Determinants of lifetime fitness in a cooperative breeder, the long‐tailed tit Aegithalos caudatus

1 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. 2 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. 3 LRS of individuals which bred successfully varied between 0 and 13 local recruits (mean 0·71 ± 0·11 SE); lambda varied between 0 and 2·54 (mean 0·28 ± 0·04). The measures were highly correlated, and their distributions were strongly skewed. Helping by individuals contributed little to their fitness, but one‐fifth of birds that accrued fitness did so only through helping. In general, individuals that gained fitness from helping did not gain fitness directly. 4 Both LRS and individual fitness were significant predictors of the number of grand‐offspring that an individual had, but they accounted for only about one‐third of the variation. 5 When variance in LRS was partitioned between length of breeding life span, average fecundity and offspring survival, the latter component was the most important in accounting for variance in LRS. 6 Offspring local survival was positively related to the provisioning effort of mothers, but was unrelated to that of fathers. As a result, the fitness of females was positively related to their provisioning effort. 7 Immigrant birds tended to be more reproductively successful than philopatric ones. Among females, only immigrant birds accrued any LRS or individual fitness. 8 The probability that an individual had at least one offspring recruit to the local breeding population varied among cohorts, probably as a result of variation among years in offspring local survival. This resulted in variation among cohorts in the individual fitness of females, but not in their LRS, nor in the LRS or individual fitness of males.