What’s happening to migratory seabirds? Biologists are worried about a twofold problem: Commercial fishing is reducing their food source, and climate change is causing fish to seek colder waters, according to a bulletin released Tuesday by the U.S. Fish and Wildlife Service.

“We’ve seen a 40 percent decline of Arctic terns in the last 10 years,” said Linda Welch, a Fish and Wildlife Service biologist at the refuge. Arctic tern pairs in Maine have fallen from 4,224 pairs in 2008 to 2,467 pairs last year, the Fish and Wildlife Service said.

Biologists at the Maine refuge are not sure whether herring sought colder waters elsewhere or went deeper, but they are no longer on the surface, from which Arctic terns pluck them. While other birds can dive deep for food, Arctic terns cannot.

“They’re not getting herring, so they bring butterfish that the chicks can’t swallow,” Welch said. “So they starve to death. You have thousands and thousands of chicks dying. It’s very sad.”

On the Machias Seal Island, the largest tern colony on the refuge’s 50 islands, a shortage of fish prompted 3,000 pairs to abandon their nests in 2007. “They haven’t raised any chicks since,” Welch said.

Arctic terns arrive at the Maine islands after a month of flying from the Antarctic, about 470 miles a day — 14,000 total — low on energy, longing for a bite. If they lack food and energy, “they can’t keep the gulls off them,” Welch said. Gulls eat terns.

In the past two years, Welch said, biologists at the refuge went to the most productive foraging grounds where seabirds, whales and dolphin prey on herring, and spotted fishing trawlers.

“When [the trawlers] come out, the whales and birds disappear, and you don’t see them again,” Welch said. “I think it’s hard to deny that they don’t have an effect on these birds.”

Recently the Mid-Atlantic Fishery Management Council lowered the amount of herring and shad that trawlers can take next year in an effort to save the species. The effect on seabirds was not a prominent factor in that decision.

Migratory birds “are among the most vulnerable groups of species to climate change,” said Doug Inkley, a senior scientist for the National Wildlife Federation who studies them. “Migratory birds need suitable areas to breed when they’re migrating and on their wintering grounds. Through climate change, if you affect any of these habitats, you’ve broken a link in the chain. That puts the species in peril.”

Climate change also threatens a shorebird, the red knot. As temperatures warm, they are leaving the southern tip of Brazil later for a 9,000-mile journey back to their Arctic breeding ground. Timing is key, because red knots might miss the peak of Delaware’s horseshoe crab spawn, where they gorge themselves on eggs and double their weight.