Common Myna Acridotheres tristis is considered to be among the world’s most damaging invasive species through disturbance, predation, competition pathogen introduction to native birds and other taxa. Claimed impacts on native birds have often been based on anecdotal reports. More substantive evidence of interference with small-island endemic birds has been reported, but impacts have rarely been quantified or subjected to experimental manipulation. On Denis Island (Seychelles), up to 10% of Seychelles Warblers Acrocephalus sechellensis, and small numbers of Seychelles Fodies Foudia sechellesis and Seychelles Paradise Flycatchers Terpsiphone corvina had head injuries following myna attacks, stimulating an eradication of the mynas. Populations of four species of Seychelles’ endemic birds, introduced to the island to establish insurance populations, were estimated before, during and after the completion of the eradication, permitting assessment of the impact of myna removal on populations of the endemics. Numbers of all four endemics increased following introduction, but increases in the numbers of Seychelles Magpie Robins Copsychus sechellensis and Seychelles Paradise Flycatchers accelerated after >90% of the mynas had been removed. All endemic populations continued to increase during and after completion of the eradication in 2015, and injuries to Seychelles Warblers, Seychelles Fodies, and Seychelles Paradise Flycatchers ceased. Habitat management within a designated conservation zone on the island, into which the endemics were released and subsequently spread to occupy most of the island, also contributed to the endemic birds’ global populations and to their improved conservation status. This study confirms that mynas negatively impact small island populations of endemic birds and suggests that their potential impact has been underestimated. Myna eradication should be considered vital before endangered endemic birds and other taxa susceptible to their negative impacts are translocated to small islands for conservation reasons.

Endemic island species are of conservation interest as unique taxa, often with restricted populations, but many are data poor. The Mangaia kingfisher Todiramphus ruficollaris, known locally as the tanga‘eo, is endemic to the island of Mangaia in the Cook Islands, and categorized as Vulnerable on the IUCN Red List. The population size has not been estimated since 1996, despite concerns over habitat loss and competition with an invasive species, the common myna Acridotheres tristis. We provide new population estimates for both the tanga‘eo and the common myna, using the same methodology as previous estimates. During December 2018–February 2019 we surveyed with distance sampling along 73 line transects walked across the six habitat types on Mangaia. We estimate there are 4,106 tanga‘eo on Mangaia (95% CI 3,191–5,283), a dramatic 7–8 fold increase compared to the previous estimate of 393–764. We estimate there are 13,350 common myna (95% CI 10,998–16,206), a slight increase, although densities in the two most favoured habitats for myna have declined. There is no evidence that the common myna poses a threat to the viability of the tanga‘eo population, as the latter has increased despite a much larger population of common myna. Presumed declines in the tanga‘eo population in the past were probably a result of habitat loss as a result of the cultivation of pineapples Ananas comosus for export, an industry that collapsed in the 1980s. We recommend a review of the IUCN Red List status of the tanga‘eo.