Thornbills where their ranges come into contact. around the SA gulfs. They concluded that there Accordingly, the aim of this paper is to review the two species are locally sympatric and that specimen evidence to clarify the nature of these there are no intermediate forms. Higgins and interactions but the scope is restricted to two Peter (2002) agreed although Matthew (2002) zones of contact in South Australia (Meningie recognised a single specimen of intermediate area and east Gulf St Vincent). Specifically, we character. Paton, Carpenter and Sinclair (1994) ask whether these closely related and similar but recorded Brown Thornbills in the Mount Lofty not sister species (Nicholls et al. 2000, Nicholls Ranges but not on the Adelaide Plains, and 2001, Gardner et al. 2010) occur sympatrically Inland Thornbills in coastal habitats north with little or no hybridisation, or whether there of Adelaide. On the other hand, Black (2015) is intergradation between them, either from reported field observations from this area that secondary contact and hybridisation, or as a questioned the presumption that only Inland primary cline. Thornbills occupy the mangroves of eastern Gulf St Vincent, Brown Thornbills also being reported Short (1969) outlined two kinds of zones of from them as well as in adjacent habitats, secondary contact between differentiated 20 South Australian Ornithologist 41 (1) populations: the zone of overlap and We examined a total of 232 specimens of both hybridisation, and the hybrid zone, with a species in the South Australian Museum, strategy for distinguishing them. He defined the Adelaide (SAMA), including those in the S. former as containing both parental phenotypes A. White Collection held there but not seen by as well as a variety of intermediate forms and, Black (2015), and Australian National Wildlife because it suggested a degree of reproductive Collection, CSIRO, Canberra (ANWC). This isolation, he argued that it characterises included all SAMA material of both species interactions between distinct species. The latter from throughout most of their ranges (A. pusilla: contains intermediate individuals showing Queensland (n = 3), NSW (5), Victoria (11), progressive variation across the zone and implies SE (35), MLR (31); A. apicalis: south-western a degree of reproductive compatibility between Australia (18), EP and Gawler Ranges (38), YP the interacting populations. Distinguishing the (13), MN (7), Flinders Ranges (5), inland SA (10) two is rendered problematic when differences and MM/USE (32)) and the following from the between parental forms are subtle and hybrids two contact zones of focal interest: Meningie area are difficult to recognise as a consequence. Endler (n = 9; eastern margin of Lake Albert) and east (1977) on the other hand used the term hybrid GSV (n = 15; Outer Harbor and St Kilda to Port zone for the former and cline for the latter and Gawler and the Light River). The broader SAMA stressed that clines might be steep or gentle, and ANWC material included specimens of both stepped or smooth, narrow or broad, no matter species from immediately adjacent populations whether primary or secondary intergradation in the MLR, USE, MN and YP. was involved. Hewitt (1989) referred to clines between parapatric subspecies as hybrid We assessed the variation of colour and tone zones. There are thus many variations in the of skin specimens, as shown by Schodde terminology and definitions applied to the and Mason (1999) and/or Higgins and Peter subject of hybridisation and hybrid zones. Short (2002) to vary among subspecies of Brown (1969) regarded very gradual intergradation and Inland Thornbills. Colour perception and across a broad zone as a primary cline