Species turnover in forest bird communities on Fiordland islands following predator eradications

Recent advances in the control of mammalian predators have begun to reveal interspecific competition as a key driver in the structure of New Zealand forest bird communities once predation pressure is reduced. We present evidence that, when at high densities, South Island robins (Petroica australis) may be responsible for declines in a suite of smaller native and introduced songbird species. Bird surveys undertaken on 47 islands in Breaksea Sound and Dusky Sound, Fiordland, during 1974 to 1986, were repeated on the same islands in 2016 or 2019. During the first block of surveys, Norway rats (Rattus norvegicus) were present on two islands, and stoats (Mustela erminea) were known or presumed to regularly reach 43 of the remaining islands. The rats were eradicated in 1986 and 1988, and stoats have been controlled to zero density since 2001 on all 28 of the islands surveyed in Dusky Sound, and since 2008 on 12 of 19 islands surveyed in Breaksea Sound. Bird species that apparently benefited from pest mammal eradications included South Island robin (Petroica australis) and kākā (Nestor meridionalis), both of which are endemic. Species recorded less often after the eradications included tomtit (Petroica macrocephala), grey warbler (Gerygone igata), silvereye (Zosterops lateralis), dunnock (Prunella modularis), and chaffinch (Fringilla coelebs) – a mixture of endemic, native, and introduced species. We hypothesise that these five species have been outcompeted or displaced by the now widespread and abundant South Island robin, probably through aggressive interactions.