In a population of long-tailed tits Aegithalos caudatus all birds attempted to breed monogamously, but some helped to provision the nestlings of another pair if their own breeding attempt failed. An individual's investment rules will determine the fitness consequences of helping for breeders and helpers, and the aim of this study was to examine how individual investment varied with the number of carers. Provisioning behaviour was investigated in two ways. First, observations of natural variation showed that males and females reduced their provisioning effort when aided by one helper, but showed no further reduction with additional helpers. By contrast, helpers did not adjust their provisioning effort in relation to the number of helpers at a nest. Secondly, the reaction of parents to the presence of helpers was tested experimentally by the temporary removal of helpers. Both parents significantly increased their provisioning rate when helpers were absent, and the total provisioning rate was unaffected by the manipulation. The reactions of breeders and helpers to the number of carers, and the fitness benefits for breeders and helpers are discussed.