The effect of helpers on reproductive success in the laughing kookaburra

1. In co-operatively breeding birds and mammals that live in family groups, helpers may gain indirect fitness benefits by increasing the number of breeding attempts in a season, by increasing the success of each nesting attempt, and by increasing the survivorship of related breeders. 2. The effects of helpers were examined in an analysis of reproductive success in the laughing kookaburra (Dacelo novaeguineae), where a monogamously breeding pair is assisted by offspring of both sexes. 3. Helpers could not increase the number of attempts in a season, because kookaburras are single-brooded. 4. In a paired analysis controlling for pair and territory quality, increases in group size were not matched by increases in the number or weight of fledglings in each breeding attempt, even though brood reduction was the major source of productivity loss. This concurs with previous results showing that group size does not affect overall provisioning levels to broods, because all group members reduced their provisioning effort as group size increased. 5. Because kookaburras prefer to reduce workloads during breeding rather than raise larger broods, the costs of caring for young must be high. Thus breeders with helpers may enjoy energetic savings and enhanced survivorship. 6. Helpers had different effects on nest success depending on their sex. Whereas male helpers had a neutral effect on fledging success and fledgling weight, additional female helpers reduced fledging success. Females are relatively poor helpers, and other group members may not properly compensate for the female's 'inadequate' help. 7. High-quality kookaburra pairs, or pairs on good territories, were more likely to be in larger groups because they produced more potential recruits for their groups than relatively 'poor' performers. This was demonstrated by comparing the results of the paired comparisons with those of a correlational analysis that showed that in groups of two to six birds, group size was positively correlated with fledging success and fledgling weight (which affects the probability of juvenile survival). 8. However, unusually large groups of seven or more birds suffered drastically reduced nest success because eggs were damaged during incubation.