Responses of Yellowhammer Emberiza citrinella, Pine Bunting E. leucocephalos and Their Hybrids to Playbacks of Con- and Heterospecific Songs and Calls in a Hybrid Zone

Yellowhammers Emberiza citrinella and Pine Buntings E. leucocephalos hybridize in a c. 2500 km wide sympatric zone in northwest Asia. In the centre of this zone, over the last 50 years males with Yellowhammer and intermediate (i.e. hybrid) phenotypes have been replacing the Pine Bunting phenotype. In the southern part of the zone by contrast, the Pine Bunting phenotype is still common. In this study, we asked if there is a behavioural asymmetry between males depending on their phenotypes and whether this asymmetry can explain phenotypic compositions observed in different parts of the hybrid zone. This study was performed in 2017–2019 in Altai Republic, Russia, in the southern part of the hybrid zone. Songs of Yellowhammer and Pine Bunting are generally similar, although they differ in details. The species also differ in usage of call repertoires: the ‘see’ call is more characteristic for Yellowhammer, while the ‘zieh’ call is used more frequently by Pine Bunting. We performed playback experiments using contrasting stimuli to evaluate responses towards (1) Pine Bunting song vs. Yellowhammer song and (2) songs coupled with ‘zieh’ calls vs. songs coupled with ‘see’ calls. Experiments showed that Yellowhammer and hybrid males were more aggressive towards other males than were birds with the Pine Bunting phenotype. Therefore, the latter might be at a disadvantage in territory acquisition and maintenance. At the same time, males of any phenotype responded more strongly to Yellowhammer playback than to Pine Bunting playback, and to the ‘see’ call than to the ‘zieh’ call. This might counterbalance the competitive ability of different phenotypes in territorial competition, thus allowing Pine Bunting males to avoid being displaced by more aggressive Yellowhammer and hybrid males in the southern part of the hybrid zone.