Erosion Threatens Birds’ Nesting Sites

HARRISBURG, Pa. —  Two black-crowned night herons sit on a tree that has crashed from Wade Island into the Susquehanna River. It’s a windy day and the tree provides a sheltered perch for the birds, which typically hunt at night.

But the tree’s fall is the latest sign that the herons and their neighbors, great egrets, may be in danger.

Visible from the Interstate 81 bridge, Wade Island sits just a few feet from the Harrisburg city limits. Yet despite the birds’ proximity to the city, it’s not humans who threaten their continued existence in Pennsylvania. It’s the river itself, and the other birds that the river has attracted.

Erosion has taken about one-third of the small island’s area, causing the trees that house the birds to crash into the river. All the while, double-crested cormorants -- a thriving species that is expanding its habitat -- are starting to crowd the egrets and herons from their traditional nesting sites.

The result may be the end of Pennsylvania’s only nesting site for great egrets and one of just a handful of sites for black-crowned night herons.

Egrets were once common along eastern rivers, but their graceful, snowy-white feathers were sought by hunters, who sold them for decorations.

“Historically, it did nest more places,” said Cindy Dunn, executive director of the Pennsylvania Audubon Society. “Their numbers are still rebounding from the turn of the century, when their numbers were decimated by the plume trade, the hat trade.”

With its towering silver maple and river birch, Wade Island provides ideal nesting for great egrets, a state endangered species, and black-crowned night herons, considered at risk in the state. The clear, shallow water of the Susquehanna affords ample food for both species of wading birds.

Wade Island has shrunk to just 2.5 acres, from about 3.8 acres in 1985. As the river washes away the edges of the island, trees lose their footholds and crash into the river, decreasing the available nesting space.

Except for the egret and heron nests, Wade isn’t that much different from nearby Sheets Island or the smaller islands that make up the Sheets Island Archipelago, which also provides habitat for yellow-crowned night herons, blue herons and other species.

So why haven’t the egrets and black-crowns expanded to nearby islands?

“That’s what we’ve been trying to figure out,” said Cal Butchkoski, a game technician with the Pennsylvania Game Commission. “Some of the other islands have similar trees. But some of it is history. The young come back -- and so do the adults -- to the same spot, and they get attached to it.”

And another species is becoming attached to Wade Island. Double-crested cormorants, more common along the Eastern Seaboard, have begun expanding up the Susquehanna, drawn by its plentiful food supply.

“Pennsylvania was not thought to be double-crested cormorant habitat, and you wouldn’t expect to really see them. They dive 20 to 40 feet deep to eat fish, so you’d never think they’d take off in the Susquehanna,” Dunn said.

But the cormorants are thriving, and their expanding populations need new places to feed. And they’re adapting, learning to eat from the shallow waters of the Susquehanna instead of the deeper waters that they’re used to.

Ironically, it may have been the success of the egret and heron colony on Wade that attracted the cormorants.

“Colonial nesters intuitively attract other colonial nesters because they recognize there’s safety in numbers,” said Dan Brauning, a Game Commission biologist. “But in this case, that’s unfortunate, because Wade has limited -- in fact, eroding -- accommodations.”

That’s bad news for the wading birds. Although the species don’t really compete for food, the cormorants do compete for nesting space with the egrets and herons. It’s a competition that the wading birds aren’t likely to win.

“Cormorants can form some fairly large colonies and if that occurs here, there’s a good possibility they could take over the nesting habitat on the island,” Butchkoski said. “They’re more aggressive than these wading birds and once they set their nest, that’s their area. The egrets and the herons aren’t going to push them off.

“We haven’t noticed any aggression toward the existing nests of egrets and herons from the cormorants. But the trees that the cormorants are in, in the past, they had egrets there. They’re definitely displacing some of the egret and heron nests.”

Some 249 egret and heron nests exist on Wade Island, down from almost 400 in 1990, but still substantially higher than in the early 1980s.

Although cormorant populations are strong, as a migratory bird, they are protected by federal law. Because of that, the state would need permission from the federal government before taking any action that would hurt the birds.

But Butchkoski said the state is not yet at that point. For now, officials will simply monitor the birds and do what they can to encourage the herons and egrets.

“I think we’re happy with the numbers right now when you figure that there’s only so much habitat,” Butchkoski said.