Can mixed singing facilitate coexistence of closely related nightingale species?

Knowledge of the mechanisms facilitating the coexistence of closely-related competing species is crucial for understanding biodiversity patterns. The concept of convergent agonistic character displacement (ACD) suggests that interspecific interference competition may lead to convergence in territorial signals between species, which helps to establish interspecific territoriality and thus facilitate the species coexistence. Despite a strong theoretical background, however, empirical evidence for convergent ACD in nature is still scarce. Here we tested whether mixed singing (i.e., copying of elements from songs of a closely related sympatric species) in the Thrush Nightingale (Luscinia luscinia) in a secondary contact zone with the Common Nightingale (L. megarhynchos) could represent a case of convergent ACD. Using playback experiments, we measured the intensity of physical and vocal territorial responses of Common Nightingale males to three stimuli: conspecific song, pure heterospecific ...