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P. Dee Boersma, a biologist at the University of Washington, has been watching the Magellanic penguins of Punta Tombo, in Argentina, for almost 30 years. For most of that time, their numbers have been declining: breeding pairs are down 22 percent there since 1987, she writes in Tuesday’s issue of BioScience.

But the dwindling numbers do not just mean the birds are suffering, Dr. Boersma writes. Because penguins are “marine sentinels,” their decline is a blunt message that their marine environment is in trouble, chiefly from overfishing and pollution from offshore oil operations and shipping.

Now, though, Dr. Boersma writes, they are also threatened by climate change, which is reducing sea ice and, as a result, the abundance of the marine creatures the birds eat. Magellanic penguins can swim almost 100 miles a day, she said in an e-mail message, but to get enough to eat now they must venture as much as 40 miles farther from their nests than they did a decade ago.