In cooperatively breeding species, helpers can alleviate reproductive constraints by assuming the role of primary carers to first-born young, liberating breeders to invest in subsequent broods. However, evidence on how first-born young are transferred to helpers is currently lacking. We propose that breeder-offspring aggression might facilitate inter-brood division and test this idea using data from a wild population of cooperatively breeding pied babblers (Turdoides bicolor). After second-brood young hatch, breeders become increasingly aggressive to first-brood fledglings and attack them when they beg for food. After an attack, fledglings reduce begging. Helpers are much less aggressive to begging fledglings and fledglings subsequently tend to target helpers, rather than breeders, when begging for food. In this way, first-born dependent young are transferred to helpers, resulting in a partitioning of tasks among breeders and helpers. Task partitioning in eusocial insects is thought to be determined by the morphological or physiological characteristics of individuals. This complementary study suggests that flexible behavioural strategies may also result in specialized roles in cooperatively breeding vertebrates.
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