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

Robert M Mcelderry,Andre V Nguyen,Eben H Paxton,Shane R Siers

doi:10.3389/fcosc.2021.683964

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.

Highlights

The accidental introduction of Brown Treesnakes (Boiga irregularis; BTS) in the 1940s to the Pacific island of Guam precipitated an unprecedented collapse of the island’s entire forest avifauna (Savidge, 1987; Wiles et al, 2003; Rodda and Savidge, 2007)
Focusing on seven promising candidate bird species for reintroduction, we developed a questionnaire to answer four broad questions: (i) Relative to a baseline BTS presence, how likely is each bird species to be encountered by a BTS? (ii) Which life stages within a family unit may be at risk during a single BTS encounter? (iii) How likely is an individual to be killed during an encounter? (iv) How do various bird and snake life stages/sizes affect the outcome of an encounter? Our results provide a metric by which bird species may be ranked for reintroduction according to their vulnerability to predation by BTS
Respondents agreed on specific attributes that varied among bird species and contributed to their risk of being encountered by a BTS (Figure 1)

Summary

Introduction

The accidental introduction of Brown Treesnakes (Boiga irregularis; BTS) in the 1940s to the Pacific island of Guam precipitated an unprecedented collapse of the island’s entire forest avifauna (Savidge, 1987; Wiles et al, 2003; Rodda and Savidge, 2007). There are at least two initiatives in which avifauna once native to Guam and the Mariana Islands are being reared in captivity with the intent of reestablishing wild populations on Guam (Haig et al, 1990; Brock and Beauprez, 2000; MAC Working Group, 2014; Trask et al, 2021). These goals to reestablish avifauna once native to the island are hindered by the continued presence of BTS.

Objectives

Methods

Results

Conclusion