A U.S. Forest Service biologist confirmed Monday, days after glacial drainage brought high water to Mendenhall Lake, that the Mendenhall Glacier Visitor Center’s famed nesting colony of Arctic terns failed this year — it was gone before the flooding even started.

Wildlife technician Gwen Baluss, who surveys bird populations at the lake, said the Arctic terns abandoned their nests just days before rising waters covered their nesting grounds in front of the Visitor Center.

“It’s possible that a few eggs and young birds were washed away, but probably not, because the terns just didn’t have a good year this year,” Baluss said. “They started their nests very late, and they were experiencing problems with predators, such as ravens.”

When Baluss did her survey of the Arctic tern colony on June 24, she said she saw no tern eggs or chicks at the three nests of which she had been keeping track, but observed raven tracks around some of the nests. She also said she heard at least one report on June 28 of a visitor claiming to have seen ravens eating eggs.

“Ravens will eat eggs and young chicks that can’t run away,” Baluss said.

The raven problem, Baluss added, has been especially severe this year.

“The ravens this year are more numerous and aggressive around the Visitor Center than they have been in the past,” said Baluss. She said visitors feeding the birds, despite food at the Visitor Center being prohibited, could be to blame, adding, “It’s not a good idea to feed wildlife, including ravens.”

Baluss reported seeing some 25 Arctic terns on her survey June 28, with some landing in areas out of her sight where she suspected there might be additional nests. When she did her survey last Wednesday, she said, she saw no Arctic terns at all.

The next day, rising waters caused by the glacial drainage event, known as a jökulhlaup, probably wiped out whatever remained of the terns’ nests, said Baluss. She said she does not know if any eggs or chicks remained at the colony by then, but does not believe any terns born at the colony this year survived.

“I think most of them were predated or abandoned when they were still eggs, or perhaps very newly hatched,” Baluss said.

The Visitor Center affords an uncommon opportunity for both researchers and members of the public to view Arctic terns, a species which prefers to nest on the ground in open, even barren areas. The birds’ nesting grounds are fenced off to prevent people or pets from disturbing them.

“It is a really unique opportunity to witness the breeding cycle of a very spectacular bird,” said Visitor Center interpreter Laurie Craig. “They seem to associate this place with enough cushion of safety that they will let people get close to them.”

Baluss said many bird species, including mew gulls, spotted sandpipers and semipalmated plovers, live near the Visitor Center. But, she added, the Arctic terns are “kind of the star attraction out there.”

Last July, another jökulhlaup flooded low-lying areas around Mendenhall Lake. That time, the Arctic terns had also left their nests by the time high water arrived, but not because their colony failed.

“Last year, the case was that they already had some young birds that were ready to fly and probably left the area of their own accord before that,” said Baluss. She added, “I estimated that they had … at the most, six around the Visitor Center and probably five across the lake. So there were potentially 11 young last year, and they would have left the area before the jökulhlaup. So last year wasn’t a bad year.”

The Arctic tern is notable, said University of Alaska Southeast assistant professor of marine biology Heidi Pearson, for having the longest known migration of any species. The birds winter annually in the Southern Ocean, near Antarctica.

“Every year, they do this migration pole-to-pole, which is pretty amazing,” Pearson said.

Arctic terns have been nesting at Mendenhall Lake since at least 1945. Baluss said the terns moved to the Visitor Center site about 20 years ago and she thinks they will be back despite this year’s failure.

“I would be very surprised if they didn’t try again next year,” Baluss said. “But their rate of success has not been real high in the last five years, so that doesn’t look good for the long-term health of the colony.”

• Contact reporter Mark D. Miller at 523-2279 or at mark.d.miller@juneauempire.com.