On a morning late in June, mist drips from the needles of Douglas-fir and western red cedar on the outer coast of Washington, near Cape Flattery, the uttermost end of the earth. The cries of gulls and seabirds echo above the slow drumming of the surf in sea caves eroded from the cliffs. The fog horn on Tatoosh Island plays counterpoint.
On the dead branch of a Douglas-fir a Bald Eagle perches, its feathers glistening with salt spray from the surf. The tree might have died by blight or fire but still stands above the surrounding forest. It offers the eagle a commanding view of Tatoosh, little more than a half mile offshore. (The eagle’s relentless vision can pluck a rabbit from a field of broken cover or a river otter foraging along the tide line a mile away.) The tree’s branches are spaced widely enough to accommodate the bird’s seven foot wingspan, forming flight paths between the dead limbs, and still provide a lee when the wind freshens from the south.
From it’s perch the eagle watches Rhinoceros Auklets, Double-crested Cormorants, and Common Murres come and go, foraging at sea and returning to feed their unfledged young nesting on the island, until its own hunger becomes compelling. Hunger shapes the eagle’s behavior. Hunger drove it south from Whitehorse, in the Yukon Territory, when ice began thickening on the rivers, and hunger will drive it north again when the bodies of spawned salmon began to stack like driftwood on the rivers’ gravel banks.
With widespread wings the eagle slips from the dead limb and glides past the forest’s edge, above tide rips churning the narrow channel between island and shore, above the fenced graveyard where the lightkeeper’s children are buried, to Burning Barrel Point where the murres nest in dense rookeries. The eagle’s presence ignites panic in the rookeries. Adult murres abandon their nests and flee toward the water. If they reach the sea they are safe from the eagle’s hunger.
The Glaucous-winged gulls and Northwestern Crows have been loitering with a purpose at the rookeries’ edge. When the eagle flushes the adult murres, they move quickly among the crevices and salmonberry bushes, plucking unguarded eggs or helpless nestlings and carrying them away. It is heartless but efficient.
Although only 2% of the murres that breed on Tatoosh Island fall prey to eagles, their population has steadily declined 3% per year since 1991. The decline is not a direct effect of eagle predation but rather an indirect one. Indirect effects occur "when the impact of one species on another requires the presence of a third species." One species changes the intensity of the relationship between two (or more) other species. On Tatoosh, the increased presence of Bald Eagles has made the murre rookeries more vulnerable to scavenging by gulls and crows.
Indirect effects are complex and difficult to predict but they may account for half of the interactions in ecosystems. The ascendance of the Bald Eagle leads to the decline of the Common Murre. Who knew?