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Climate change washed away bald eagles' food, research says. Now these eagles are becoming farm birds.

Bald eagles have long congregated in winter along waterways in the northwest corner of Washington state, taking advantage of the abundant food as chum salmon spawned, died and were washed onto stream banks. But climate change is forcing the eagles to adapt.

Now, those dead salmon are mostly gone – literally swept away by the effects of climate change. But the apex predators have pivoted to in-farm dining.

They've transitioned from feeding along rivers to patrolling farms – feasting on dairy farm discards rather than dead salmon. Now their preferred dishes are cow placentas and stillborn calves.

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Here's what's happening to the bald eagles:

What did the bald eagles eat before?

Salmon die after they spawn, providing a rich source of food and nutrients for local ecosystems, including bald eagles.

  • How it used to work: Dead salmon gently washed up onto river banks for the eager eagles.

  • What's happening now: The salmon carcasses are being swept downstream by winter high waters.

  • Why? Salmon are spawning earlier because the rivers and streams have warmed. And winter high waters are occurring at a different time too.

  • The end result: Dead salmon are no longer an easy food source for bald eagles.

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Many eagles counted on salmon for food

"The eagle congregations along the rivers are truly one of the coolest things to experience up here," said Ethan Duvall, who's been studying them for over a decade.

"On a peak day, we observed over 600 eagles on a stretch of the Nooksack river. It was absolutely amazing," said Duval, a Ph.D. candidate in ecology and evolutionary biology at Cornell University. He is a co-author of a recent paper about the phenomenon in the journal Ecosphere.

What happened when the salmon supply was swept away?

The number of eagles along the river started to drop.

When he investigated, Duvall found climate change was shifting things. Historically, the chum salmon would navigate upriver during the high water events and then spawn after the waters had receded.

But not for nothing do we use "eagle-eyed" to mean observant. With fewer salmon to eat, the eagles looked around for other food and found it in the rich dairy farms of western Washington and southern British Columbia in Canada.

Dairy cattle are always giving birth, meaning local farmers always had placentas and still-born calves to dispose of. When they put them out into their fields, the eagles discovered a new feasting area.

"The dismantling of a carcass occurs in about 48 hours between coyotes coming in at night and eagles during the day," said Karen Steensma, an author of the paper and professor of biology at Trinity Western University in Langley, British Columbia.

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"I've heard people talk about 50 large raptors in their fields. It's a really efficient cleanup," said Steensma, who studies wildlife interactions with agriculture and whose family has a small dairy farm on the Washington side of the border.

Is it good for farmers too?

It's a win-win for the local dairies, which have less waste to either compost or have hauled away. In addition, the eagles deter and eat birds and rodents that might come in and contaminate or eat feed stores.

It helps the eagles because it provides an important and abundant food source at the peak of winter when it's hardest for them to survive and they typically have the highest rates of mortality.

A still-born calf can weigh up to 90 pounds while a placenta is about 20. "That's a significant amount of food," said Steensma.

"This study gives me hope," said Duvall, that "moving forward, farmers, wildlife managers, and conservationists can come together to think critically about how to maximize benefits for people and wildlife in the spaces they share."

The paper published in March in the journal Ecosphere.

This article originally appeared on USA TODAY: Bald eagles adapt after climate change diminishes salmon: Experts