Helpers increase long-term but not short-term productivity in cooperatively breeding long-tailed tits

Abstract

Helpers at the nest in the cooperative breeding system of long-tailed tits Aegithalos caudatus exhibit kin preference in their helping behavior. The aim of this study was to use multivariate analyses to investigate whether helpers accrue indirect fitness benefits through their cooperation by increasing the productivity of relatives. All birds started each season breeding independently in pairs, but birds that failed in their own breeding attempt often redirected their care to help another pair provision their offspring. About half of all broods had one or more helpers, 86% of which were male. Provisioning rates increased and there was a corresponding increase in the mass of nestlings within broods as the number of helpers increased. Helpers had no significant short-term effect on productivity because nest predation, nestling survival, and brood size were unaffected by the presence of helpers. However, in the long term helpers had a highly significant effect on the recruitment of fledglings, the positive effect of helpers being linear within the range of helper numbers that we observed. We found no evidence to suggest that these results were confounded by the effects of individual or habitat quality. We conclude that long-tailed tits accrue indirect fitness benefits by helping kin. Nevertheless, the inclusive fitness benefit from helping is substantially lower than that of independent breeding, showing that helpers are making the best of a bad job.