Hierarchical distance sampling reveals increased population size and broader habitat use in the endangered Bahama Oriole

Abstract

The Caribbean is home to over 20 passerine species listed by the International Union for Conservation of Nature as vulnerable, endangered or critically endangered. The Bahama Oriole (Icterus northropi) is listed as critically endangered and is now restricted to Andros, a single island complex in The Bahamas. We investigated this species' habitat use and produced the first statistically robust estimate of population size. We conducted point counts during the Bahama Oriole's breeding season in May and June 2017, extensively surveying the northern part of North Andros over a study area covering 713 km². Hierarchical distance sampling models estimated 1269-2765 individuals within our study area, a substantially larger population on North Andros than was indicated by the previously published estimates. Earlier studies, which disproportionately sampled anthropogenic and coppice habitats, likely underestimated this species' abundance in pine forest. We found that the Bahama Oriole is widespread, most abundant in pine forest, and not dependent on developed habitats during the breeding season. These findings provide a better outlook for the species' persistence and indicate that conserving pine forest would benefit this critically endangered species. Our results also emphasize the importance of rigorously evaluating habitat use when developing conservation plans for endangered species. Systematic population counts and statistical analyses that account for detection probability are needed for endangered and vulnerable endemic birds across the Caribbean, especially in the face of increased hurricane strength and sea level rise due to climate change. L'échantillonnage fondé sur la distance couplé à une approche hiérarchique révèle une taille de population plus élevée et une utilisation de l'habitat plus vaste chez l'Oriole des Bahamas, espèce en danger RÉSUMÉ. Les Caraïbes abritent plus de 20 espèces de passereaux classées par l'Union internationale pour la conservation de la nature comme vulnérables, en danger ou en danger critique d'extinction. L'Oriole des Bahamas (Icterus northropi) est classé comme en danger critique d'extinction et est désormais limité à Andros, un complexe insulaire unique des Bahamas. Nous avons étudié l'utilisation de l'habitat de cette espèce et calculé la première estimation statistiquement solide de la taille de la population. Nous avons effectué des dénombrements par points d'écoute pendant la saison de reproduction de cet oriole, en mai et juin 2017, en sillonnant la partie nord d'Andros Nord sur une aire couvrant 713 km². Des modèles hiérarchiques à partir des dénombrements fondés sur la distance ont permis d'établir le nombre d'individus à 1 269-2 765 dans notre aire d'étude, soit une population nettement plus importante sur Andros Nord que ce qui avait été rapporté précédemment. Les études antérieures, qui ont échantillonné de manière disproportionnée les milieux anthropiques et les taillis, ont probablement sous-estimé l'abondance de cette espèce dans les pinèdes. Nous avons constaté que l'...

Comparing global and local maps of the Caribbean pine forests of Andros, home of the critically endangered Bahama Oriole

“…Therefore, we conservatively estimate that there may be between roughly 2000-8000 individuals on all of Andros. Assuming the orioles are present in pine forest and developed habitats in similar densities, as suggested by Rowley et al (2021), we estimate that only a very small percentage (1.7%) occur in developed habitats, while the majority of the population occurs in pine.…”

“…While the Bahama Oriole is also known to use developed habitats (Price et al, 2011), we did not focus on this class for the present analysis given that its distribution is small (1.7% of total suitable habitat), well known, and likely supports only a small percentage of the Bahama Oriole population. For example, on their 713 km 2 study site on North Andros, Rowley et al (2021) estimated that there were between 1200 and 2800 orioles. Their study site included 480 km 2 of potentially suitable oriole habitat, most of which was pine (414 km 2 ) with only 18 km 2 of developed habitat (Rowley et al, 2021).…”

“…For example, on their 713 km 2 study site on North Andros, Rowley et al (2021) estimated that there were between 1200 and 2800 orioles. Their study site included 480 km 2 of potentially suitable oriole habitat, most of which was pine (414 km 2 ) with only 18 km 2 of developed habitat (Rowley et al, 2021).…”

“…Given that the total area of pine we mapped was 1388 km 2 , we estimate that the Rowley et al (2021) study site contained approximately one-half to onethird of the total area of suitable habitat (pine and developed) across Andros. Therefore, we conservatively estimate that there may be between roughly 2000-8000 individuals on all of Andros.…”

“…The animal and plant biodiversity of these pinelands are notable, including several endemic species such as the critically endangered Bahama Oriole (Icterus northropii; Myers et al, 2004). This bird species, endemic to The Bahamas and once found on both Abaco and Andros, is now found solely on Andros, having been extirpated from Abaco during the latter half of the twentieth century (Stonko et al, 2018;Rowley et al, 2021). The association of the Bahama Oriole, among other species, with the pine forests on Andros highlights the critical role that a single, restricted habitat may play in conserving global biodiversity (Stonko et al, 2018;Yancy et al, 2020;Rowley et al, 2021).…”