Effects of previous intrusion pressure on territorial responses in Nightingales

In territorial animals, establishing and defending a territory against rivals is commonly a prerequisite for successful reproduction. Yet, often, non-territorial males that are seeking to establish their own territory may intrude into occupied territories and persistently challenge residents in order to test their resource-holding potential. Such challenges may have long-term consequences for the territorial behaviour of the residents. Here, we tested whether territorial Nightingales (Luscinia megarhynchos) use information on the intrusion pressure of rivals in territory defence. Using multi-loudspeaker playback experiments, we simulated rivals that either sang from different song perches within occupied territories (persisting intruders) or that immediately left territories after previous intrusions (retreating intruders). In a final playback 1 h later, we then simulated the same rivals singing from a different location at the territory boundary. During this final playback, residents approached the loudspeaker closer, changed their song perches more often, sang more songs in close proximity to the loudspeaker, and sang more trills when rivals were previously simulated as having stayed in their territory without retreating. Our findings show that songbirds can integrate the level of threat from rivals differing in their intrusion behaviours in subsequent territorial encounters. This study emphasises the importance of considering how territorial defence is affected by previous intrusion pressure from rivals.