Contact rates with nesting birds before and after invasive snake removal: estimating the effects of trap-based control

Abstract

Invasive predators are responsible for almost 60% of all vertebrate extinctions worldwide with the most vulnerable faunas occurring on islands. The brown treesnake (Boigairregularis) is a notorious invasive predator that caused the extirpation or extinction of most native forest birds on Guam. The success of avian reintroduction efforts on Guam will depend on whether snake-control techniques sufficiently reduce contact rates between brown treesnakes and reintroduced birds. Mouse-lure traps can successfully reduce brown treesnake populations at local scales. Over a 22-week period both with and without active snake removal, we evaluated snake-trap contact rates for mouse- and bird-lure traps. Bird-lure traps served as a proxy for reintroduced nesting birds. Overall, mouse-lure traps caught more snakes per trap night than did bird-lure traps. However, cameras revealed that bird-lure traps had a snake contact rate almost 15 times greater than the number of successfully captured snakes. Snakes that entered bird-lure traps tended to be larger and in better body condition and were mostly captured in bird-lure traps, despite numerous adjacent mouse-lure traps. Traps placed along grid edges caught more snakes than interior traps, suggesting continuous immigration into the trapping grid within which bird-lure traps were located. Contact between snakes and bird-lure traps was equivalent before and after snake removal, suggesting mouse-lure traps did not adequately reduce the density of snakes that posed a risk to birds, at least at the timescale of this project. This study provides evidence that some snakes exhibit prey selectivity for live birds over live mouse lures. Reliance on a single control tool and lure may be inadequate for support of avian reintroductions and could lead to unintended harvest-driven trait changes of this invasive predator.

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

“…The risk of being encountered by a BTS may vary among avifauna due to one or more of many possible predator and prey trait combinations. We asked respondents to rank encounter rates among avifauna relative to a baseline rate, such as might be determined by the BTS contact rate at live lure camera traps in an area (e.g., Siers, accepted;Yackel Adams et al, 2019). Live lure camera traps use a live bird or mouse to lure BTS.…”

“…Unfortunately, because BTS and most of the native Guam bird species no longer co-exist, much of the necessary information on ecological interactions and predation risks does not exist and cannot be studied. Nonetheless, there is a need for informed predictions about predation risks to birds so that future bird reintroductions into areas inhabited by BTS at some suppressed level have higher chances of success (Yackel Adams et al, 2019). Therefore, we used expert opinion to provisionally bridge knowledge gaps.…”

Brown Treesnake Mortality After Aerial Application of Toxic Baits

“…Multiple factors specific to individual snakes may have reduced foraging activity, thus susceptibility to baits, such as inactivity following ingestion of a large meal or prior to ecdysis (Rodda et al 1999 b , Siers et al 2018). Brown treesnakes may also exhibit individual heterogeneity in response to rodent‐based carrion, given heterogeneity in capture probabilities in traps using rodent lures (Tyrrell et al 2009) and evidence of only capturing some individuals in traps containing avian lures despite being surrounded by rodent lure traps (Yackel Adams et al 2019). Additional work is needed to explore this possibility, but if a subset of the population, independent of body size, exhibits little interest towards rodent‐based carrion, then we might observe a pattern of decreased mortality rates during future treatments as that segment becomes a larger proportion of the population (Zavorka et al 2018).…”

Evaluation of camera trap‐based abundance estimators for unmarked populations

“…2009, Yackel Adams et al. 2019]). Brown treesnakes have size‐structured prey preferences, where smaller snakes (<700 mm) largely avoid mammalian prey, partly because they are gape limited (Savidge 1988) and partly because of a preference for lizard prey (Lardner et al.…”

“…Channeling animal movement to break a near infrared beam is useful for sampling some species (Hobbs and Brehme 2017), though not possible for all species (e.g., arboreal species) and thus sampling frequently requires the automated time‐lapse feature to be used on cameras (Yackel Adams et al. 2019). Depending on the selected time interval between photographs and the length of the study, this results in thousands or millions of photographs to process.…”