Systematics of Zoothera thrushes, and a synthesis of true thrush molecular systematic relationships.

The true thrushes (Turdinae; Sibley and Monroe, 1990) are a speciose lineage of songbirds, with a near-cosmopolitan distribution. Following the systematic placement of true thrushes as a close relative of Old World flycatchers and chats (Muscicapinae) by the DNA–DNA hybridization work of Sibley and Ahlquist (1990), a number of molecular systematic studies have focused on various aspects of true thrush relationships. These studies have included phylogenetic assessments of genera membership in true thrushes, assessments of relationships among and within true thrush genera, and the recognition of ‘‘new” species (Bowie et al., 2003, 2005; Outlaw et al., 2003; Voelker and Spellman, 2004; Klicka et al., 2005; Miller et al., 2007; Voelker et al., 2007). Despite improved hypotheses of true thrush relationships, several problems remain. One problem involves several genera that are either monotypic (Hylocichla, Ridgwayia, Chlamydocaera), that contain just a few species (Cochoa, Sialia, Neocossyphus), or that appear to be distantly related to other genera (Sialia, Myadestes, Neocossyphus). While each of these genera have been phylogenetically placed within well supported true thrush subclades (Klicka et al., 2005), their sister relationships are not well supported within those clades. This issue might be attributable to a lack of dense taxon sampling across true thrush species, in previous analyses. Another problem involves the use of improper or ‘‘too distant” outgroups, which can affect resolution within single genera. For example, in a study of species relationships within the genus Catharus, Outlaw et al. (2003) used as outgroups two species each from the genera Turdus and Zoothera. It has subsequently been shown that neither was an appropriate genus to root Catharus, as both are distantly related (Klicka et al., 2005). More recently, Miller et al. (2007) used a single member of the genus Sialia to root their analyses of the genus Myadestes, despite both morphological and molecular analyses which suggested that Neocossyphus is probably a close relative, if not the sister taxon to Myadestes (Ames, 1975; Olson, 1989; Pasquet et al., 1999; Klicka et al., 2005).