A double-crested cormorant drying its wings. The birds dive for fish, and must dry afterward. (Special to the Star Tribune/The Minnesota Star Tribune)

Double-crested cormorants -- large, migratory, fish-eating birds that nest in colonies at this time of year -- have returned to the same island on Lake Waconia for years.

Not in 2012.

University of Minnesota researcher Linda Wires spotted only two of the protected birds when she flew over Coney Island late last month. That's down from 470 cormorant nests -- each with two birds -- in 2010 and 324 nests last year.

"I would have expected at least some to come back," said Wires, who since 2004 has been monitoring the 32-acre island in Carver County. "It's very odd."

Speculation is that sharpshooters hired in past years to legally reduce the bird's population -- long viewed as a nuisance by anglers who say they eat too many fish -- worked a little too well: More than 900 cormorants were shot in the past two years. Or that partygoers who sneaked onto the private island played loud music, set off fireworks or otherwise disturbed the birds.

Whatever the reason, some are just glad they're gone.

"Since we're not seeing the big [cormorant] flocks this year, the bait fish will be in better shape, they won't get eaten up by the birds, and I think it will be a good year for fishing on the lake," said Waconia City Councilman Jim Sanborn.

So many birds have nested in the area in recent years that more than 3,000 Waconia residents signed a petition asking that private landowners be allowed to shoot them. That prompted two Minnesota congressmen to introduce a bill, heard in committee in late March, to give the state more leeway in controlling the federally protected birds.

Legal kills since 2008

Lake Waconia, which hosted the Governor's Fishing Opener last weekend, has a rich diversity of fish species. The trees on Coney Island are also full of other fish-eating birds, including great blue herons, great egrets and black-crowned night herons. Those migratory species are back on the island and nesting as usual this year.

Wires, who works in the Department of Fisheries, Wildlife and Conservation Biology, said she's mystified about what happened. Cormorants typically return to nest in the same place each year, she said, and have done so this year at several other places in Minnesota that she monitors, except for Lake Waconia.

It's unclear if the cormorants simply didn't show up this year, she said, or arrived and for some reason chose not to stay.

As for the ire the birds have earned from anglers, state natural resource officials have said there's no evidence that cormorants and other fish-eating birds in the area have hurt fish populations.

Although the bird is protected, the U.S. Fish and Wildlife Service grants permits allowing cormorants to be shot if they're damaging private land.

Norman Hoffman, owner of Coney Island, has claimed that dung from the birds is killing trees, and has received permits since 2008 to have the birds killed legally. The permits require that federal sharpshooters do the work, because they're trained in how to minimize disturbance to protected egrets and herons that often share the same trees with cormorants.

Sharpshooters killed 187 cormorants on Coney Island in 2008, 368 in 2009, 600 in 2010, and 309 in 2011, according to records.

Hoffman paid for the work and carcass disposal, which cost nearly $3,500 in 2011.

Two theories

It's possible that the sustained killing during the past four years has broken the cycle of cormorants returning to the island each year, said Gary Nohrenberg, Minnesota director of Wildlife Services, part of the U.S. Department of Agriculture. But it's also possible, he said, that some sort of disturbance on the island may have occurred in April just as the birds returned.

Because no one lives on the island, he said, it occasionally attracts partygoers. "People may have been out there doing certain activities that at the right time, if that occurs, could result in the abandonment of a colony," he said.

Nohrenberg said that he and a team of sharpshooters went to the island last May and were surprised to find fewer birds than expected. Days later they heard stories of partying on the island, including fireworks, that occurred a few days before their visit. If true, he said, it probably scared birds off for a few days, resulting in fewer being available to shoot.

Wires has counted cormorants on Coney Island for the past two years, before and after the legal killings. She said it's common for cormorants that escape the shooting to return, and perhaps hook up with other birds that are nesting late. Last year she counted 77 nesting pairs on Coney Island in late May, after the killings.

Access denied

Wires said she would like to explore the island to look for clues about what might have happened to cormorants this year, but Hoffman has refused to give her permission. Hoffman did not respond to requests for an interview.

Hoffman also changed his mind about having sharpshooters kill the birds this year, after requesting and receiving a permit in February.

Nohrenberg said he called Hoffman in mid-April to confirm that he'd be back in early May to shoot the birds.

"The landowner informed me that he'd decided not to do it, and that was that," Nohrenberg said. "The birds hadn't really even come back then, so he would have no way of knowing what was going to be present or not."

Steve Lewis, biologist in the Fish and Wildlife Service's Division of Migratory Birds, said he informed Hoffman that he would need an amendment to the permit if anyone else was going to kill the birds. "We have not received any word from him," Lewis said.

Lewis said it's a "real big puzzle" that almost none of the cormorants are nesting on the island, and that this year's odd spring weather adds yet another complicating factor. "There's just a lot we don't know about the behavior of these birds," he said.

Sanborn doesn't know what happened to the birds, but thinks they may have simply moved to a different lake, where they'll just become a problem and a nuisance for someone else.

Waconia city administrator Susan Arntz said she was too busy working on the Governor's Fishing Opener to pay much attention to the birds.

"Just because they're not here now doesn't mean they won't be here later," she said. "It's going to be interesting to see a month from now what we hear from the fisher folks, and whether they've seen any changes."

Tom Meersman • 612-673-7388