Disentangling site and mate fidelity in a monogamous population under strong nest site competition

Site and mate fidelity are important aspects of animal mating systems and have implications for animal population structures. In birds, they are often related to the age, breeding experience, prior residence, pair duration or breeding performance of individuals. These variables are often correlated, and site and mate fidelity may also be associated with each other. Most previous studies focused on either site fidelity or mate fidelity, and the relationship between them remains to be explored. In this study, we used path analysis to examine the correlates of site and mate fidelity of Lanyu scops owls, Otus elegans botelensis, and to investigate the relationship between site and mate fidelity. Site fidelity of female owls was related to previous breeding success, following a ‘win-stay, lose-switch’ model. Site fidelity of male owls increased with both previous breeding success and years of prior residence. Mate fidelity was related to female site fidelity and pair duration. Although mate fidelity was higher after successful breeding attempts, it was a result of the breeding success-dependent site fidelity of the owls. Many studies used the positive correlation between previous breeding success and mate fidelity to support the better option hypothesis or the incompatibility hypothesis of divorce. Our study identified the most likely causal pathway for the complex site and mate fidelity pattern in the Lanyu scops owl, and demonstrated that it is important to identify the underlying driving forces to avoid erroneous conclusions.