Reproductive consequences of mate retention and divorce in a short-lived migratory passerine

In socially monogamous birds, pair bond duration varies widely across species, from single-breeding associations to long-lasting, multi-year bonds. Studies on mate retention and divorce have predominantly focused on long-lived species, while research in short-lived and migratory species is limited. Consequently, the fitness consequences of divorce or remating in these species remain unclear. Here, we used a long term (1987–2023) and individual-based dataset to investigate the reproductive consequences of mate retention and divorce in pied flycatchers (Ficedula hypoleuca), a short-lived, long-distance migratory passerine. First, we described the overall patterns of divorce and remating within the population. Then, we analyzed whether reproductive success (laying date, clutch size and number of fledglings) differed between individuals that remate and those that divorce, while controlling for confounding factors, such as age or breeding dispersal. Specifically, we compared the reproductive consequences in the season before remating or divorce (year t) and the subsequent season (year t+1). We found that remating rates were low (3.52%) compared to divorces (26.8%), and both remained stable over the study period. In terms of reproductive performance, pair bond duration (remating vs. divorce) did not explain variation in laying date or clutch size. However, divorce was associated with an increase in the number of fledglings in the year following separation for both males and females, while remated individuals produced more fledglings in their first breeding season. The fitness consequences of divorce/remating persisted after accounting for confounding factors, supporting a direct influence of pairing status on reproductive success. These findings highlight the complex dynamics of mate relationships in short-lived migratory birds and emphasize the need for further research into the adaptive potential of pair-bond stability.