Symmetrical discrimination despite weak song differentiation in 2 suboscine bird sister species

Song mediates territorial competition and mate choice in birds and population divergence in this signal can have important evolutionary consequences. For example, divergent songs can act in specific recognition and limit gene flow and, hence, have a fundamental role on the origin and/or integrity of evolutionary lineages. Especially interesting systems to test the role of song in specific recognition are species pairs that present small structural differences in this signal. Here, we perform song play-back experiments on males of a long-diverged sister pair of Neotropical Suboscine species, the squamate antbird (Myrmoderus squamosus) and the white-bibbed antbird (Myrmoderus loricatus), which occur in parapatry in the Atlantic Forest and that overlap extensively in song variation. Previous evidence indicates that genetic introgression between these species is either absent or negligible, suggesting that vocal discrimination or other mechanisms function as effective barriers to gene flow. Our results show that responses to heterospecific songs were symmetrical and intermediary compared with responses to conspecific songs in both species. A stronger response to conspecific territorial songs suggests that conspecific individuals pose greater competitive threat than heterospecifics. An important implication of our study is that even small song differences can play an important role in specific recognition. Experiments with 2 related bird species with slightly divergent songs show that males are able to recognize individuals of their own species. This study shows that the evolution of even minor song differences between species can be effective in specific recognition.