Mate-switching is not associated with offspring fitness in a socially monogamous bird

In many species, individuals form socially monogamous pair-bonds lasting multiple breeding seasons, or even whole lifetimes. Studies often suggest social monogamy to be adaptive, but this is usually quantified through the survival and annual reproductive success of the partners. However, beyond the number of offspring produced, parental partnerships may also affect their offspring's phenotype, health and ultimately fitness. Using multigenerational data on the Seychelles warbler (<i>Acrocephalus sechellensis</i>), we investigated the impact of parental pair-bond tenure (pair-bond duration) and pair-bond ending (pair-bond ended across breeding seasons) on offspring fitness components. First, we addressed juvenile-stage fitness components using indicators reflecting physiological state (haematocrit, telomere length and body condition). Second, we assessed long-term fitness components using offspring lifespan and lifetime reproductive success (LRS). We found no consistent evidence of pair-bond tenure or pair-bond ending effects on short-term (telomere length, haematocrit and body condition) or long-term (lifespan and LRS) fitness components. To our knowledge, this is the first study quantifying long-term parental effects of pair-bond tenure and pair-bond ending on offspring fitness components in wild populations. This work provides insights into the lack of intergenerational implications of long-term socially monogamous partnerships.