Tony Cenicola/The New York Times

Bald eagles tend to eat salmon carcasses that wash up on the river shoreline

April 12, 2023

Bald Eagles, Dairy Farmers Share Symbiotic Relationship, Cornell Study Finds     

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A recently-published study, “A Win-Win Between Farmers and an Apex Predator: Investigating the Relationship Between Eagles and Dairy Farms,” by Ethan Duvall grad investigates the relationship between dairy farmers and bald eagles. This newly-discovered relationship will increase human-bald eagle interactions across various contexts.

Duvall, who studies ecology and evolutionary biology, began his research in Washington state, where he found that bald eagle populations concentrated around the Nooksack River migrated to nearby dairy farms. The dairy industry produces natural byproducts, such as cow placenta and carcasses. Although waste to the farmers, these byproducts offer nutrients and energy to eagles when they are unable to find the food they need at the river.

However, eagles are not intentionally migrating to farms. 

“It’s not just that they’re moving directly to the farms — they’re dispersing away from the rivers to these other human-dominated areas where anthropogenic resources are available,” Duvall said. 

The exact reason for the decrease in salmon is still unknown, but Duvall attributes it to a response to hydrology, the study of the movement and distribution of water, and temperature.

Bald eagles traditionally scavenge for salmon carcasses when they wash up on river banks, because they provide an ample supply of food during cold winter months. However, due to global warming, the timeline of the salmon’s life cycle is changing dramatically. 

Climate change is disrupting the life cycle of salmon and timeline of river water levels, as the salmon would previously end their life cycle when the river water receded, allowing the bald eagles access to their carcasses. This is no longer true, as the salmon are completing their life cycles earlier, when the water levels are at their winter highs, therefore causing a decline in the number of salmon carcasses that the eagles are able to obtain and leading them to search for food in other places, like dairy farms. 

“Even if you have the same amount of salmon in the river that are actually spawning, the amount of carcasses that are being available to the eagles is being reduced,” Duvall said. 

Duvall further explained that, in the past 50 years, salmon have been returning to their spawning grounds, or birthplace, to die a month earlier — though the reason for this phenomenon is still unknown — indicating that salmon are now dying at different periods of the river’s cycle.

“So you see a total reduction in salmon carcasses nonetheless,” Duvall said. “And that’s forcing eagles that have historically relied on this abundant food source to seek other food sources during the winter, which are the hardest periods to survive.” 

As a result, eagles have adapted to seek food from anthropogenic, or man-made, structures like farms and landfills rather than riverbanks. 

This increase in interactions with humans is having observable effects on bald eagle populations. For example, since bald eagles are now more likely to inhabit areas populated with humans, unprecedented rates of avian flu have been observed, killing large numbers of bald eagles.

There has also been an increase in lead poisoning among eagles in the Pacific Northwest, as these birds are scavenging in locations where hunters use lead bullets, such as forests. 

The direct impact of humans on bald eagles will increase as bald eagles become more exposed to anthropogenic structures as a result of rising global temperatures. 

“It’s not just as simple as [the bald eagles’] moving to the farms. It becomes a much more complicated interaction between [humans and] eagles across the board now,” Duvall said.       

The future of these birds seems to be heavily intertwined with that of humans as these birds inhabit human populated areas, which Duvall said makes this interaction much more complicated and important to understand.

“For a half-century, as we’ve continued to put effort into conserving eagles, [we are] going to be continuing to discuss and make informed management decisions that enable conservation of eagles but also potentially maximize benefits for humans,” Duvall said.