PORTLAND – Ten pairs of federally protected, endangered roseate terns are nesting on Outer Green Island in Casco Bay, the first time they have settled there in nearly a century.

The seabirds’ presence on Outer Green five miles from Portland is important given the bird’s declining numbers on other Maine islands.

The birds were probably attracted to Outer Green by a growing colony of common terns that have made their home on the rocky, state-owned island over the past two years, said Stephen Kress, director of the National Audubon Society’s seabird restoration program.

Lured by Audubon’s decoys and recordings of tern vocalizations, the population of common terns on Outer Green has soared to 510 pairs this summer, up from just 98 pairs last year. Such a large common tern colony provides good shelter for the rare roseate tern.

“There’s a lot of roseates that are sort of moving around right now, which made it ideal for them to find Outer Green,” Kress said. “I wouldn’t be surprised if there’s more before the summer’s up.”

This summer, for the first time, biologists have seen a decline in roseate tern nests at Eastern Egg Rock in Muscongus Bay, partly because of competition from laughing gulls. The number of nests dropped from 163 last summer to 109 this year.

The population also has been declining on Stratton Island in Saco Bay because of predation by a night heron and a mink. Stratton was home to 40 roseate pairs last year, but just 11 pairs this year.

There are also two pairs of roseates nesting on Jenny Island in western Casco Bay, and nine pairs on Pond Island at the mouth of the Kennebec River.

“Roseates are very susceptible to disturbance from predators,” Kress said. “They really get spooked. And their memory permits them to remember these events from one year to the next, so if there was a mink on Stratton Island last year, a lot of them won’t even try nesting there this year.”

A tern colony thrived on Outer Green during the late 19th century, but was driven out of Casco Bay by 1914 when herring and great black-backed gulls began taking over the territory.

The new colony on Outer Green is protected from predators by its remote location and a group of Audubon interns who live on the island throughout the nesting season.