Bald Eagles’ predation decimates Columbia’s Tern colony

Friday, June 10, 2011
Posted by: OWC

Harassed in recent weeks by bald eagles, the world’s largest Caspian tern colony has “collapsed entirely” with the last of some 5,000 nests plundered.

Repeated evening/night-time disturbances at the East Sand Island colony over the past few weeks by bald eagles has made the terns fidgety and willing to flush frequently. When the terns take flight and abandon their nests, gulls that share the island have been swooping in and robbing the nests.

“The real damage is caused by terns leaving their nests,” said Ken Collis of Real Time Research, who is co-principal investigator for the long-running research project along with Dan Roby of Oregon State University.

The lower Columbia River island’s double crested cormorant colony, which is also believed to be the world’s largest, has also been besieged this spring by bald eagles, peregrine falcons and great horned owls.

“…these disturbances and nest predation events coupled with heavy rainfall resulting in significant flooding on the colony has caused unprecedented nest failure at the tern colony; the number of active tern nests with eggs has declined from 5,000 to less than 500 over the past two weeks,” according to a research update. The updates are posted on Bird Research Northwest’s web site:

The last remaining egg-laden nests were looted “during a series of mid-morning flushes” on Wednesday, June 1, according to information updated through June 5.

“Individuals are still laying eggs, but nests rarely persist for more than 30 minutes.”

“Gull pressure on newly formed nests has been very intense and one individual was seen taking an egg from under an incubating tern.”

The level of “disturbance” caused by the bald eagles is unprecedented, according to researchers who have been monitoring the island since the late 1990s. The adult terns being taken by the eagles are being “completely consumed” except for the wings and head, Roby said.

Bird Research Northwest gathers data on the size, distribution, status, productivity, diet composition, smolt consumption, and factors limiting the size and productivity of various piscivorous waterbird colonies located in Oregon, Washington, and California. A primary goal is to evaluate the birds’ impact on salmon and steelhead stocks that are protected under the Endangered Species Act.

Aggregations of birds, Caspian terns and double-crested cormorants in particular, have found a home in the lower Columbia on manmade islands such as Rice and East Sand. Both islands were created with the deposition of materials dredged from the Columbia River navigation channel, which is maintained by the U.S. Army Corps of Engineers.

“I think it would be premature to write the epitaph” for this year’s East Sand tern reproductive season, Roby said. The researchers have seen chicks hatched out as late as the first half of July so persistent terns could still complete their task.

“The food has to be available and the predators have to cut them some slack,” Roby said.

The island’s double crested and Brandts cormorant colonies took a beating too in recent weeks.

“Groups of eagles observed walking through the colony during the day to take cormorant eggs,” the May 30 update says. Up to 70 percent of the double-crested cormorant nests west of a tower blind were abandoned, including almost all nests in satellite colonies; all cormorant nests on the rip rap east of the tower blind have also been abandoned.

“Double-crested cormorants are attempting to re-nest in these areas but face continued pressure from bald eagles causing adult cormorants to flush and leave nests unattended for entire day,” the May 30 update says.

But, last week “disturbances were relatively uncommon” at the cormorant colony and many of the big birds began to resettle at their original site on the west end of the island and at the West Jetty, which had not been used for several years.

The double-crested cormorant colony has shown steady growth in recent years and has become the island’s largest presence. Last year an estimated 13,600 breeding pairs nested on the island and they gobbled up about 19 million young salmon and steelhead, according to preliminary estimates made by the research team.

The research project is a collaborative project between Oregon State University, Real Time Research Inc. and the USGS-Oregon Cooperative Fish and Wildlife Research Unit. Support for this research project has come from the Bonneville Power Administration; the U.S. Army Corps of Engineers – Walla Walla District; Portland District; the Bureau of Reclamation; the U.S. Fish and Wildlife Service – Pacific Region, Migratory Birds and Habitat Programs; NOAA Fisheries; and the Northwest Power and Conservation Council.

Columbia Basin Bulletin

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