Audubon Magazine / Winter 2018

Are Starlings the Key to Making Guam’s Forests Sing Again?

Decades after the brown tree snake wiped out Guam’s birds, biologists have an ambitious plan to bring native species back to the stunning island.

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A Micronesian Starling perches at the entrance of a nest box on the Andersen Air Force Base in Guam. Once widespread on the island nation, the species now numbers in the hundreds. Their human-made dwellings are hung on smooth concrete poles to prevent brown tree snakes from climbing up and eating the nestlings. Photo: Nancy Borowick

Words by Eric Wagner

Published Winter 2018

The forests of Guam are eerily quiet. Ever since birds disappeared a few decades ago, only the hum of insects and rustling of leaves float on the humid air of this 210-square-mile island in the Western Pacific Ocean. If a bold avian reintroduction project is successful, however, the mountainous terrain might once again brim with song.

The story behind Guam’s near-silent forests is a classic in the annals of ecological invasion. It begins around 1949, when brown tree snakes hitched a ride on U.S. military craft from Manus Island in Papua New Guinea. The reptiles, which can grow up to eight feet long and have a native range from Australia to the nearby region of Melanesia, multiplied and ate their way through Guam’s avifauna. By the time biologists grasped the problem in the 1980s, snakes had rendered 13 of the island’s 22 breeding birds extinct in the wild, including six endemics. The 12 forest species were hit especially hard, with only the Island Swiftlet and Micronesian Starling still clinging to existence on the snake-free grounds of a U.S. Air Force base. Others, such as the Guam Rail and Guam Kingfisher, survive only through captive-breeding programs.

Iowa State University biologist Haldre Rogers and her team are working to resurrect some of those lost species—and in doing so, help rebuild the larger ecosystem. In the absence of birds, Guam’s karst limestone forests, which cover a third of the island, have changed significantly. The density of cobwebs can be up to 40 times greater on birdier isles, hinting that spiders—which compete with avian predators—now overrun the woods. Many plants, meanwhile, have declined. Seventy percent of tree species, including the berry-packed åplokhateng and åhgao, rely on birds to gorge on their fruits and spread the seeds far and wide. (Some are up to four times likelier to germinate after passing through a bird’s gut.) Rogers found that because of the snakes, the number of seedlings for two common species plummeted by as much as 92 percent, thinning once dense canopies.

By the time biologists grasped the problem, snakes had rendered 13 of the island‘s 22 breeding birds extinct in the wild.

Enter the Micronesian Starling. Known locally as Sali, it’s a glossy black bird with a yellow eye and a clear song. “It’s probably the most effective fruit disperser of the birds that used to be here,” Rogers says. “This means it’s the best candidate for restoring ecosystem function.”

Historically widespread, in 2016 fewer than 1,000 Sali survived on Guam. As cavity nesters, the birds are easy pickings for the tree-bound snakes. So the first step in bolstering their numbers was to protect the starlings until chicks fledge. Rogers’s collaborators hit upon a simple solution: They hung PVC nest boxes on poles too slippery for the reptiles to scale. Since they installed the contraptions around Andersen Air Force Base in 2015, 589 young have fledged.

The next stage is to build new boxes—with help from Guamanian students in shop classes—outside of the friendly confines of the base. That’s at least a year off and a considerable challenge, given that 2 million brown tree snakes still slither around the island. The population has been stable since the 1990s, despite localized eradication efforts that include airdropping mice laced with acetaminophen, which is toxic to reptiles.

“Snakes are definitely still a concern,” says Rogers. But she is not content to wait while communities of trees wither without their birds. Even if a fraction of the young starlings make it to adulthood, it will help stem, and maybe even reverse, the decline of swaths of forest. “It’s a small step,” Rogers says. “But an important one to produce a functioning, more diverse ecosystem.”

Her hope is that ultimately the lessons her team learns from the starlings can pave the way to reintroduce the Guam Kingfisher, another cavity nester that’s being raised in the Philadelphia Zoo and other U.S.-based institutions. Like the Sali, they’re waiting in the wings, ready to come back home.

A pair of starling chicks hunker down inside one of the nest boxes, custom-designed by Colorado State University biologist Julie Savidge and her team. These chicks are almost fully feathered and will soon fledge out of their home. They typically spend about 25 days in the nest. Photo: Nancy Borowick

Scientists and starling-squad members Martin Kastner (top) and Henry Pollock open a nest box on Andersen Air Force Base as an adult bird flies in to observe. The structures—modified from chickadee dwellings—are a valuable tool for tracking reproduction, and also provide researchers with easy access to starlings for banding and population monitoring. Photo: Nancy Borowick

Iowa State University biologist Haldre Rogers identifies a vine in a mature limestone forest. Rogers, who first visited Guam in 2002 as part of a U.S. Geological Survey team to halt the brown tree snake’s spread, was struck by the silence of its forests. She has spent the last decade studying the ecological consequences of bird loss on the island. Photo: Nancy Borowick

Chewed-up fruits of Pandanus, a common understory tree on Guam. The seeds were probably regurgitated by pigs, but sometimes they're also deposited by Mariana fruit bats, which are endangered in the country. In the absence of birds, the bats are one of Guam’s last remaining frugivores. Photo: Nancy Borowick

Kastner holds a starling chick just removed from its nest box. In its foot the bird holds pieces of broadleaves that makes up the nest cup. A fecal sample, on Kastner’s palm, will provide valuable dietary information. Photo: Nancy Borowick

Pollock and Kastner open a mist net in a forest. They are trying to capture free-living juvenile and adult starlings so they can collect valuable information about their diet, behavior, movements, and habitat use. Photo: Nancy Borowick

A brown tree snake rises out of the “snake pit” at a U.S. Department of Agriculture laboratory on Andersen Air Force Base on Guam. To control the predators' spread—as many as two million may still slink around the island—the reptiles are captured in modified minnow traps and brought back to the lab for processing before they are euthanized. Photo: Nancy Borowick

Kastner (right) demonstrates how a nest box works at Sågan Tinanom Nature Park in northern Guam. Ultimately, the team hopes to deploy these nest boxes around the island to help the native starlings expand their range. The species is probably the most effective avian fruit disperser on Guam, and in the absence of it and other birds, some tree seedlings have plummeted by as much as 92 percent. Photo: Nancy Borowick

Researchers Natalie Myers and Ed Perez draw a census of understory seedlings. As part of Rogers’ Ecology of Bird Loss project, they monitor native-plant growth and survival on Guam, as well as the brown tree snake-free Mariana Islands. The purpose is to show how the reptilian predator affects not just the birds they eat, but the habitats in which those birds used to forage and live. Photo: Nancy Borowick

Micronesian Starlings, like most species of their kind, are cavity-nesters. Adults will aggressively defend their young, but they're no match for brown tree snakes. Only two of Guam's twelve forest birds—the starling and the Island Swiftlet—still live freely in the island's wilds. Photo: Nancy Borowick

A subadult starling with a freshly captured insect perches with its younger kin. The juvenile, which fledged from one of the nest boxes, wears three color bands and a metal band on its legs. Unique color combinations like this allow scientists to follow individual birds over time and learn about plumage development, survival, and changes in movement and habitat use. Photo: Nancy Borowick

Correction: The chewed-up fruits in photo no. 5 were originally misidentified as being left behind by fruit bats. Some captions were also edited to clarify that the nest boxes were designed and installed by a separate team from Colorado State University.

This story originally ran in the Winter 2018 issue as “Island of Silence.” To receive our print magazine, become a member by making a donation today.