Endangered Mariana Crows Return to Island Forest

San Diego Zoo Global Researchers Help Rear and Release Aga on Rota

For more than 2 million years, the native forests on the islands of Guam and Rota were home to several thousand crows of a species found nowhere else on Earth. But over the last 60 years, the Mariana crow—called the aga, in the Chamorro language—has completely disappeared from the island of Guam and rapidly declined on neighboring Rota. Today, there are only about 175 Aga left on the planet.

“Aga are a critical strand in the ecological and cultural web that make up the forests of Guam and Rota,” said Anthony Benavente, secretary of the Commonwealth of the Northern Mariana Islands Department of Lands and Natural Resources. “Without drastic measures, we could lose this part of our natural and cultural heritage forever.”

To ensure the survival of the species, scientists from the University of Washington and San Diego Zoo Global are partnering with the Commonwealth of Northern Mariana Islands Department of Lands and Natural Resources (CNMI-DLNR) and the U.S. Fish and Wildlife Service on a bold new project that they hope will stabilize the population of aga on Rota.

In 2016, researchers began collecting eggs from wild aga nests, to be reared in captivity. The captive-reared birds would be raised past the critical period of highest mortality and then released. This project has the potential to greatly increase reproductive output, because the wild aga pairs normally lay another set of eggs when a nest fails—thus, this project will double the number of broods in a year while increasing the survivorship of the captive-reared birds.

On Sept. 28, the first cohort of five captive-reared aga were released on public lands on Rota. An additional five birds will be released into the same area later in the year. Researchers will continue to monitor and support the birds for approximately one year after the release to ensure their continued success in the wild.

“Today, there are many species in decline all over the world, and we are really proud to be part of an effort to bring a species back from the brink of extinction,” said Renee Robinette Ha, research associate professor at the University of Washington and director of the university’s aga research efforts since 2005. “Our research has determined some causes of mortality in Mariana crows, and by working collaboratively with our partners, we have been able to start turning that around.”

One of the notable recent successes in aga conservation is the ability of conservation experts to successfully hand rear the species using techniques perfected with other species at San Diego Zoo Global’s Hawaii Endangered Bird Conservation Program, at its Keauhou and Maui Bird Conservation Centers. "Although we are very experienced in such work, it is always a challenge to attempt to rear a chick from a new species in a new place," said Susan Farabaugh, Ph.D., associate director of Recovery Ecology at the San Diego Zoo Institute for Conservation Research. "In the case of the aga, we were extremely successful. We had 100-percent hatch success and 96-percent rearing success."

The aga was driven to extinction on Guam by the brown tree snake, an invasive predator from Australia and Melanesia. The brown tree snake decimated most of the endemic bird population on Guam after it arrived there in the 1950s, but the reasons for the birds’ decline on Rota are less clear.

Predation by feral cats, persecution by humans, nest loss from typhoons, habitat degradation, inbreeding and disease are all considered threats to the birds. Over the past 22 years, the population of aga on Rota has decreased by more than 80 percent, and only about six out of 10 of juvenile aga survive their first year—a much lower percentage than in healthy populations of other crow species.

In 2005, the CNMI-DLNR asked the University of Washington to determine the causes of the population decline and the current status of the species. At that time, very few birds were banded for research, and there had been no consistent tracking of breeding pairs since the late 1990s.

An intensive nest searching and monitoring program was implemented, in addition to a radio telemetry program to determine causes of bird mortality. At the same time, federal biologists worked with island residents to encourage maintenance of bird habitat, especially through a landowner incentive program. With these efforts, the population appears to have stabilized—albeit at a very low number.

“This has been an ongoing learning process for everyone,” said Manny Pangelinan, director of the CNMI-DLNR Division of Fish and Wildlife. “This current effort highlights the importance of commitment to the long-term goal of bringing the aga back to a stable population.”

“Recovering threatened and endangered species takes dedicated partnerships,” said Mary Abrams, field supervisor for the U.S. Fish and Wildlife Service. “We are working to ensure the long-term survival of aga, but that can’t happen in a vacuum. There is a whole ecosystem at work here, and the cooperation that has brought us this far is a great example of the kind of work that is necessary in conservation.”

Aga in the wild have a high mortality rate for the first two years of their life, so the goal of the project is to boost the population of aga by helping birds survive to adulthood. The “capture, rear and release” approach has the potential to greatly increase the population, because the wild aga pairs will have the opportunity to lay another set of eggs.

“By closely monitoring nests of the Mariana crow, we realized we had the opportunity to intervene and double the output of a wild pair by pulling one clutch of eggs early in the season for captive rearing, and allowing the pair to produce a second clutch in the wild,” said Ha. The University of Washington and CNMI also formed a collaboration with the Smithsonian Institution and Binghamton University—a relationship that, in the future, may allow University of Washington scientists to target specific genetically healthy pairs for collection of eggs that San Diego Zoo Global biologists could rear and release.

“Each year, we break new records on the number of new birds that are banded and genetic samples that are collected, and the number of nests found,” said Ha. “So, we are getting better and better at understanding and studying this population.”

The project does not end with the release of the birds. Researchers will continue to monitor and track the released birds as they work to ensure a future for the aga. “As stewards of our islands' unique marine and terrestrial habitats and species, it’s vital that we work together to make sure that they thrive and can continue to enrich our lives in the future,” said Pangelinan.