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

Abstract

The avifauna of Guam was devastated by the introduction of the Brown Treesnake, and the restoration of native birds would need to address the problem with eradication or suppression of BTS. With eradication of the snake unlikely in the near term, and suppression capabilities limited to specific finite areas, key information for reintroductions is how low BTS abundance will likely need to be for each bird species to be re-established based on their vulnerability to BTS predation. Here, we estimate vulnerability, which can no longer be measured directly, so biologists who are familiar with one or more of seven Guam birds were surveyed to obtain their knowledge and produce quantitative vulnerability estimates. As is typical of birds adapted to islands devoid of predators, respondents judged that our focal species exhibit few predator avoidance and tolerance traits, leaving body size as the prime determinant of vulnerability. Respondent opinion also holds that any behavior that reduces the likelihood of an encounter by BTS, e.g., roosting/nesting in palm crowns, cavity nesting, and in particular urban dwelling, substantially reduces vulnerability. Our results can help inform species-specific decisions about when it may be safe to consider the release of birds on Guam depending on the relative vulnerability of each species to predation by BTS.

Predation thresholds for reintroduction of native avifauna following suppression of invasive Brown Treesnakes on Guam

“…We selected seven birds for our study that might be considered candidates for release on Guåhan in the future, referring to them by their local Chamorro name: Ko‘ko‘ (Guam rail) Gallirallus owstoni , Åga (Mariana crow) Corvus kubaryi , Totot (Mariana fruit dove) Ptilinopus roseicapilla , Såli (Micronesian starling) Aplonis opaca , Sihek (Guam kingfisher) Todiramphus cinnamominus , Chichirika (Rufous fantail) Rhipidura rufifrons , and Nosa‘ (bridled white‐eye) Zosterops conspicillatus . We list these species in order of decreasing body size (Table 1 ), an important indicator of predation risk (McElderry et al, 2021 ), with Ko‘ko‘ almost 30 times heavier than Nosa‘.…”

“…Although some growth rates have been reported either in the wild or in captivity for our species, we ultimately could not know exactly how quickly any of these birds would multiply if released on Guåhan, but the demography of some birds simply generated more rapid population growth than others. However, when we analyzed the sensitivity of our results to the assumed population growth rate, we simply recovered the vulnerability rankings reported by McElderry et al ( 2021 ).…”

“…If this threshold cannot be reached, our results indicate that bird reintroductions will fail. Despite seemingly large differences among birds in vulnerability to BTS (McElderry et al, 2021), the range of tolerable predation rates is rather narrow relative to the amount of reduction in current BTS levels required.…”

“…Having either higher survival odds or higher reproduction rates may allow a particular bird species to become established and persist in BTS‐suppressed areas, but we cannot directly measure these characteristics in situ given the absence of Guåhan's avifauna. To bridge this knowledge gap, knowledge from a panel of experienced Guåhan bird and BTS biologists was elicited to predict the outcomes of individual bird–snake encounters (McElderry et al, 2021 ). Consensus indicated notable differences among bird species and life stages in their vulnerability to BTS, especially body size and a few other physical and behavioral attributes that were likely important for determining species‐specific rates of predation by BTS.…”

“…Although expert knowledge can provide insights into main factors that affect prey vulnerability (McElderry et al, 2021 ), population models evaluate interactions of multiple factors, providing a synergistic assessment of vulnerability over all life stages and allows for sensitivity analysis, such as the sensitivity of population growth to changes in each demographic rate that is affected by BTS. Highly sensitive species will respond more strongly to different levels of BTS suppression.…”