Clinal pattern of morphological variation in Sakesphorus luctuosus (Lichtenstein, 1823), with comments on the enigmatic Sakesphorus hagmanni Miranda-Ribeiro, 1927 (Passeriformes: Thamnophilidae)

Sakesphorus luctuosus (Glossy Antshrike) is endemic to Brazil, harboring two subspecies. The nominal subspecies is widespread in the Amazon region, and S. l. araguayae is endemic to the Rio Araguaia valley. In this paper we investigated the hypothesis that S. l. araguayae represents the end of a cline. We examined 588 specimens housed in Brazilian, North American and European museums and based our conclusions on plumage coloration and morphometric characters. We found no significant differences in morphometric characters between subspecies, except for culmen length, which was slightly shorter in S. l. araguayae, but with wide overlap in measurements. The main proposed diagnostic character between subspecies (length of the white tips to tail feathers) showed clinal variation, with northern birds showing more white on the tail, and southern birds showing less white on the tail, irrespective of hydrographic basin. The other diagnostic character proposed (width of the white edges to the outer webs of the scapular feathers) showed high individual variation and no taxonomic consistency. We conclude that S. l. araguayae is a junior synonym of the nominal taxon. On the other hand, S. hagmanni, known only from the holotype and long thought to be a junior synonym of the nominal taxon, proved to be a unique specimen that should be considered as species inquirenda until further investigations determine its status.