Partners coordinate territorial defense regardless of distance between paired intruders in a duetting bird

Year-round territorial defense is crucial for reproductive success in many tropical birds. In these species, partners often coordinate their songs in duets to defend a shared territory. In this scenario, the pair members may behave according to different motivations and interests, or through essentially cooperative strategies. However, few studies have examined whether sex-specific roles in leadership and cooperation vary with the sex and spatial arrangement of intruders. The aim of this study was to experimentally investigate how the distance between intruders influences partner coordination in territorial defense in the Rufous Hornero (Furnarius rufus). We conducted playback experiments simulating intruder duets broadcast either from adjacent speakers (0.1 m apart) or from spatially separated speakers (∼12 m apart). We recorded the number, duration, and latency of songs, as well as the latency to approach within 20 m and 5 m of the speakers and the time spent by each individual within 5 m of them. Partners showed strong coordination across all approach and vocal behaviors, jointly approaching a single speaker when the speakers were spatially separated. Pairs responded more cautiously when the speakers were adjacent; however, responses did not differ according to the sex simulated by each speaker. Notably, when females led the vocal response, pair defense was biased toward female intruders. Overall, our findings support the joint territory defense hypothesis, highlighting the importance of pair cooperation and coordination in Rufous Hornero territorial defense.