ACADIA NATIONAL PARK – Two pairs of peregrine falcons have returned to the cliffs at Acadia to begin their annual spring courtship and breeding rituals.

Every spring for the past decade, two or three pairs of the endangered birds have nested on the park’s seaside granite mountainsides.

Acadia’s steep, exposed cliffs make a perfect falcon nesting ground because few land predators can reach the nests, and the falcons can spot approaching airborne predators – such as eagles or owls – from miles away, park biologist Bruce Connery said recently.

This year’s mild winter made many of the traditional nesting sites more accessible to hikers, however, so park officials have closed hiking trails near the Precipice more than a month earlier than in recent years, to protect the birds.

“We don’t have the ice and snow we usually do, and people have just been out a lot more,” Connery said.

Paths in the nesting areas, such as the popular Precipice Trail, and those on the east side of Champlain Mountain will remain closed until this year’s falcon chicks leave the nest, typically in mid-August, he said.

Throughout the spring and summer, the falcons become a major attraction. Over the next month, the birds will swoop around the cliffs, flying in patterns that send signals to one another as well as to other birds that might be in the area.

The falcons protect a wide zone around their nesting area long before the season’s eggs are laid, to assure that predatory birds and those that might steal eggs, such as ravens and crows, are not in the vicinity, Connery said.

“They’re very aggressive toward other birds of prey,” he said. “If there’s an eagle within a half-mile, they’ll see it and fly right out to it and attack.”

Given the opportunity, nesting falcons will kill any smaller birds that invade their territory.

Just before the breeding season begins, falcon pairs fly patterns around their chosen nesting cliff. Sometimes one bird will drop a fish or other bit of prey that it has caught, and its partner will catch the meal in midflight. At other times, the two birds fly at one another, and join their feet on the prey, Connery said.

“They’re strengthening their bond,” he said.

Sometime in April, the female falcons typically lay a clutch of eggs. Just over a month later, two to four chicks hatch.

Unseasonably warm weather could prompt the birds to breed earlier than usual, however, which would create problems once the chicks hatch. Falcons depend on the spring return of other bird species for ready prey to feed their chicks.

“If the other birds aren’t here yet, they’ve shot themselves in the foot because there’s nothing to feed the little ones,” Connery said.

Connery already has spotted falcons at Precipice and Valley Cove. Pairs typically arrive together at Acadia in February or March, and scientists have observed that the same male and female pair often breeds in consecutive years.

Even if their mates change, however, males tend to return to the same nesting grounds where they were born. Females occasionally fight, even to the death, for a superior nest site, Connery said.

During the winter, Maine’s falcons fly as far south as Mexico and Costa Rica, although Connery has heard unsubstantiated reports of some falcons that remained on the Cranberry Isles during this mild winter.

In the 1970s and 1980s, few falcons were spotted at Acadia, although its cliffs make ideal nesting sites. The birds were federally designated as endangered species, and the decline in their numbers was linked to the loss of habitat as well as the harmful effects of pesticides such as DDT.

In 1984, Acadia was selected as one of several sites in Maine where young, captive-bred falcons were released into the wild in hopes of re-establishing a native population. The first successful nesting pair was spotted in the park in 1991.

Over the past decade, several chicks have been born every year, and the Acadia program is considered a tremendous success, Connery said.

“It teeters between two and three pairs each year,” he said. “We’re probably at or above what has ever been expected.”