# Hard to Swallow: Scaling Relationships Between the Size of Avian Prey and the Overall Size and Maximal Gape of Brown Treesnakes

## TL;DR

This study shows that brown treesnakes attempt to eat birds much larger than they can swallow, expanding the threat they pose to bird populations.

## Contribution

The study expands the known range of avian prey sizes that brown treesnakes attempt to eat, revealing a greater threat than previously understood.

## Key findings

- RPA significantly predicted the probability of birds being eaten, with an upper limit near the snake's maximal gape.
- Snakes attempted to eat birds with RPA up to 447%, far exceeding what they could swallow.
- Prey size decreased after ingestion, allowing snakes to consume more mass than expected based on initial size.

## Abstract

Snakes are a useful model for gaining insights into the relationships between predator and prey sizes and resource utilization because their anatomy limits the size of prey that can be swallowed whole. However, data are sparse regarding how commonly gape‐limited predators eat or attempt to eat prey with sizes up to or exceeding their maximal gape. Thus, for an invasive predator, the brown treesnake (
Boiga irregularis
), we fed captive snakes dead birds with an extremely large range (17%–447%) of relative prey area (RPA = prey cross‐sectional area/snake gape area) to test the predictive value of RPA for snakes attempting to ingest or successfully ingesting prey. As expected, RPA significantly predicted (logistic regression p < 0.0001) the probability of birds being eaten, with an upper size limit similar to the maximal gape of the snakes. Although RPA also significantly predicted (p = 0.003) the probability of attempting to eat a bird, it was less accurate in predicting attempts than successes, and many snakes attempted to eat birds too large to swallow. Twenty‐five snakes attempted to eat birds with RPA ranging from 130% to 447%. The longest durations of unsuccessful feeding attempts were often for values of RPA near 100% rather than the extremely large values. For six large birds with mean measured RPA = 93%, the prey diameter soon after ingestion averaged 14% less than that measured prior to ingestion, which can allow snakes to consume 30% more mass than would otherwise be possible. Our findings complement a recent field study that concluded brown treesnakes regularly attempt to eat live birds too large to swallow. Our results also greatly expanded the known range of avian prey sizes that these snakes attempt to eat. Consequently, brown treesnakes pose a risk to birds with sizes well beyond the limit on prey size imposed by gape.

The anatomy of snakes and other gape‐limited predators imposes an upper limit on the size of prey that are swallowed whole, and maximal gape was a good predictor of the maximal size of prey eaten by the invasive brown treesnakes in our study. However, gape had modest predictive value for the size of prey that snakes attempted to eat as some attempts involved birds more than four times larger than what could be swallowed. Hence, the threat of brown treesnakes to avian prey is far greater than just the size of birds small enough to be eaten.