"Penguins are in trouble," Boersma, whose study appears in the journal BioScience, said in a telephone interview on Tuesday. "They certainly are canaries in the coal mine."

For the past 25 years, Boersma has tracked the world's largest breeding colony of Magellanic penguins located at Punta Tombo on Argentina's Atlantic coast. She said that since 1987 she has observed a 22 percent decrease in the population of these penguins at the site.

Boersma said the decline appears to have begun in the early 1980s after the population at the site peaked probably at about 400,000 breeding pairs of Magellanic penguins between the late 1960s and mid-1970s. Today's total is half of that.

The world's warming climate is only one of the causes of the penguins' problems, she said. They also are threatened by oil pollution, depletion of fisheries, becoming entangled in fishing nets, and coastal development that eliminates breeding habitats, according to Boersma.

"Penguins are sentinels of the marine environment, and by observing and studying them, researchers can learn about the rate and nature of changes occurring in the southern oceans. As ocean samplers, penguins provide insights into patterns of regional ocean productivity and long-term climate variation," Boersma wrote in the study.

Most scientists recognize 17 species of penguins, and they live in Earth's southern hemisphere. Penguins are beautifully adapted to life in the ocean, residing in places as different as the warm Galapagos islands and icy Antarctica.

While a bit ungainly on land, they gracefully knife through the water, feeding on fish and other sea delicacies.

But many species have been experiencing population declines in Antarctica, Africa, South America, Australia, New Zealand and the Falkland Islands, Boersma said.

The number of Galapagos penguins, the only species with a range that inches into the northern hemisphere, has slipped to around 2,500 birds, about a quarter of its total in the 1970s.

Anton Seimon of the Wildlife Conservation Society, which backed Boersma's work, said the findings illustrate the disruption that people have caused to penguins' ecosystems.

"These disruptions introduce instability into what had been somewhat stable populations. That instability means we don't really know what's going to be happening in the future. In many instances it does signify declines that may result, in the most drastic case, in extinctions," Seimon said. (Editing by Julie Steenhuysen and Eric Walsh)