Clarifying the systematics of an enigmatic avian lineage: what is a bombycillid?

Avian Tree of Life (TOL) projects using multilocus sequence data have begun to reweave Sibley and Ahlquist’s (1990) “tapestry” of the systematic relationships among birds, which was based on DNA-DNA hybridization. Many of these studies have focused on relationships among members of the most species-rich avian order, Passeriformes, or the perching birds (e.g., Barker et al., 2002, 2004; Cibois and Cracraft, 2004; Voelker and Spellman, 2004; Alstrom et al., 2006). Bringing direct sequence-based character evidence to bear on the systematic relationships among passerine birds has demonstrated that monophyly cannot be assumed for many lineages (Barker et al., 2004), from suborder to tribe, proposed by Sibley and Ahlquist (1990) and Sibley and Monroe (1990). However, many of the paraphyletic passerine lineages identified by modern sequence-based studies represent taxa identified as difficult to place or otherwise problematic by Sibley and Ahlquist (1990). One of the consistently problematic groups has been the Bombycillidae (waxwings and allies). Sibley and Ahlquist (1990, p. 630) ultimately placed the Bombycillidae within the superfamily Muscicapoidea, but they were not confident of this placement. Recent studies have only confirmed the ambiguous placement of Bombycillidae, removing the family from the “core Muscicapoidea” because of poorly supported (low Bayesian credibility and bootstrap values) relationships with the core Muscicapoidea (Barker et al., 2002, 2004; Cibois and Cracraft, 2004; Voelker and Spellman, 2004), but keeping it in the superfamily. A potentially confounding factor for placing the Bombycillidae within the passerine phylogeny is that membership in and relationships among members of the family have long been controversial; therefore, a more thorough sampling of the group is desirable. Many early taxonomies recognized the morphological similarities among waxwings (Bombycilla, 3 spp.), silky-flycatchers (Phainopepla, Phainoptila, and Ptilogonys, 4 spp.), and the palmchat (Dulus, 1 sp.) and proposed them as members of a single family Bombycillidae (Arvey, 1951; Beecher, 1953). However, studies of egg white proteins in the waxwings, silky-flycatchers, and palmchat indicated a close relationship between the former two, but not the latter (Sibley, 1970). The DNA-DNA hybridization analyses of Sibley and Ahlquist (1990) supported the close association among these three groups as indicated by morphology, leading these authors and Sibley and Monroe (1990) to maintain these species as members of the Bombycillidae, while also recognizing their differences by labeling them as distinct tribes (Bombycillini, Ptilogonatini, and Dulini; Table 1). Other modern taxonomies of the bombycillids (Voous, 1977; Cramp, 1988; AOU, 1998) have followed Wetmore (1930) in emphasizing skeletal differences among the three groups and elevating each to the family level (Bombyicillidae, Ptilogonatidae, and Dulidae). If the waxwings, silky-flycatchers, and palmchat are considered the “core” bombycillids (but relationships among the core groups are still unknown), then the remaining ambiguity lies in placement of the only other species ever to be considered among the bombycillids, the grey hypocolius (Hypocolius ampelinus). The grey hypocolius is generally considered to belong to a monotypic family, Hypocoliidae; however, at various points in its taxonomic history it has been considered closely allied with shrikes of the families Prionopidae and Laniidae (morphological characters; Mayr and Amadon, 1951), cuckoo-shrikes (jaw musculature; Campephagidae, Beecher, 1953), bulbuls (morphology; Pycnonotidae, Sibley and Ahlquist, 1990; Sibley and Monroe, 1990; Clements, 1991), and the Bombycillidae (plumage and morphological characters; Lowe, 1947; Delacour and Amadon, 1949). The most recent taxonomic treatment of waxwings, silky-flycatchers, palmchat, and grey hypocolius listed each as a separate but closely related family (Bombycillidae, Ptilogonatidae, Dulidae, and Hypocoliidae), reflecting the continued ambiguity of relationships among the groups (del Hoyo, 2005). Table 1 Specimen information.