Endangered herons build nests near dangerous predators

CHICAGO - Small and stocky birds perch, hunched over, on the treetops and branches of Lake Okeechobee on the edge of Florida’s swampy Everglades for the better part of winter. The alligators swarming the murky waters below don’t faze the black-crowned night herons — in fact, the birds count on them for protection. And when some fly away for the summer, they seek out other animals that can do the same.

This heron is the world’s most widely distributed species of its kind, found on every continent except for Australia and Antarctica. But it’s been endangered in Illinois since the 1970s as the population has declined across the Great Lakes region because of human harassment and disappearing wetlands.

For the past 15 years, however, Chicago has become a popular summer hub and the location of the last remaining breeding colony of the species in the state, specifically atop the red wolf enclosure at the Lincoln Park Zoo. Hundreds of black-crowned night herons flock there starting in mid-March every year, migrating from nearby Indiana and Kentucky, and from farther away like Louisiana, Florida and Georgia, and more recently, even Cuba.

“[It’s] a story that most people find a little bit ironic, that they settle above this particular predatory species. But it seems that they have this really great — what’s called a facultative relationship — mutually beneficial relationship,” said Henry Adams, wildlife management coordinator at the zoo’s Urban Wildlife Institute and resident expert on black-crowned night herons. “They experience so many different wildlife species along their migratory pathways and have built those relationships.”

In Chicago and, in the obvious absence of alligators, the birds have found the largest carnivorous animals and apex predators around. They rely on the red wolves to scare away smaller threats, such as raccoons and birds of prey, from their nests. The wolves remain undisturbed and, for the most part, uninterested.

Toward the end of April, zoo staffers counted some 768 adults in the colony, breaking the previous record of 751 from May of 2022.

On a recent morning at the Lincoln Park Pritzker Family Children’s Zoo, the treetops teemed with innocuous nests where pointed bills gave away the resting herons inside. Staffers close off the children’s zoo to the public for the nesting season to ensure that the nesting pairs and their chicks aren’t disturbed — and so the birds don’t poop on visitors.

“There’s one of our red wolves,” said Adams, who uses the pronouns they/them, pointing and gazing straight ahead. Through the glass, the curious predator eyed them back. Another one looked on from the shade of a cave. They are 3-year-old Poco and 9-year-old Becca.

The wolf species — once common in most of the eastern and southern United States — still exists but is critically endangered, partly because wolves in the wild often mate with coyotes and produce hybrid offspring. That has led to low genetic diversity and weak evolutionary fitness. Just under 20 red wolves exist in their wild, native habitats today. The captive breeding population is around 240 that live in 45 facilities across the country.

Occasionally, chicks fall into the wolf enclosure, which the predators find to be a tasty snack.

“But the keepers are very, very diligent to make sure that they [eat] the birds as little as possible,” Adams said. “We want to be able to have as much control and monitoring of their diet so they maintain the best health possible. We make sure that their diet is not heavily comprised of night herons.”

The staff has studied the nests closely: how high they are off the ground and how densely — or not — they are built in a single tree.

“So we have both presence and absence data,” Adams said. “The birds have a strong preference to nest either inside the red wolf exhibit or at least proximate to it. We see nest density significantly declining as you move away. They really like to maintain that close relationship.”

Nearby, another predator offers the birds some protection: American black bears Katai, a female, and a younger male, Birch, who recently made headlines munching on pansies in a video. Fewer herons nest near their habitat, though, likely because the massive animals, as opposed to the wolves, can climb trees.

“Presumably, these herons don’t want to be too close to them,” Adams said. “But we are continuing to see this colony expand southward and utilize more of this nesting habitat on the south end of the zoo.”

On the robust branches of a red maple, a dozen or so nests support some 20 black-crowned night herons. “This is one of their absolute favorite trees,” Adams said.

Ever thoughtful about nesting, the herons prefer to do so on the branches of six tree species of the 43 kinds scattered around the outdoor exhibits: red maple, sugar maple, river birch, white mulberry, American hornbeam and common hackberry. They love gathering sticks from elm and oak trees across Stockton Drive, Adams said, but mostly nest on zoo property.

“This beautiful understory, I’m sure, also makes them feel a lot more at home, a lot [safer],” they said. “One of the reasons that we think they came to settle here — of course, they have the protection of the red wolves and the black bears — is this is really a beautiful re-creation of a more natural Illinois woodland ecosystem. All of the trees that are planted back here are either indigenous to Illinois or are naturalized species.”

In Chicago, the zoo has been part of a collaborative effort to band and put radio transmitters on adult black-crowned night herons to track their foraging grounds in the city and their full migratory routes.

During the summer, the herons find their food by the lake and the river, specifically at Montrose Harbor and the Wild Mile in the North Branch. Evidence of their diet can be found near their nests, where in the air hangs what Adams called a “mild fishy aroma.” They also regurgitate crustacean exoskeletons like crab claws, which they can’t digest.

Male and female parents share roles and switch duties, including foraging for food, incubating the eggs and rearing the chicks, which tend to start fledging in June. In fact, nesting and rearing season takes a village and involves whole colonies.

“Individuals will assist in the rearing of chicks that may not necessarily be their own,” Adams said. “At a particular time when all the chicks are fledged, they’re all climbing around the tree, they all kind of need support, and sometimes we see nonparent herons helping out.”

But the success of this colony is not without concerns.

Herons that nested nearby in trees by the Chicago History Museum since at least 2014 were displaced when construction began in 2021 for a trail nearby. Some experts believe they might have joined the Lincoln Park Zoo colony. Now with so many of the state-endangered birds concentrated in this one location, they have sounded the alarm about the species’ vulnerability: One traumatic event, like a hailstorm or avian flu, could threaten essentially the entire population.

That worry became more real earlier this year as bird flu spread nationwide, killing poultry in farms and waterfowl in the wild. However, studies have found that this species is generally not the most efficient host for the pathogen that causes the disease, according to Adams, who is also a disease ecologist.

Zoo staffers did some oral swabs and blood tests on the colony’s adults and juveniles last summer that indicated they weren’t actively infected and didn’t have any antibodies for the disease — meaning they hadn’t contracted it in the past year and mounted an immune response to it.

“It was wonderful to be able to share that with our veterinary staff,” Adams said. “From the population health perspective, it was really fabulous to see.”

The birds’ reliance on their own, as well as on other creatures, indicates a flourishing community and larger ecological success.

“We think of urban spaces as not necessarily being the most hospitable areas to wildlife, but we have, right in front of us, this beautiful example of when habitat is created with wildlife in mind, even endangered species can thrive in urban areas,” Adams said.