Bald Eagle Diet Complicates Conservation Efforts

bald eagle profile photo

Image credit: Beverly & Pack/Flickr

Since the ban on DDT—an egg-shell thinning endocrine disruptor and pesticide—in 1972, the iconic bald eagle has battled it's way back from the brink of extinction. Now, conservationists have their sites set on difficult habitats, like California's Channel Islands, that have thwarted past reintroduction attempts.

A new study mapping the historical changes in the eagle's diet, however, shows that such efforts may be far more complicated than previously imagined.

bald eagle roost photo

Image credit: Peter Sharpe, Institute for Wildlife Studies

By mapping the concentrations of specific isotopes found in eagle bones, researchers were able to establish a rough understanding of how the diet of bald eagles on the Channel Islands has changed over the last 30,000 years.

In a prehistoric sense—from 1850 to as far as 30,000 years ago—eagles on the island fed primarily on large fish and smaller shore birds. This changed, however, with the introduction of ruminants—sheep and cows—in the middle of the 19th century. At this point, the survey showed, the birds' diet changed from primarily marine-based prey, to terrestrial prey.

Seth Newsome, who led the study, explained:

Reintroducing bald eagles to the Channel Islands has had mixed understanding of their diet is critical to successful reintroduction, so we looked for chemical traces of the foods the eagles consumed over many millennia. Since bald eagles are extremely opportunistic, they can quickly adapt to changes in the prey base of the diverse ecosystems they inhabit.

On the Channel Islands, however, this versatility may actually make reintroduction more difficult.

A Fragile Bounty

Conservationists believe there are three key sources of prey potentially attractive to bald eagles on the Channel Islands: seal or seal lion carrion, seabirds, and the island fox. The first option, however, is not likely because, much of the seal and sea lion carrion is contaminated with pollutants. This leaves local seabirds—which are threatened due to pollution and diminished coastal fish populations—and the already-endangered island fox.

Removing the invasive ruminants from the island was an intensive effort that took the National Park Service years to accomplish. The aliens took with them, it seems, a source of carrion that might have made immediate reintroduction of bald eagles possible.

Now, conservationists may have to wait for the island's fragile ecosystem to rebuild itself before the bald eagle can come home.

Read more about bald eagles:
The Bald Eagle is Back in the Black
Desert Nesting Bald Eagles Set to Lose Protected Status
Beheaded Bald Eagle Traced Back to National Eagle Morgue
Santa Cruz Island Bald Eagles Proud Parents of New Hatchlings, Web Cam Shows Happy Family

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