Revealing cryptic diversity: vocal and morphological evidence for a new species of Chamaeza (Aves: Formicariidae)

Chamaeza campanisona (Lichtenstein, 1823) is a medium-sized, ground-dwelling passerine bird widely distributed in South America, inhabiting Neotropical forests across the Amazon Basin, Atlantic Forest, and Andean regions. Across its range, 12 subspecies are recognized, predominantly distributed along altitudinal and longitudinal gradients in the Andes. Within the Atlantic Forest, two subspecies are currently recognized, although C. c. tshororo Bertoni, 1901 (described from southeastern Paraguay) has been proposed as inseparable from the nominotypic form. Given the limited taxonomic studies on this genus and the demonstrated utility of vocal analysis in suboscine taxonomy, we investigated bioacoustic variation among eastern South American taxa of this species complex. We analyzed 183 recordings representing the full geographic range of these populations, combining traditional bioacoustic methods with machine learning algorithms. Our results revealed three distinct loudsong patterns: a southern variant occurring from southeastern Paraguay and northeastern Argentina north to the state of Bahia, Brazil; an isolated population in the state of Alagoas, northeastern Brazil, represented by a single recording and lacking voucher specimens; and a northern pattern restricted to the state of Ceará, northeastern Brazil. The loudsongs typically consist of a rapid series of closely spaced notes exhibiting population-specific variation, particularly in pace and in the presence of a terminal series of slowed, descending grunting notes. The southern vocal type corresponds to C. campanisona, under which C. c. tshororo should be treated as a junior synonym. The Alagoas population requires additional study given the limited available material. The Ceará population represents a previously undescribed taxon, which we name and describe herein. This new species is morphologically diagnosable, as it lacks the black frontal spot seen in all other populations, as well as vocally distinct. It is restricted to the Baturité Mountains of Ceará, where it persists in relictual montane rainforests surrounded by the seasonally drier Caatinga xerophytic forest in the municipalities of Guaramiranga and Pacoti.