Asymmetric response to song dialects among bird populations: the effect of sympatric related species

Abstract

Given that birds discriminate heterospecific songs from conspecific ones, bird songs have a function in species recognition. Even in the same species, local populations often have somewhat different songs (i.e. dialects). Birds might discriminate nonlocal dialects from local dialects due to the differences in acoustic characteristics between them. Discrimination of nonlocal dialects from individuals' own dialects has the potential to promote reproductive isolation. Individuals in populations with sympatric related species that sing similar songs might discriminate not only heterospecific songs but also the nonlocal dialects of conspecifics. In populations without sympatric related species, individuals may not need to distinguish local conspecific songs from similar types of songs and may not discriminate nonlocal dialects from their own dialect. I investigated this hypothesis using Japanese tits, Parus minor, and varied tits, Poecile varius, in the Ryukyu Archipelago, Japan. I performed playback experiments to simulate territorial intrusions by forming pairs of two populations: one lived with, and the other without, the related tit species. The results from two and three paired populations of Japanese and varied tits, respectively, showed that males living with other sympatric species responded strongly to their own dialects but weakly to foreign dialects of conspecifics. Additionally, males living without a related species did not respond differently to the dialects. This asymmetric response pattern indicates that sympatric related species have important effects on response to conspecific song dialects, which may have implications for gene flow and reproductive isolation among populations with differing dialects.