Phenotypic variation and weak behavioral isolation in a putative hybrid zone between Andean Atlapetes brushfinches (Passerellidae)

Individuals in the contact zone between hybridizing species often exhibit intermediate phenotypes that vary smoothly in a clinal fashion from one species to the other. However, many tropical hybrid zones remain poorly documented due to limited specimens, small species ranges, and logistical challenges of accessing key localities within contact zones. We investigated phenotypic variation and species recognition between two endemic Andean brushfinches that replace each other geographically along the Central Cordillera of the Colombian Andes: Atlapetes fuscoolivaceus (Dusky-headed Brushfinch) and A. flaviceps (Yellow-headed Brushfinch). We scored phenotypic traits from 111 georeferenced photographs of live individuals and 40 museum specimens. Using cline-fit analyses for two hybrid indexes (face color pattern and overall body color), we tested whether phenotypic transitions were abrupt (suggesting a barrier, or a sharp replacement) or gradual (indicating hybridization) across their contact zone. We also conducted playback experiments to assess song discrimination as an indication of potential prezygotic reproductive barrier. Plumage traits varied clinally, with a center near Inza, Cauca (2.55{degrees}N), and with notably different cline widths between facial (12 km) and body-color traits (73 km). This difference likely reflects greater sampling density for facial traits and possibly stronger selection on face coloration. Territorial A. flaviceps individuals responded to songs of both species (no difference in latency); however, response duration was longer when exposed to local songs, indicating weak behavioral isolation. Our results suggest that these two ecologically equivalent brushfinches maintain a hybrid zone where their ranges meet, with a smooth transition from a pure phenotypic form to the other, which is likely facilitated by weak prezygotic behavioral barriers to reproduction. This study represents the initial step in investigating phenotypic variation, speciation genomics, behavior and interaction at the hybrid zone between Atlapetes brushfinches in the Colombian Andes.