COS 137-6 - High rate of unsuccessful predation attempts by brown treesnakes offers insight into the risks and rewards of consuming large prey

Background/Question/Methods

Snakes must swallow prey whole, and many species have evolved adaptations to consume prey that is very large relative to their body size. However, snakes may subdue and kill prey that is too large to swallow, and this behavior carries inherent risk for the snake, including potential injury from prey defense or asphyxiation during prey ingestion. Therefore, there is debate in the literature whether these cases are dangerous miscalculations by the snake, or worthwhile gambles with potentially high payoffs. The actual frequency of snakes attempting to consume oversized prey in nature is not known. We radio-transmittered a large number (n > 300) of fledgling Såli (Micronesian starling, Aplonis opaca), a locally-endangered prey item of the invasive brown treesnake (Boiga irregularis) on the island of Guam in the Western Pacific, allowing us to document rates of predation as well as collect morphological data on snakes that successfully consumed prey. We also conducted visual surveys for brown treesnakes across the study area to collect demographic and morphological data on the overall snake population.

Results/Conclusions

We found that of the >100 fledgling Såli killed by snakes, fewer than 50% were successfully consumed. All unconsumed Såli had a layer of mucus on the head and body of the carcass, indicating that snakes had started swallowing the bird, and then abandoned the attempt. Brown treesnakes that successfully consumed Såli represented the largest individuals within the population, suggesting that failed predation events may be due to medium-sized snakes physically capable of subduing and killing Såli but unable to swallow such large prey. Two snakes that successfully consumed Såli died while attempting to regurgitate their meals (in a captive setting), but no snakes were found dead in the field following failed predation attempts. Brown treesnakes have been observed attempting to predate bird species much larger than Såli and even infant humans. Together, it appears that predation attempts on oversized prey are routine for brown treesnakes. These results have important implications in terms of the predation risk posed by snakes of smaller size classes to birds and other wildlife of conservation concern. Furthermore, they suggest that attempting to consume oversized prey, at least those with limited defensive abilities, may present enough sufficient reward to warrant the risks entailed to snakes.