Breeding biology and population dynamics of the Norfolk Island green parrot (Cyanoramphus cookii)

Abstract

Considering their land mass, islands support a disproportionately large number of endemic species. However, the restricted ranges and long-term isolation of island endemics make them particularly vulnerable to change, exacerbating the impacts of anthropogenic threats such as habitat loss and invasive species. Furthermore, small founding groups and persistently small population sizes can limit the genetic diversity within these populations, restricting their ability to adapt to change. The Norfolk Island green parrot Cyanoramphus cookii (hereafter 'green parrot') is a prime example of an island endemic, threatened by habitat loss, a lack of secure breeding sites and predation by invasive mammals. The population has drastically contracted since the 1700s, falling below 100 birds more than once in the last 50 years. Predator-proofing of nests and broadscale predator control have been used to support breeding birds for over 30 years, yet the species remains listed as Endangered under the EPBC Act. This thesis aims to examine the breeding biology and population dynamics of the green parrot to inform future conservation management. Chapter one serves as a general introduction for the thesis, discussing the challenges faced by island-dwelling species, introducing the green parrot and presenting the thesis rationale. Chapter two provides important context by presenting a systematic review and meta-analysis of global efforts to protect wild bird nests from predation using non-lethal methods. The results show that non-lethal nest protection research is biased towards ground nesting birds in North America and that physical interventions preventing predator access are significantly more effective than behavioural interventions. Chapter three examines the use of predator-proofed nests by green parrots and their primary nest competitor, the crimson rosella Platycercus elegans. Only 54% of nests were used by green parrots, their nest use was associated with thicker nest walls, and they did not use artificial nests. Crimson rosellas used 42% of nests and did not display any nest preferences. These findings show that management efforts may support non-target species, highlighting the need to consider species-specific preferences when designing management actions. Chapter four uses stochastic population models to simulate population growth from 2013 to 2021. The models indicate that a large amount of nesting in natural, unprotected nests is required to explain field observations from 2021. These findings suggest that unprotected nests have played an important role in recent population recovery, possibly due to improved broadscale predator control. Chapter five compares the success, productivity and characteristics of predator-proofed nests with unprotected nests. It finds that predator-proofing significantly increases nest success, but that these nests have significantly deeper nest chambers than their natural counterparts. It also presents green parrot breeding parameters and observations regarding the seasonality of breeding and a decline in nest use in recent years. These findings show the value of predator-proofing natural green parrot nests, but also indicate areas for improvement and emphasise the need for complementary management measures. Chapter 6 examines the species' population genetics finding that nearly 20% of birds were highly inbred, that the effective population size was just 43.8, and that there was no evidence of genetic structure. These findings further stress the value of complementary management actions that can support the breeding population as a whole, and ensure genetic variation is preserved. Chapter seven provides a general conclusion for the thesis, discussing implications for the future management of the species and future research directions. While much remains to be discovered about the species, the knowledge gained from this thesis will contribute greatly to the conservation of the Norfolk Island green parrot. Show more