Estimating the population sizes of rainforest birds of the Tweed Caldera

Climate change is likely to negatively impact populations of rainforest birds along Australia’s eastern seaboard. To this end, accurate estimations of species’ current population sizes are necessary for future monitoring and conservation. Here, we used distance sampling protocols to generate abundance and density estimates for 11 species of birds occupying the Gondwanan rainforests of the Tweed Caldera on the Queensland–New South Wales border. Data were collected along two elevational gradients in subtropical rainforest between September 2015 and October 2016. Half-normal functions with cosine adjustments were used to calculate the density of each species per km 2 . We used a GIS to determine the total area of suitable habitat available in protected areas adjacent to the sampling locations (~425 km 2 ) and then extrapolated the density estimates to generate estimates of population size for each target species in the study area. Estimated population sizes ranged from 473 719 (Yellow-throated Scrubwren Sericornis citreogularis ) to 21 841 (White-throated Treecreeper Cormobates leucophaea ). The standard error (SE) associated with the population estimate of the White-throated Treecreeper was large, as was the SE associated with the Eastern Yellow Robin Eopsaltria australis (estimated population 235 084), Green Catbird Ailuroedus crassirostris (69 731) and Grey Fantail Rhipidura fuliginosa (124 534). Accurate abundance estimates (SE <20% of total abundance) were obtained for the Brown Gerygone Gerygone mouki (193 919), Brown Thornbill Acanthiza pusilla (198 801), Golden Whistler Pachycephala pectoralis (72 730), Lewin’s Honeyeater Meliphaga lewinii (190 389), Silvereye Zosterops lateralis (92 268), White-browed Scrubwren Sericornis frontalis (94 426) and Yellow throated Scrubwren (473 719). Species specific surveys would provide more accurate results, but given the declines expected for many species in the region, and the current lack of estimates of population sizes in these protected areas, the baseline data presented here are of importance from ecological and conservation perspectives.