The responses of New Zealand's arboreal forest birds to invasive mammal control
Authors:   , Fea Iona 
Published: November 2021

<p>Introduced mammalian predators are responsible for over half of contemporary extinctions and declines of birds. Endemic bird species on islands are particularly vulnerable to invasions of mammalian predators. The native bird species that remain in New Zealand forests continue to be threatened by predation from invasive mammals, with brushtail possums (Trichosurus vulpecula) ship rats (Rattus rattus) and stoats (Mustela erminea) identified as the primary agents responsible for their ongoing decline. Extensive efforts to suppress these pests across New Zealand’s forests have created "management experiments" with potential to provide insights into the ecological forces structuring forest bird communities. To understand the effects of invasive mammals on birds, I studied responses of New Zealand bird species at different temporal and spatial scales to different intensities of control and residual densities of mammals. In my first empirical chapter (Chapter 2), I present two meta-analyses of bird responses to invasive mammal control. I collate data from biodiversity projects across New Zealand where long-term monitoring of arboreal bird species was undertaken. The projects cover a range of treatments including fenced sanctuaries, offshore islands, forests treated periodically and sites lacking significant mammal control. I found that New Zealand bird species exhibit complex responses to the varied and sustained management effort that has occurred across New Zealand’s landscape in the last fifty years. Some species show significant positive outcomes to control, notably the larger endemic species, while others, including highly endemic species, consistently decline after control. In Chapter 3, I estimate the responses of bird populations in the central New Zealand region to changes in ship rat densities. I collaborated with scientists from the Department of Conservation (DOC) and Greater Wellington Regional Council and collated biodiversity data from four restoration projects located across the central New Zealand region. I constructed multiple density impact functions (DIFs), where the effect of a change in density of a pest on a valued resource is quantified, to describe the impacts of ship rat population dynamics on native bird populations. These responses were then modelled in a meta-analysis to provide overall effects for bird populations when rat abundance increases. I identified two taxa that exhibit significant negative responses across the region: the native parakeet species (Cyanoramphus spp.) and the tomtit (Petroica macrocephala). Evidence from single projects also showed that two other species were negatively affected by increases in rats: the South Island kaka (Nestor meridionalis) and the North Island rifleman (Acanthisitta chloris). Conversely, populations of the recently introduced silvereye (Zosterops lateralis) were resilient to rat population recovery as silvereye counts significantly increased the year after an increase in ship rat populations was observed. In Chapter 4, I monitored bird species through a 1080 mammal-control operation in the southern Wairarapa. This operation coincided with a heavy beech mast, an irruptive event that occurs every 2-6 years. Most likely because of the abundance of seed, suppression of ship rats and possums appeared to be short-lived, and detections of these two mammals returned to pre-control levels within one and two years, respectively. Short-term responses of native birds to the control operation were positive: initially, for the small-medium sized bird species (i.e. the bellbird (Anthornis melanura), rifleman, tomtit, and tui (Prosthemadera novaeseelandiae) with a delayed positive response of the largest species 2.5 years after control (the New Zealand pigeon (Hemiphaga novaeseelandiae). In my final data chapter, I focus on the nesting outcomes of a common endemic species, the North Island fantail (Rhipidura fuliginosa placabilis), to different densities of ship rats. Through intensive monitoring of over 100 fantail nests, I estimated the outcomes of nesting attempts and formulated a DIF where nesting success was modelled as a function of the abundance of ship rats at the nest micro-site. Nesting attempts suffered higher failure rates at sites with higher rat abundance however, in this study I also identified a feature of nest placement that apparently limits predation from mammals. Nests placed on thinner branches were more likely to survive rat predation, a neat trick that perhaps only the smallest of birds can manage. My thesis identifies some species as particularly vulnerable to invasive mammalian predation while others are more resilient. Understanding resilience and vulnerability in New Zealand’s bird species sheds light on historical extinctions and the processes that continue to mould New Zealand's avifauna. I quantified responses of New Zealand forest bird species, to different levels of invasive mammal management and residual densities of mammals, with consideration of climate and forest productivity. These estimates could be applied by conservation managers to more effectively gauge future threats to native avifauna according to the attributes of bird species and present and future management scenarios.</p>