The Bird That Builds Nests Right By Its Worst Enemy
“You can understand the predator wanting to be near the prey, but not really the other way around.”
To watch a bald eagle raid a nesting colony of great blue herons is a gut-churning experience. “The herons have a progression of alarms,” explains Ross Vennesland, a researcher with Environment and Climate Change Canada. “They start with a chortle, and quickly move to really hideous screaming as the eagle swoops in and lands on the nest.” The adult herons are usually forced to flee, while the eagle cracks open an egg or flies away with a chick. “It’s a pretty horrible scene to witness,” he says.
You’d think the herons would want to build their nests as far away from bald eagles as possible. But you’d be dead wrong. Research on the southwest coast of British Columbia shows that herons are deliberately seeking out nesting pairs of eagles—and building right next to them.
“You can understand the predator wanting to be near the prey, but not really the other way around,” Vennesland says. “We were amazed. We call it the mafia-protection racket.”
A heron’s decision to build right next door to such a dangerous predator is a delicate trade-off. Bald eagles are territorial and will chase off other eagles. A heron colony with a neighboring eagle pair may lose some young to them, but the carnage would be greater without their protection.
This tactic may be helping great blue herons cope with the renewed threat from bald eagles. In British Columbia, coastal herons are a unique subspecies numbering an estimated 4,000 to 5,000 individuals; they’re classified as being of special concern by the Committee on the Status of Endangered Wildlife in Canada.
Bald eagles, meanwhile, have been rebounding following a brush with extinction in the 1960s; their comeback is partly due to bans of the toxic DDT in both the U.S. and Canada and restrictions on hunting. But herons have paid the price for this renewal, in some cases abandoning nesting sites in the face of widespread predation.
“The eagles weren’t around for decades, and the herons pretty much had a free rein,” says ornithologist Rob Butler, author of The Great Blue Heron. “Then the eagle numbers started going up, and we’d see them going into the colonies. That’s when we got really concerned.”
The situation raised fears that the herons might themselves become a threatened species. But continued research suggests that the eagle population leveled off around 2005, and that heron survival has improved over the past decade. Some of that gain may be due to the herons’ unexpected nesting strategy.
The tactic, called the predator-protection hypothesis, is seen in other species. Arctic geese, for example, are known to nest close to raptors such as snowy owls and peregrine falcons, and in Italy, wood pigeons nest alongside hobby falcons. “It’s probably some sort of ancient behavior,” Vennesland says of the herons. “Or it could just be that they figured out quickly their best bet was nesting near eagles.”
However the behavior came about, it seems to be working.
A 2009 master’s thesis by Iain Matthew Jones at Simon Fraser University in British Columbia looked at 1,165 heron nests within 15 colonies. He found that 70 percent of those nests were found in the three largest colonies, all of which had long-term eagle nests no more than around 650 feet away.
One heron colony crossed an international border to seek out an active eagle nest, relocating from Point Roberts, Washington, to nearby Tsawwassen, British Columbia.
Vennesland says that, later on, those same eagles uprooted—and the herons followed. “They surrounded the eagle nest to the point there was a heron nest on the same tree right under the eagle nest. Maybe the eagles didn’t like that very much, so they moved just a couple hundred meters down the slope—and the whole heron colony moved with them.”
The Tsawwassen colony remains British Columbia’s largest, with more than 400 nests. But why have the resident eagles not wiped out the entire heron colony? To answer that, one must consider what else is nearby. The colony’s located in a prime foraging area next to the Fraser River delta, where the eagles’ favored prey—fish—does not come with agitated parents. Waterfowl in winter also provide the eagles a cleansing of the palate.
Research at a second site—the Great Blue Heron Nature Reserve in the upper Fraser Valley—supports the Tsawwassen findings. Vennesland says a bald eagle nest there fell from its tree in 2019, and when its owners rebuilt in 2020, sure enough, the herons followed. Earlier research at the same site showed that for herons, average reproductive success was 1.62 fledglings per active nest when an eagle pair was present, and 1.11 after winds took out the eagles’ nest.
Vennesland is hopeful for the great blue herons: “The optimistic voice thinks that maybe some sort of equilibrium is finally being reached between these long-dueling species.”
This post appears courtesy of Hakai Magazine.