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Growing number of bald eagles find murres easy meals

Growing number of bald eagles find murres easy meals

The recovery of bald eagle populations in Oregon is an environmental success story that has resulted in a resurgence of this iconic symbol in the state, which is good news – unless you happen to be a common murre living at the coast.

Scientists at Oregon State University who are studying the seabird have documented how the increase of bald eagles – especially along the central Oregon coast – is having a significant impact on the murre’s reproductive success. It is developing into a fascinating ecological tale of which the ending has not yet played out.

What has happened, the researchers say, is that bald eagles have taken up a seasonal residence near Yaquina Head and forage on the murres, which have a major nesting colony there. The predation of an occasional adult murre isn’t the issue, the researchers point out – it is the encroachment of “secondary predators” that is having a negative impact on the murres’ reproductive success.

“An adult eagle that swoops down and grabs an adult murre may disrupt the colony for a minute or two, but things get back to normal rather quickly,” said Robert Suryan, an OSU seabird expert at the university’s Hatfield Marine Science Center. “The problem arises when the eagles – especially juveniles that are not yet accomplished hunters – land on the colony and send the adult murres scurrying.

“That opens the door for brown pelicans and gulls to come in and grab the eggs, or even the murre chicks, and the results are pretty devastating,” Suryan added. “They literally will destroy hundreds of eggs in just a few minutes.”

The OSU-led project is supported by the Bureau of Land Management, the Yaquina Head Outstanding Natural Area and the U.S. Fish and Wildlife Service.

Suryan and his colleagues conducted studies of the Yaquina Head colony in 2007-10 and documented reproductive success of 55 to 80 percent – even with some eagle disturbance. By 2011, however, when more eagles began hunting at this colony, that success dropped to 20 percent. And it has gotten worse since after brown pelicans arrived last year.

Cheryl Horton, an OSU graduate student working with Suryan on the project, said the eagles affect the colony in other ways as well.

“When juvenile pelicans or eagles land on the rocks, all of the birds scatter,” said Horton, a master’s candidate in fisheries and wildlife. “We documented some 300 murre chicks that washed up dead on the beach last summer after a single pelican disturbance. They no doubt panicked and slipped off the rock and weren’t yet able to swim.”

Horton said in past years, one or two bald eagles would perch in the trees above Yaquina Head and swoop down to prey on the murres. This year, the number has grown to as many as a dozen – many of them juveniles.
The eagles’ appearance is a reflection of protective measures adopted more than three decades ago, Horton said. In 1978, researchers documented 101 bald eagle breeding sites in Oregon; in 2007, that number had climbed to 662 sites.

Suryan said the eagles’ predation hasn’t had an apparent impact on the overall population of murres at the colony, but if the reproductive failures of the past couple of years continue, that will change.
“During the past 2-3 years, we are not only seeing more eagles, but the disturbances are lasting longer – into July – and more juveniles are hanging out at the colony,” Suryan said. “The implications really are quite interesting. Is the predation of the eagles on murres a learned behavior, or are they missing another food source?

“In Alaska, eagles feast on dead salmon on the streambanks, but when salmon numbers are low, they head over to the coast and decimate seabird colonies,” added Suryan, an associate professor of fisheries and wildlife at OSU. “What we’re seeing at Yaquina Head could just be a natural rebalancing of predators and prey as eagles recover, or it might be that the eagles are recovering into a system that is different than the one they previously occupied.”

As Yaquina Head is turning into an outdoor laboratory for this evolving ecological puzzle, the researchers are learning more than they ever imagined, Horton said.

“We captured video of a pelican grabbing a murre chick and shaking it until it regurgitated a fish that its parents had fed it,” Horton said. “Then the pelican dropped the chick and gobbled down the fish. Why were juvenile pelicans doing this? It seems like such a desperate way of finding food.”

Photo: Hatfield Marine Science Center

 

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