Murre-Eagle Interaction

ARTICLE Intro Eagles and Murres by RL 512x219

As Oregon's Bald Eagles prosper, nesting Common Murres take a beating. How does a resurgent raptor spell reproductive disaster for a colonial seabird? It's not as cut-and-dried as you might think.

Bald Eagles in the lower 48 states have made a remarkable comeback in the last forty years. Thanks to the passage of protective legislation banning or restricting the use of noxious chemicals such as DDT, along with massive conservation efforts nationwide, populations south of Alaska have rebounded from an all-time low of 417 nesting pairs in 1963 to almost 10,000 pairs in 2007. That year, Bald Eagles were officially removed from the federal Endangered Species List.

Breeding eagles have recently returned to Oregon’s north and central coasts, much to the delight of conservationists. In 1978, only 101 active nests were documented throughout the state; by 2007 there were 661. But the eagles’ recovery is nothing but bad news for their prey species, one of which is the Common Murre.

Colonies of murres nest on Oregon’s offshore rocks and islands during the spring and summer. These crowded, exposed rookeries make obvious targets for hunting eagles. At Newport’s Yaquina Head, one of the largest murre colonies in Oregon has attracted the interest not just of eagles but seabird researchers at Oregon State University. Their studieson the impact of increased eagle presence on murres’ reproductive successcombine recent field observations with prior census data taken by U.S. Fish and Wildlife Service and U.S. Geological Survey biologists. 

Led by OSU associate professor Rob Suryan and graduate researcher Cheryl Horton, a seven-year survey conducted at Yaquina Head found that, from 2007-2010, murre reproductive success—their ability to raise chicks to fledging age—ranged between 55 and 80 percent, even with a few eagles present. But by 2011, as both eagle populations and their predatory disturbance increased near the colony, that success dropped to 20 percent—meaning some 80 percent of murre pairs failed to rear young. Last summer only 0.17 chicks per pair were raised there, the lowest rate recorded to date.

The researchers found that this sharp decrease wasn’t simply a result of eagles killing parents and chicks. Eagles, however voracious, can only eat so much murre. More devastating was the disturbance wrought by their predatory swoops: each time an eagle alit on the colony, terrified murres fled to the water, leaving their eggs and chicks exposed to so-called “secondary predators” such as gulls, ravens and, increasingly, pelicans. Sometimes, in the mad dash to escape the eagles, not-yet-fledged chicks plummeted from the rocks and drowned, unable to swim.

It remains to be seen what sort of long-term effects emerge from this interaction. Murres reach sexual maturity at age 4 to 5 and can live more than twenty years—a long window of fecundity. The population of breeding adults hasn’t diminished significantly with the eagle resurgence. But they’re an aging flock of baby boomers—if these adults consistently fail to rear young, there will be no recruitment, no future generation of teeming alcids. The murres, for their part, seem content to relocate their colonies away from the disturbance, wherever it occurs. But how long until the eagles find these new redoubts and commence their raids?

For abstracts of these studies, go to OSU’s Seabird Oceanography Lab page.