Recent recovery and expansion of Guam’s locally endangered Såli (Micronesian Starling) Aplonis opaca population in the presence of the invasive brown treesnake

Abstract

Summary Assessing the impacts of invasive predators on the demography and distribution of native species is critical for understanding mechanisms of species persistence and informing the design of recovery programmes. On the oceanic island of Guam, the introduction of the predatory brown treesnake Boiga irregularis after World War II caused the near-total loss of the native forest avifauna. Localised snake control measures have been implemented since the early 1990s, yet it remains poorly understood how they have impacted Guam’s remaining native bird populations. To address this question, we combined intensive area searches of Andersen Air Force Base (AAFB) with island-wide transect surveys and opportunistic sightings to provide a comprehensive update on the distribution and abundance of Såli (Micronesian Starling, Aplonis opaca) – one of Guam’s last extant native bird species. Area searches of AAFB, where the largest remnant of the Såli population persists, revealed a 15-fold population increase since the last survey in the early 1990s, and transect surveys and opportunistic sightings indicate incipient recolonisation of other urbanised areas of northern and central Guam. We estimate the current island-wide population size at ~1,400 individuals. The population increase can likely be attributed to a combination of snake control measures and the Såli’s ability to exploit urban refugia for nesting and roosting. Although these trends demonstrate some population recovery, a skewed age ratio (>90% adults and subadults) at AAFB and a highly urbanised distribution and low abundance outside AAFB indicate that snake predation continues to strongly impact the population. More intensive snake suppression efforts, particularly in forested areas, may allow for the Såli population to attain its former distribution and abundance on Guam. More broadly, our findings reinforce the importance of urban areas as refugia for some threatened species.

Distilling Professional Opinion to Gauge Vulnerability of Guam Avifauna to Brown Treesnake Predation

“…Såli appears to be effectively avoiding BTS by adapting to urban areas, where snake densities are lower than in forests, and by nesting in cavities that may be more difficult for BTS to access. Cavity nesting itself does not seem to deter nest predation by snakes (e.g., Berkunsky et al, 2011), but cavities in large diameter poles without adjacent branches, e.g., solitary coconut palms, tall telephone poles, and other anthropogenic structures may be inaccessible to BTS (Savidge et al, 2018;Pollock et al, 2021). Sihek also nests in cavities, but exclusively in the forest in decaying wood or termite nests (Marshall, 1989;Kesler and Haig, 2005), not in inaccessible/artificial cavities like Såli.…”

“…Among the avifauna extirpated from Guam by BTS predation, we focused here on the following: Ko'ko' (Guam Rail) Gallirallus owstoni, Åga (Mariana Crow) Corvus kubaryi, Totot (Mariana Fruit-Dove) Ptilinopus roseicapilla, Sihek (Micronesian Kingfisher) Todiramphus cinnamominus, Chichirika (Rufous Fantail) Rhipidura rufifrons, and Nosa' (Bridled White-eye) Zosterops conspicillatus. Såli (Micronesian Starling) Aplonis opaca still persists on Guam, but only on a small fraction of its historic range, and largely in urban habitats (Pollock et al, 2021). These seven candidate species are being considered for reintroduction due to the maintenance of captive breeding programs or appropriate source populations on nearby islands (Haig et al, 1990;Brock and Beauprez, 2000;MAC Working Group, 2014;Trask et al, 2021).…”

Nest defense, personality, and fitness of a locally endangered island passerine

“…Habitats on AAFB include urbanized areas, a golf course, and mixed secondary and primary limestone forest. The majority of Guam's Såli population (>90%; Pollock et al, 2021) nests and roosts on AAFB. Starting in 2015, nest boxes were installed on AAFB as part of a conservation project to increase Såli nest success (Savidge et al, 2022).…”

“…The Pacific island of Guam experienced devastating losses to its native avifauna when the invasive brown treesnake ( Boiga irregularis ) was introduced following World War II, resulting in the total extirpation of 10 of 12 native forest bird species (Savidge, 1987; Wiles et al, 2003). The Såli is one of two such species that has persisted and has been making a recovery on Guam due to its tolerance for urbanization coupled with localized snake control measures, in particular on Andersen Air Force Base in northern Guam (Pollock et al, 2021). Såli eggs and nestlings experience low predation rates due to adults choosing relatively safe nest site substrates in urbanized areas, such as predator‐resistant nest boxes and streetlamp posts (Savidge et al, 2022).…”

“…The Såli is one of two such species that has persisted and has been making a recovery on Guam due to its tolerance for urbanization coupled with localized snake control measures, in particular on Andersen Air Force Base in northern Guam (Pollock et al, 2021). Såli eggs and nestlings experience low predation rates due to adults choosing relatively safe nest site substrates in urbanized areas, such as predator-resistant nest boxes and streetlamp posts (Savidge et al, 2022).…”