Communication across territory boundaries: distance-dependent responses in nightingales

In communication, vocal signals are often used for long-range signaling. Yet, little experimental evidence is available on the role of territorial signals across territory boundaries and their effectiveness at different propagation distances. In many songbird species, song overlapping and rapid broadband trills are used and perceived as agonistic signals, yet they differ in their propagation distance. Trills degrade quickly over distance, suggesting that their agonistic function may decrease faster over distance than that of song overlapping. Here, we tested whether different signaling distances of a rival affect singing responses of a territorial male and whether such distance effects differ when a rival uses rapid broadband trills or song overlapping. We exposed male nightingales (Luscinia megarhynchos) to songs of simulated rivals broadcast from 2 distances outside their territories. Each subject was exposed either to a moderate alternating playback without trills or to an agonistic playback, that is, to an alternating playback with trills or to an overlapping playback without trills. Irrespective of the treatment, males sang more songs containing trills in response to near than to far playback. As expected, males responded more strongly to the 2 agonistic treatments than to the moderate treatment. However, males did not clearly decrease responsiveness to playback containing trills broadcast from afar. This indicates that trills maintain their agonistic function even at distances at which information encoded in frequency bandwidth is degraded. Taken together, our results show that information encoded in signals used for resource maintenance is important also in communication across territory boundaries.