Glucocorticoid correlation between barn owl (Tyto alba) breeding partners and its association with reproductive success

In biparental species, reproductive success is influenced by the quality of the parents, the care each provides, environmental factors, and the cooperation between parents in sharing reproductive tasks. Hormones like corticosterone, which modulate physiological and behavioural functions associated with reproductive success, likely play a critical role in reproductive success through hormonal compatibility between breeding partners. Here, we investigate how similarity or dissimilarity in corticosterone levels between barn owl (Tyto alba) breeding partners are related to reproductive success. Using data from 2004 to 2018, we analyzed baseline and stress-induced corticosterone to explore correlations between partners' corticosterone levels and their association with key fitness parameters including clutch size, offspring number, and rearing success. We found that while partners' corticosterone levels do not predict clutch size, they are a significant predictor of offspring number and rearing success. Pairs with dissimilar baseline and stress-induced corticosterone produced more fledglings than pairs with similar corticosterone levels. To evaluate the potential advantage or disadvantage of growing up in large broods, we further examined the effect of brood size on offspring quality and survival. Nestlings from smaller broods had better body condition than those from larger broods, and individuals with better condition were more likely to survive their first year. These findings suggest that barn owl reproductive success is influenced by the combined corticosterone profiles of both parents and indicate a potential trade-off between offspring quantity and quality. This study highlights the importance of considering both parents' hormonal profiles when evaluating corticosterone's role in reproduction among biparental species.