Researchers have concerns about the relation between the bald eagles’ rise and a receding blue heron population in the state.

by Olivia Wilson, Community News Service After years of absence, the most patriotic bird in the sky returned to Vermont — but it might’ve come at another’s expense.

Vermont finally took the bald eagle off of its endangered species list in 2022 following years of reintroduction efforts starting in the 2000s. Since that reintroduction, researchers have concerns about the relation between the bald eagles’ rise and a receding blue heron population in the state.

“Much like many predator reintroductions, there are controversies and conflicting views in the bald eagle project,” said Allan Strong, dean of University of Vermont’s Rubenstein School and professor in the wildlife and fisheries biology program. 

“Missisquoi National Wildlife Refuge has reported that their blue heron populations have dramatically dropped,” he said. “Bald eagles are not killing these birds, but rather they are bullying and harassing them out of their territories in Vermont.” 

Revival efforts

Since the late 1970s, a number of states across the Northeast added the bald eagle to their endangered species lists and began the work to take it back off. Vermont, however, took a while to catch up. 

In 2003, Vermont was the only state in the lower 48 without any breeding bald eagles. A year later, the Vermont Department of Fish and Wildlife launched a three-year reintroduction project at a wildlife management area in Addison County. By 2006, 29 young eagles were back in the sky. 

Today there are 45 bald eagle breeding pairs — or couples — being monitored, said Margaret Fowle, the senior conservation biologist at Audubon Vermont, who took charge of the state’s eagle efforts around 15 years ago. 

“Once they started to come, they really started to expand and do really well,” Fowle said. “It just took a while to get them here.”

The threatened status for bald eagles ends at around 19 nests producing an average of 19 young, Fowle said. The amount it takes to remove them from the endangered species list is around 28 nests, producing an average of 28 young. The population in Vermont met that goal when Audubon Vermont observed over 40 pairs in the state in 2022. The species was officially delisted in February that same year.

But amid all the excitement, researchers are asking the question: Are we trading one bird’s prosperity for another? 

Falling herons

Missisquoi National Wildlife Refuge has been known for its prosperous great blue heron population. People travel across state lines to kayak and look for the heron before the trees fully fill in for summer. 

But now they may be harder to find.

A Vermont Fish and Wildlife report on species conservation from 2015 warned that bald eagles were a potential threat to the great blue heron population.

“Attempted predation by bald eagles is suspected of causing colony abandonment,” the report says. “Potential eagle nesting near colony sites could result in the loss of the colony.”

The wildlife refuge observed its first bald eagle nest in 2012, which has stayed on the site from then on — excluding one year when it briefly blew over. The refuge has since added three nests that consistently and actively produce young, according to Ken Sturm, the wildlife refuge manager at Missisquoi National Wildlife Refuge. He said it's been at the expense of the herons.

“Since that 2012 nest occurred, we had a complete crash of our great blue heron rookery, which had a high of over 400 nests, and an average of 275 or 300 nests per year,” said Sturm. He added, “Since that time, they’ve plummeted, and we had several years with very low numbers of nests.” 

The number of great blue heron nests dropped from 410 in 2011 to 150 in 2012, according to data provided by Sturm. A year after that first eagle nest emerged in Missisquoi, the herons attempted to set up a new rookery, or gregarious bird breeding colony, in a different location. But they quickly abandoned it, and in 2019, the population fell again, to the lowest it's ever been: 12 nests.

Potential for harmony

Sturm said a similar thing happened in the Pacific Northwest a few decades ago. The reintroduction of the bald eagles into ecosystems there completely disrupted the great blue heron rookeries. The herons left their nests abandoned. 

Over time, however, he said the herons began nesting closer to well-established bald eagle nests, and the results proved positive. 

A 2013 article published in Canadien Science analyzed the effects of great blue herons nesting near bald eagles. It found that “70% of heron nests and 19% of heron colonies were located within 200 meters of eagle’s nests with high reproductive success. These herons had greater reproductive success than those nesting far from eagles’ nests.” 

To Sturm, bald eagles defend their territory like territorial gangs — if you’re in their neighborhood, you automatically have a layer of protection. 

Does that have a chance of playing out at Missisquoi? It’s unclear, but Sturm thinks there’s potential for future growth. 

“In theory we could even have three or two other rookeries established near those other eagle nests and have a much larger heron population on the refuge than we do now,” said Sturm. “To me, that’s the real interesting part of this whole story, not just the fact that the eagles came and the herons left, but this dynamic predator exclusion.”

The Community News Service is a program in which University of Vermont students work with professional editors to provide content for local news outlets at no cost.