Territorial responses of male blue tits to simulated dynamic intrusions: effects of song overlap and intruder location

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Territorial songbirds frequently challenge each other with intrusions and close-range singing interactions. The timing of songs in such interactions may signal aggressive intentions and experiments have shown that song overlapping may indicate a higher willingness to escalate an interaction than song alternating. A territory owner should therefore perceive an intruder overlapping its songs and staying inside its territory as a greater threat than one alternating and exiting the territory quickly. To test this hypothesis we used playback to interact with territorial male blue tits, Parus caeruleus, by varying the amount of overlap of a subject's song; we also used multiple loudspeakers to simulate an intruder changing song posts. The territory owners approached and followed simulated intruders that changed song post inside their territories more slowly when more of their songs had been overlapped at the first loudspeaker, and they did not follow an intruder outside their territory. Nevertheless their responses remained high throughout playback and interaction terms between proportion of songs overlapped and loudspeaker position inside or outside the territory indicated that male blue tits combined and used information from both singing and intrusion patterns.