Determinants of lifetime fitness in a cooperative breeder, the long-tailed tit aegithalos caudatus

Long-tailed tits (aegithalos caudatus) are a cooperatively breeding species in which helpers often invest effort in the provisioning of young that are not their own. We quantified the lifetime reproductive success (LRS) and the individual fitness, lambda, of 228 long-tailed tits using 8 years of field data. Calculation of lambda took account of the effect of helpers on reproductive success, and thus lambda estimates the inclusive fitness of individuals. We examined the relationships between the fitness estimators and the provisioning effort, cohort, body size and dispersal status of individuals. LRS of individuals which bred successfully varied between 0 and 13 local recruits (mean 0 71