Flying Mice Target Tree Snakes

For years, Guam’s skies have been nearly empty. The brown tree snake, an invasive reptile that snuck onto the island in the cargo of Navy airplanes shortly after World War II, decimated most bird species decades ago. But last September the sky was not vacant: it was raining mice.

Navy helicopters criss-crossed Guam’s jungle as mice were shot from specially-designed contraptions, all in the latest strategy to beat back the brown tree snake and allow the reintroduction of native bird species. Dead before ejection, the rodents act as tiny Trojan horses tossed into the jungle canopy. Inside their tasty exteriors scientists slip acetaminophen — Tylenol’s active ingredient also poisons these tree snakes. This strategy is the culmination of years of research by the United States Department of Agriculture.

“No one else is doing this,” says William Pitt of the USDA, who headed the research team tasked with devising the technique to scatter poisonous snake bait over Guam’s jungle.

A major goal of Guam’s snake control program is keeping the snakes from hopscotching to other islands and wreaking similar havoc on equally vulnerable ecosystems like Saipan’s or Hawai‘i’s. Currently, trained dogs sniff outgoing cargo to head off any sneaky snakes attempting to catch a free ride on Navy planes leaving Andersen Military Base. When a snake is sighted on neighboring islands, the U.S. Geological Survey’s Brown Tree Snake Rapid Response Team flies in to help local authorities capture it and determine whether there’s a burgeoning snake population.

While the diversity of species on other Pacific islands attests to the success of these measures, they are labor intensive. Most snake control happens at specific locations, like the cargo holds of airplanes or along the fences encircling the military base. This means snakes that remain in the jungle mostly slither free. Scattering mice over the forest would be much easier and quicker than slogging through the forest baiting and checking traps — the snakes would essentially get rid of themselves.

The new method takes advantage of their location and basic biology: acetaminophen is poisonous to the snakes. The mice are stuffed with just 80 milligrams of acetaminophen — equal to a child’s dose of Tylenol — then glued to cardboard strips. Paper streamers tangle in the small branches after the mouse bundles are catapulted from helicopters. “The whole idea is that they get caught in the canopy,” where only the snakes go, says Pitt.

Brown trees snakes have dominated Guam’s treetops since shortly after their arrival in the 1940s, fundamentally altering the forests. “It’s eerie. It’s very, very quiet,” says Robert Reed, a wildlife biologist with the geological survey, who works to combat brown tree snakes but was not involved with the latest aerial drop. Only a few birds remain near Anderson Air Force base, where stringent controls keep snake numbers down.

After laying waste to the birds, the snakes went after the rodents. Reed estimates that the rodent population on Guam is ten times lower than on nearby islands where the snake has not spread. The brown tree snake’s ability to persist after overwhelming its prey populations is one key to its insidious success. Rodents are rare and the birds are gone, but the snakes slither on. As Reed explains, “The snakes are just marvelously good at taking a small amount of resources in the form of prey and turning it into more snakes.”

But lobbing tasty toxic treats at the snakes will not eradicate them, Pitt says—there are far too many to poison them all. He explains that the goal is to create snake-free areas where bird species can be reintroduced. One possible bird is the Guam rail, a small flightless bird (not unlike a kiwi in looks) that is being successfully bred in zoos worldwide. Reed is optimistic. “It’s a promising technique,” he says. “Maybe we’ll be able to restore some native species on Guam some time in the near future.”

Data is already being gleaned from the aerial mouse drop. It is imperative, Pitt explains, that the poisonous mice only target brown tree snakes because the snakes are not the only scavengers on Guam. To make sure the mice aren’t going to waste, researchers glued radio transmitters to the bellies of some mice, so they can follow the signals to see whether the bait ends up in the bellies of snakes, or innocuous species like coconut crabs.

Another advantage of the aerial baiting technique is its potential for targeting large numbers of snakes at minimal cost. “People think of the helicopters as being really expensive,” says Reed, “But the alternative is to have human employees walking around and checking traps or checking bait stations all over the island” — a more costly alternative.

Haldre Rogers, a botanist at the University of Washington in Seattle, is studying the indirect effects of brown tree snakes on forest diversity. Many tree species need birds to disperse seeds to new areas and help speed their germination. In neighboring islands like Saipan, seeds travel far from their parent trees. In Guam, she says, seeds fall straight to the ground. Rogers predicts that Guam’s current forest of intermingling species will decline, as these seeds grow into clumps of the same species of tree.

Next up for the new technique: automation. Putting together the mouse baits is currently a time-consuming process, requiring human volunteers wielding hot glue guns, says Pitt. Wide-scale drops won’t become feasible until the entire process, from bait assembly to expulsion from helicopters, is mechanized.

Meanwhile, Rogers says, the aerial drop is “a good tool to have in the arsenal.” Occasionally, she encounters a rare starling near one of the military bases, when it breaks into song.

In the hushed forest, the sound, she says, is “shocking.”