Mixing the waters: a linear hybrid zone between two riverine Neotropical cardinals (<i>Paroaria baeri</i> and <i>P. gularis</i>)

Amazonian rivers have been more frequently conceptualised as barriers rather than as habitats for birds with their own ecological and biogeographic histories. However, many river-restricted bird species have differentiated within the formidable network formed by the Amazon and its tributaries. Here we demonstrate that the riverine-distributed Crimson-fronted Cardinal (<i>Paroaria baeri</i>) is narrowly distributed along the middle Rio Araguaia basin, where it comes into contact and hybridises with the geographically widespread Red-capped Cardinal (<i>P. gularis</i>). This one-dimensional hybrid zone, which is situated over <i>ca</i>.160 km along the Araguaia and Javaés Rivers, appears to be of recent origin. Admixed individuals between the non-sister <i>P. baeri</i> and <i>P. gularis</i> are phenotypically intermediate between the parental species, and superficially resemble the geographically disjunct and phylogenetically distant Masked Cardinal (<i>P. nigrogenis</i>). Two phenotypically admixed specimens were confirmed as such based on sequences of the mitochondrial Cytb and the Z-linked MUSK gene. Field observations and genetic data indicate that <i>P. baeri</i> × <i>P. gularis</i> hybrids are capable of producing viable offspring, but more data are necessary to confirm hybrid viability and fertility. The non-sister hybridising species <i>P. baeri</i> and <i>P. gularis</i> last shared a common ancestor 1.8–2.8 mya (uncorrected genetic p-distance of 4%), which corresponds closely to when the Araguaia/Tocantins river basin last discharged directly into the Amazon.