Fenced sanctuaries deliver conservation benefits for most common and threatened native island birds in New Zealand.
Island species are disproportionately threatened with extinction, and invasive species are the primary driver of biodiversity loss. Globally, eradicating invasive mammals from small oceanic islands has led to the recovery of threatened populations, but eradicating mammals from large islands and continents is more challenging. In New Zealand, conservation organizations have established a large network of fenced sanctuaries that use predator-proof fencing to exclude invasive mammals and conserve native flora and fauna. Yet, critics question if sanctuaries meet these targets, given a lack of evidence on outcomes. We surveyed birds in three sanctuaries and three paired sites on New Zealand's North Island to investigate whether sanctuaries increase bird population densities relative to sites with minimal mammal control. Densities of nine endemic bird species were higher in sanctuaries compared to unprotected sites (0.27-9.00 more birds/ha), but we found no significant difference in mean population densities for introduced and biogeographically recent native species. These findings provide compelling evidence that fenced sanctuaries effectively conserve native island bird populations, and affirm predictions that native species are more likely to benefit from invasive mammal eradications than introduced species. New Zealand's novel approach to recovering rare species holds great promise for conserving biota at risk from invasion in other global hotspots of endemism.