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

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 logos, 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 islandwide transect surveys and opportunistic sightings to provide a comprehensive update on the distribution and abundance of Sali (Micronesian Starling, Aplonis opaca) - one of Guam's last extant native bird species. Area searches of AAFB, where the largest remnant of the Sali population persists, revealed a 15-fold population increase since the last survey in the early logos, 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 similar to 1,400 individuals. The population increase can likely be attributed to a combination of snake control measures and the Sali'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 Sali 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.

Automated summary

Assessing the impacts of invasive predators on native species is crucial for understanding species persistence. In Guam, the introduction of the brown treesnake led to the near-total loss of native forest avifauna. Control measures have led to an increase in the population of the Micronesian Starling, but skewed age ratios and continued impacts from snake predation indicate ongoing challenges for population recovery.