Avian Responses to Novel Landscapes in Aotearoa

The alteration of natural landscapes for human use creates a mosaic of different habitats, varied in how much they have been modified from a natural baseline. These novel landscapes can have reduced habitat availability for native fauna which is the primary cause of global biodiversity decline. Faunal decline is exhibited in the well-studied taxonomic group birds and is most severe on oceanic islands. Evidence suggests that many bird species have traits linked to tolerance of novel landscapes, such as high levels of foraging versatility and larger relative brain sizes. In Aotearoa New Zealand, 83% of native, resident bird species are ‘At Risk’ of or ‘Threatened’ with decline, largely driven by predation by introduced mammals. While there has been considerable research into bird species’ responses to predation and the removal of that primary threat, little is understood about native Aotearoa New Zealand bird species’ tolerance to modified habitats, nor their responses to aspects of habitat degradation. Thus, now Aotearoa New Zealand has major landscape-level initiatives in place to address mammalian predation, research is needed on how species-specific traits of native birds interact with one another and influence birds’ abilities to live throughout novel landscapes. Additionally, research is needed on how native birds respond to specific habitat covariates beyond presence of mammalian predators, such as landcover types and human population densities. In this thesis I investigated how foraging versatility (Chapter Two) and bone morphometrics (Chapter Three) in case study bird species interact with each other and tolerance to novel landscapes, and how species’ encounter rates relate with habitat covariates, such as landcover, throughout novel landscapes (Chapter Four). I used ten native Aotearoa New Zealand forest bird species with different life histories, morphologies, and known foraging behaviours as case studies. These were: kererū (Hemiphaga novaeseelandiae), kākā (Nestor meridionalis), kākāriki (Cyanoramphus novaezelandiae), tīeke (Philesturnus rufusater), hihi (Notiomystis cincta), korimako (Anthornis melanura), tūī (Prosthemadera novaeseelandiae), pōpokatea (Mohua albicilla), pīwakawaka (Rhipidura fuliginosa), and toutouwai (Petroica longipes).

In Chapter Two, I compared habitat complexity, bird foraging behaviours, and bird foraging versatility at two predator-free sanctuaries in the Wellington Region with varying levels of regeneration: Kāpiti Island and Zealandia. The older regeneration site, Kāpiti Island, had higher habitat complexity in the form of plant diversity and canopy height than Zealandia. Three bird species foraged differently between sites. All ten case study species had more versatile foraging behaviours on Kāpiti Island, likely due to Kāpiti Island’s increased habitat complexity which may in turn increase a species’ long-term resilience to habitat disturbance. Many of the case study species that were found to have versatile foraging behaviours do not occur outside of predator-free sanctuaries. While foraging versatility may increase species’ tolerance to novel landscapes, this interaction may be offset by specific foraging behaviours, such as ground foraging, or life history traits, such as cavity nesting, that increase susceptibility to predation by introduced mammals.

In Chapter Three, I characterised relative brain size, orbit area, bill width, bill length, and lengths of femur, ulna, humerus, tibiotarsus, and tarsometatarsus in case study species using three measurement methods: callipers, laser scans, and µCT (micro computerised tomography) scans. I explored relationships among these morphometrics and established how they relate to species’ diets, foraging versatility, and tolerance to urban habitats. There was no significant difference in morphometric features taken by callipers, laser scans, and µCT scans. Relative brain size was not significantly related to the percent of a species’ diet that is high-energy foods, foraging behaviour versatility, or urban occurrence. Morphometric traits associated with high dispersal ability or predator avoidance were not significantly associated with urban occurrence. Globally, foraging versatility and dispersal ability are linked to enhanced urban tolerance in birds, however, my research suggests that in Aotearoa New Zealand the threat of mammalian predation may have obscured any such relationship for the endemic case study species.

In my final data chapter, Chapter Four, I used random forest models on presence/absence data from eBird to analyse the influence of habitat covariates on the likelihood of encountering specific bird species (a subset of six of the case study species). Covariates included landcover type, human population density, presence of mammalian predator controls, and proximity to Zealandia, a significant source population for case study species. I interpreted how the likelihood of encountering species would change if the percentage of native forest cover was increased. The model results support that increasing the area of native forest positively impacted the likelihood of encountering all species in a non-linear relationship. The greatest gains in encounter rates for most species occurred when areas of currently very low levels of native forest cover were restored (i.e., when cover is increased within the 0-20% range). This thesis focussed on a limited range of species and sites. Nonetheless, there are two primary outcomes of this thesis that are likely to have implications beyond this work. First, field site habitat complexity, foraging behaviour, foraging versatility, and skeletal morphometric data collected as part of this thesis has inherent value for future research, particularly following invasive mammal removal. Second, case study species will have the largest increase in encounters when (in conjunction with predator control) the area of native forest cover is increased to a minimum of 20% in locations with low to absent native forest cover, showing the potential value of even small-scale revegetation. Indeed, small-scale revegetation may be a more efficient use of the limited resources of land managers, making meaningful habitat restoration more accessible for community groups and private landholders.