For the second time in as many days, a new report highlights trouble for American birds. While a study by the National Audubon Society points out threats from future climate change, a new multi-agency report says that many U.S. birds are already in decline. (See "Climate Change May Put Half of North American Birds at Risk of Extinction.")

The current state of birds in the U.S. is a mixed bag, according to a report released Tuesday morning by the North American Bird Conservation Initiative U.S. Committee. Some species, like the bald eagle, have recovered enough to be taken off the U.S. list of endangered species. Others, like all 33 native Hawaiian bird species, are either endangered or are likely to become endangered.

Human activities can have enormous impact on bird species—even the abundant ones, said Pete Marra, head of the Smithsonian's Migratory Bird Center in Washington, D.C., at a press conference.

Birds in the U.S. are hit with myriad threats, including invasive species introductions, new diseases, habitat loss, and climate change. Conservation efforts and habitat restoration can still save birds, Marra said, but must be implemented quickly.

The passenger pigeon is perhaps the most famous example of a common species driven out of existence with breathtaking speed, and the report marks the hundredth anniversary of their extinction. Flocks of billions of the birds darkened skies in the mid-1800s. Hunters slaughtered them until the last passenger pigeon was spotted in the wild in 1900. The species clung to life in captivity until 1914 when Martha, the last of her kind, died in the Cincinnati Zoo. (See "Century After Extinction, Passenger Pigeons Remain Iconic—And Scientists Hope to Bring Them Back.")

The current report is the fifth annual "State of the Birds," and is the latest to take a comprehensive look at the status of birds in the U.S., explained Dan Ashe, director of the U.S. Fish and Wildlife Service, in an interview. Researchers used long-term bird population data from 1968—or 1974, in the case of shorebirds—to 2012 to make their assessments. That included data sets like Audubon's annual Christmas Bird Count and the U.S. Geological Survey's Breeding Bird Survey.

In a State

The report authors classified birds according to their habitat: grasslands, arid landscapes like chaparral and deserts, forests, wetlands, coasts, open ocean, and islands. Every habitat saw declines in bird populations with the exception of wetlands.

"The good news is wetland birds," said Marra. Over 80 species have shown population increases since 1968. And those increases aren't being driven by a couple of species, he added. "This is being driven by a large group of species."

The encouraging picture is due to conservation and habitat restoration efforts in the nation's wetlands, Marra explained.

However, the overall picture for birds leaves plenty to be concerned about. All the other habitats examined by researchers saw continued declines in bird populations, with islands being especially worrisome. (See all the birds on the report's "Watch List.")

Invasive species such as cats and snakes, and introduced diseases such as avian malaria, have taken a heavy toll on island birds. Hawaii's native birds have been especially hard hit, comprising one-third of all U.S. federally endangered bird species. Hawaii can be considered the "extinction capital of the world," Marra said. (Watch a video on the endangered state bird of Hawaii.)

View Images A long-billed curlew—also on the report's "Watch List"—skims across a wetland in California. Photograph by Hal Beral, Corbis

Health Conscious

Despite successful conservation attempts and policies like the U.S. Endangered Species Act (ESA), it's better not to let bird populations get to the point of needing such protections, said Marra. Not only are healthy bird populations an indication of a healthy environment, they're also beneficial economically, he said: Conservation efforts can get expensive.

Species like the California condor and the bald eagle have been helped immensely by the ESA, said the Fish and Wildlife Service's Ashe. "But the Endangered Species Act is like the emergency room," he said. Comprehensive health care strives to keep costs down by avoiding an emergency room in the first place.

We should all be striving to keep bird species from ending up in the emergency room, Ashe said.