You have to hike a pretty long distance if you hope to see the critically endangered bird known as the palila (Loxioides bailleui), but if you're lucky and work hard, you can walk their entire habitat in a single day. That's because these beautiful yellow-headed birds live in just one place on Earth: the upper slopes of the dormant Mauna Kea volcano on the island of Hawaii.

A century ago Palila habitat included most of the volcano as well as large areas of the western half of the island but today they live in just 5 percent of that range. Their decline is linked to that of another species, the māmane tree (Sophora chrysophylla), the immature seeds of which make up the bulk of the palila's diet. The palila is the only species that can eat māmane, which pack a potent poison that kills just about anything else.

The trees are vulnerable to feral goats and sheep, which overran the Hawaiian islands centuries ago. They long since ate all of the māmane at elevations lower than 2,400 meters above sea level. Efforts to control these invasive ungulates have helped over the years but both the trees and the birds still have a long way to go before they can even approach recovery.

A massive new research document published this month by the U.S. Geological Survey (USGS) will help the conservation effort. At nearly 500 pages, "Palila Restoration Research, 1996–2012" (pdf) is a comprehensive report on the palila's ecology, how it breeds, how it selects and behaves in its nests, what it eats and the many threats the birds face, which includes predation by invasive rats and cats. It also includes information on efforts to restore the birds in other parts of the island. The combined research makes the palila, according to the USGS, the most heavily studied Hawaiian bird species.

Scientists could reach this level of detail because the palila currently has such a restricted range, explains USGS wildlife biologist Paul Banko, the co-editor of the report and the project leader for palila restoration efforts. "Because the species is now restricted to the southwestern slope of Mauna Kea Volcano, which has a relatively extensive system of dirt roads, it is possible to probe most areas of the palila's range during a day's outing , although that does not guarantee that you will see one without a good deal of hiking," he says.

The bird's small range and smaller population—currently estimated at about 1,300—made it "an excellent candidate for nearly every sort of ecological study," Banko says. That said, it still wasn't easy work. "Everything we learned about the palila came only with considerable effort, time and expense."

A good deal of the work by Banko and others during that time focused on reintroducing the birds to habitat they had previously been forced to abandon. "Some areas of former range were slowly recovering from long-term browsing damage by introduced ungulates, providing us with opportunities to assess what would happen if we translocated wild birds back or released captive-reared birds," he explains. But even though removing feral sheep helped improve situations, a long-term drought took further toll. The palila population had grown to a high of 6,463 individuals in 2003 but plummeted soon after.

Part of the problem, Banko says, is that feral sheep are still managed as a game species, so they haven't all been removed from the palila's habitat. About 47,000 sheep were removed from the Mauna Kea Forest Reserve during the 1930s and 1940s but the ones that remained continue to have an impact on māmane trees, which take decades of growth to reach the sizes large enough to attract nesting palila. "The key to stopping the palila's decline, therefore, is to promote faster habitat recovery by removing all sheep from palila critical habitat," he says.

The past few years have been better for palila. The drought has lessened and the population decline has leveled off, Banko says. Even under ideal circumstances, however, the birds still reproduce slowly. As a result, full restoration is still probably decades away. The lessons they have already learned will be enough, he hopes, to save the species from extinction.

Still, the need to preserve what is left of the palila habitat is urgent, Banko says. He hopes the new report will help: "We believe that a more complete knowledge of the palila's life history will help everyone appreciate the need to protect their natural heritage."

Photo: James St. John via Flickr. Used under Creative Commons license

Previously in Extinction Countdown: