Evaluation of counting methods for monitoring populations of a cryptic alpine passerine, the rock wren (Passeriformes, Acanthisittidae, Xenicus gilviventris)

Abstract

Developing and validating methods to determine trends in populations of threatened species is essential for evaluating the effectiveness of conservation interventions. For cryptic species inhabiting remote environments, this can be particularly challenging. Rock wrens, Xenicus gilviventris, are small passerines endemic to the alpine zone of southern New Zealand. They are highly vulnerable to predation by introduced mammalian predators. Establishing a robust, cost-effective monitoring tool to evaluate population trends in rock wrens is a priority for conservation of both the species and, more broadly, as part of a suite of indicators for evaluating effectiveness of management in New Zealand’s alpine ecosystems. We assessed the relative accuracy and precision of three population estimation techniques (mark-resight, distance sampling and simple counts on line transects) for two populations of rock wrens in the Southern Alps over six breeding seasons (2012–2018). The performance of these population estimators was compared to known rock wren population size derived from simultaneous territory mapping. Indices of abundance derived from counts on transects were correlated with territory mapping at both study areas, and performed better than either mark-resight methods or distance sampling. Simple counts on standardised line transects are a highly cost-effective method of monitoring birds because they do not require banding a population. As such, we recommend that line transect counts using the design outlined in this paper be adopted as a standard method for long-term monitoring of rock wren populations. Although species-specific testing is required to validate use of low-cost population indices, our results may have utility for the monitoring of other cryptic passerines in relatively open habitats.

Pooling robustness in distance sampling: Avoiding bias when there is unmodelled heterogeneity

“…A third case is where subpopulations exist and individuals can be assigned to subpopulation or stratum. For example, subpopulations may correspond to different habitat types or geographic strata (Lauriano et al, 2014), or different time periods (Monks et al, 2021).…”

“…A third case is where subpopulations exist and individuals can be assigned to subpopulation or stratum. For example, subpopulations may correspond to different habitat types or geographic strata (Lauriano et al, 2014), or different time periods (Monks et al, 2021). In multi‐species surveys, if sample sizes are insufficient to model each species independently, each subpopulation might correspond to a species (Anderson et al, 2015).…”

“…In our experience, this issue is largely overlooked, and it is not uncommon for a single detection function model to be fitted across strata when stratum‐specific estimates are required. Examples in the literature include Allison and McLuckie (2018), Buuveibaatar et al (2017), Edossa et al (2022), Jefferson et al (2021), Monks et al (2021), and Strindberg et al (2020).…”

Flying towards the Future: Using Drones to Detect Lizards in Remote Alpine Terrain

“…Conservation efforts and ecological research in remote or inaccessible terrain are currently hampered by a limited suite of effective survey and monitoring techniques [1,2]. Many of these ecosystems are highly sensitive to disturbance; thus, monitoring these sites can often be incredibly difficult [3].…”

“…Challenges surrounding accessibility, ease of survey, and disturbance to fauna while attempting on-foot surveys contribute to this paucity of knowledge [3]. Consequently, conservation outcomes for native fauna found in these habitats are often limited due to our restricted knowledge of their basic ecology [2,4]. The development of new and effective methods for studying rare, cryptic, or poorly understood species is a high priority to facilitate improved ecological outcomes in habitats that are difficult to survey [5][6][7][8].…”