Investigating the ecology of Norfolk Island’s endemic passerines to achieve applied conservation outcomes

Norfolk Island is one of Australia’s few remote territories, located in the western Pacific between New Caledonia and New Zealand. This unique and beautiful island, renowned for its avifauna, boasts high levels of biodiversity and endemism. Some charismatic local birds such as the Norfolk Island Morepork Owl and the Green Parrot have received considerable research attention that has led to successful conservation initiatives. Other species however, have been less fortunate. European settlement saw the extinction of seven of Norfolk Island’s 15 endemic land birds, the last as recently as 2006. Land clearing, along with introduced predators, have been implicated in these extinctions. Of the remaining endemic songbirds – a robin, gerygone, white-eye, whistler, and fantail – several are restricted to remnant blocks of forest around the island. Despite this, little is known about their population trends, and the impact of current threatening processes. Such knowledge gaps mean that effective conservation management is difficult, even though it is widely accepted that these species face imminent threats. In collaboration with Norfolk Island National Park, we aimed to rapidly quantify the current state of Norfolk Island’s endemic songbirds, providing conservation managers with the scientific knowledge necessary to make evidence-based decisions. To do this we explored causes of nest failure, sought to understand habitat requirements, and calculated population sizes to explore the potential impact of conservation translocations.

Over three breeding seasons (2018 to 2020), we followed the progress of 135 songbird nests. Invasive rats (Rattus sp.) were the most common nest predator by a large margin (80% of failed nests) substantially impacting the island’s songbird populations. We also documented two other nest predators. The first was the visiting Long-tailed Cuckoo (Eudynamys taitensis), which ‘island-hops’ along various Pacific islands on its migratory winter route. We also observed an attack from a competing robin pair, in which the competing pair killed two robin nestlings. It is possible that, in a bid for resources, the competing pair aimed to drive the established robin pair from their territory. While these are natural processes that are unlikely to drive population decline, the same is not true for the failures caused by invasive rats. We found that more rodent bait stations per hectare (i.e. rodent control effort) was correlated with higher nest survival, demonstrating that rodent control is an important management action for the protection of these endemic songbirds.

Additionally, we found that some songbird species were more at risk than others. The robin, whistler and white-eye all exhibited lower nest survival than the gerygone and fantail, and this was compounded by reduced island-wide distribution. Using habitat modelling, we determined that the more vulnerable species, particularly the robin, were heavily reliant on forested and un-cultivated areas. Therefore, the National Park remains a key protected area for these species, as habitat fragmentation elsewhere likely reduces their ability to establish and maintain sustainable populations in other areas across the island.

Increasingly, land managers are conducting conservation translocations for threatened species to ensure their survival. Predator-free islands provide the opportunity to establish additional populations, and we sought to determine whether this would be feasible for our focal species using population modelling. We conducted intensive surveys in the National Park in 2019, and subsequently estimated that the Park supported around 1500 robins, 7000 white-eyes, 3000 fantails, 3500 gerygones, and 1600 whistlers. We considered multiple scenarios in which individuals were removed (harvested) from the Park to be translocated to the nearby Phillip Island. Despite considerable variation, we determined that all five focal taxa could sustain harvesting at rates required for potential conservation translocations. As such, this could be a future complement to the ongoing management of these charming songbirds.