A high level of extra-pair paternity in the Chestnut Thrush (Turdus rubrocanus)

Extra-pair copulation (EPC) can potentially maximize individual reproductive fitness, and this process may involve sexual selection of male and female traits that reflect individual quality. Previous studies have implied that adult characteristics are associated with the probability of extra-pair paternity (EPP), but it differs between species. Moreover, there are relatively few examples of the adaptive rationale for females’ engagement in EPCs based on an examination of these more traditionally recorded traits, in combination with female flight-mediated traits such as wing length. We investigated whether EPP existed in the wild Chestnut Thrush (Turdus rubrocanus) population during three breeding seasons (2019–2021), and whether paternity was related to morphological traits of males and females. Eight highly variable microsatellite loci were used to identify paternity, and generalized linear mixed models were used to analyze the relationship between paternity and morphological traits. We found that EPP existed in the Chestnut Thrush. 53.3% (N ​= ​41/77) of the broods contained at least one extra-pair offspring (EPO), and 34.6% (N ​= ​72/208) of the nestlings were EPO. We also found that male wing length was negatively associated with the probability of EPP and the proportion of EPO. Female body length was positively related to the probability of EPP. Both female body condition and mass were negatively associated with the proportion of EPO. While other traits of male and female did not relate to the probability of EPP or the proportion of EPO. Extra-pair males had better body condition compared to the males they cuckolded. EPO did not differ from their half-siblings in terms of body size or body condition. The results suggest that body size and body condition were associated with EPP in the Chestnut Thrush. This study provides fundamental information for further studies on the evolution and maintenance of EPP in the Chestnut Thrush, and it is also useful for the comparison of EPP among Turdus species.