Disentangling the Contributions to Phenotypic Change in Conservation Breeding: A Case Study in a Breeding Program of an Endangered Migratory Songbird

ABSTRACT Conservation breeding programs play a crucial role in managing endangered wildlife, yet successful reintroduction remains challenging due to the low fitness of released individuals and complexities in conservation breeding. This study addresses these ongoing challenges by investigating morphological changes in the endangered Eastern loggerhead shrike ( Lanius ludovicianus migrans ) breeding program across multiple facilities in southern Canada and the United States, which was started in 1997. Using mixed‐effect animal models and estimated breeding values, we explore potential signals of evolutionary change in the program and assess variables contributing to morphological variation and change. Our findings reveal dynamic variation in shrike morphology across years and generations, with some evidence of evolutionary change in shrike mass. However, mass changes appear to be unrelated to selection in captivity. Additionally, recent gene flow from the wild correlates with mass and tarsus width, suggesting gene flow from the wild might drive some of the morphological change. This study provides insights into the evolutionary processes in shaping morphological traits in a conservation breeding program. The identified genetic drivers offer considerations for refining breeding practices and insights into evolutionary change in a conservation breeding program, presenting significant implications for wildlife managers and conservation practitioners. Importantly, observed temporal heterogeneities and trends are likely influenced not only by genetic changes but also by plasticity, and plasticity could mask simultaneously occurring genetic change. More research is needed to characterize and determine how and if plastic responses are directionally shaping phenotypic distributions in captivity.