Nesospiza finches are endemic to two small islands in the Tristan da Cunha Archipelago in the South Atlantic Ocean. The finches have radiated into two ecologically distinct forms at both islands: an abundant, small-billed dietary generalist, and a scarce, large-billed specialist. On Nightingale Island (area 2.6 km2), the endangered large-billed Wilkins' Finches N. wilkinsi use their strong bills to crack open the hard, nut-like fruits of Island Cape Myrtle Phylica arborea trees to access the seeds. Although Phylica fruits are available year-round, the finch population is constrained by the limited area of fruit bearing Phylica trees (c. 12 ha), which makes Wilkins' Finch one of the world's naturally rarest bird species. Prior to this study, rough estimates ranged from 30–85 pairs, but there has been no dedicated survey of the population size. Over the austral summers of 2012, 2016 and 2017 we systematically caught and colour ringed birds to better estimate the population size. Pairs were present wherever there are Phylica trees across the island, utilising even small, isolated Phylica copses. Mean territory size for 117 pairs in 2017 was 0.215 ± 0.094 ha (±SD, range 0.043–0.718) with a mean density of 9.8 pairs/ha in Phylica woodland. The total population was estimated to be 120 breeding pairs (c. 280–300 mature birds). The most pressing threat to the Wilkins' Finch population is the recent arrival of an invasive insect, the Brown Soft Scale Coccus hesperidum, and its associated sooty mould Seiridium phylicae, which reduces Phylica fruit loads and kills large trees. The potential use of a biological agent to control Brown Soft Scale infestations is being investigated, since without swift and effective mitigation, we predict the health of these woodlands will rapidly degrade with serious implications for the persistence of Wilkins' Finch. Until effective mitigation is taken, we recommend up-listing the species to Critically Endangered based on IUCN Red List Criterion B2.
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Vol. 109 • No. 1