Do the ages of parents or helpers affect offspring fitness in a cooperatively breeding bird?

Age-related changes in either the phenotypes or genotypes of care-givers can impact juvenile performance. However, rarely in wild populations have germline and non-germline transgenerational effects of ageing been separately quantified. In cooperatively breeding animals, in addition to parental ages, the age of ‘helpers’ attending the nest may also impact juvenile performance. Using a wild population of superb fairy-wrens (Malurus cyaneus), we investigated the effects of maternal, paternal, and helper ages on three measures of offspring performance: weight as a nestling, juvenile survival, and recruitment to the breeding population, using up to 4538 offspring over 30 cohorts. A mother’s age at conception negatively affected her offspring’s performance, but mothers with a longer total lifespan had offspring with higher juvenile survival. We distinguished the effects of paternal germline versus paternal environment (for offspring sired extra-pair) as well as the combined effect of paternal germline and environment (for offspring sired within-pair). Neither the ages of the genetic or the cuckolded social father impacted extra-pair offspring performance, however, for offspring sired within-pair, there was a positive effect of paternal age on the juvenile survival. Offspring performance increased most strongly and consistently with the average age of helpers. Our analyses thus revealed multiple associations between offspring fitness components and the ages of the adults around them. These associations appear to be primarily driven by age-related environmental effects, rather than age-related changes in germline.