In an environment that has a shortage of territories, helping to rear younger siblings (‘alloparenting’) is proposed to facilitate territory acquisition in two ways: (i) through group augmentation that leads to an increase of the territory with subsequent partial inheritance (budding); and (ii) through site dominance that leads to greater success when competing for the natal or a nearby territory after the death of the territory owner (complete territory inheritance). Most young Seychelles warbler (Acrocephalus sechellensis) males either show alloparenting or budding behaviour. Future budders had significantly more aggressive interactions with neighbours and assisted their parents more with territory defence than similarly aged future alloparents or non–helpers. This led to an increase of the natal territory of future budders before actual budding took place, whereas the natal territories of future alloparents remained constant in size. Alloparents never became budders and vice versa, refuting partial inheritance as an advantage of alloparenting. Natural male breeding vacancies were never inherited by alloparents born on vacant or other territories, but were inherited by budders born on the vacant territory or, if these were absent, predominantly by budders from neighbouring territories. We offer explicit experimental evidence against the ‘helping at the nest to inherit’ hypothesis. Experimentally created male breeding vacancies, with both a male alloparent and a similarly aged sibling budder present simultaneously in the vacant territory, were filled by budders only. Site dominance over territory inheritance is linked to budding and not to alloparenting.