Increased vigilance to neighbor duets broadcast from unexpected territorial boundaries in rufous horneros

Many territorial animals can distinguish neighbors from strangers when these categories represent different threat levels. This ability may stem from class-based recognition, where individuals are categorized as neighbors based on habituation or familiarity with shared sensory cues, or from true individual recognition, where specific neighbors are recognized by memorizing their unique sensory signatures. We tested whether rufous horneros (Furnarius rufus), a suboscine bird with year-round territorial pairs that produce distinctive vocal signatures, could associate acoustic and spatial information from neighbors and perceive them as less threatening than strangers, potentially indicating acoustic recognition of neighbors. We simulated territorial interactions by broadcasting neighbor and stranger duets at the neighbor boundary, inhabited by the neighbor, and at an unoccupied boundary, which lacked a neighbor or territory. Birds were more likely to fly toward the speaker in response to stranger (unfamiliar) duets than to neighbor duets at both boundaries. However, owners became more vigilant when hearing a neighbor's duet from the unoccupied boundary than from the neighbor boundary, while their responses to stranger duets were similar at both boundaries. Although birds detected mismatches between spatial and acoustic cues, their physical (flight toward the speaker) and vocal (likelihood of duetting) aggressive responses were similar toward neighbors regardless of territorial boundary, indicating no immediate retaliation for minor boundary violations. These results suggest that rufous horneros may acoustically recognize specific neighbors and provide insights into the evolution of collective signals in mediating social interactions among non-vocal learners.