Yellow-crowned Night-Heron adjusts its migration pattern to fiddler crabs

Yellow-crowned Night-Heron
An adult Yellow-crowned Night-Heron raises its back feathers to attract a mate. Photo by Judd Patterson

Thalia, an upscale neighborhood in Virginia Beach, Virginia, is lined with red-brick ranches shaded by tall loblolly pines. The community is a few short miles from the mouth of Chesapeake Bay and the Atlantic Ocean beyond. Bounded on the west by Thalia Creek and on the north by the eastern branch of the Lynnhaven River, Thalia is a magnet for homeowners seeking proximity to water. It’s also become prime real estate for marsh-loving Yellow-crowned Night-Herons.

Like many other waterfront spots in the Virginia Beach-Newport News area, “almost every residence here has a Yellow-crowned Night-Heron nest in its loblolly pines,” says Bryan Watts, director of the Center for Conservation Biology, an organization that’s part of the College of William & Mary in Williamsburg and Virginia Commonwealth University in Richmond.

Watts, an expert on the species, is conducting a breeding-season census of the herons. The resulting data are the linchpin in a three-year study of the birds’ response to an earlier start of spring — a sign of the warming climate. The project runs from 2015 through 2018, “but it really began more than 50 years ago,” says Watts, “with the work of the former first lady of Virginia, the late Constance DuPont Darden, who recorded Yellow- crowneds’ comings and goings for more than a decade. She left behind an amazing data set on these birds.” Watts is comparing his observations with Darden’s from the 1960s.

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Spring is coming sooner not only to the Virginia coast, but also to the entire North American Coastal Plain, a region that stretches along the sea’s edge from Texas to Massachusetts. The Critical Ecosystem Partnership Fund recently named the North American Coastal Plain the world’s 36th biodiversity hotspot. “Sometimes something precious is right under your nose but goes unnoticed,” says coastal biologist Reed Noss of the University of Central Florida. “Such it is that a region long explored by botanists and zoologists was recognized only recently as being a global biodiversity hotspot.”

In Thalia, one corner of the hotspot, the loblollies shoot skyward, shading houses — and night-heron nests. “In full nuptial display, the Yellow-crowned Night-Heron is one of the most exquisite of all North American wading birds,” wrote Alexander Sprunt Jr. in his 1954 book Florida Bird Life. “Its soft grays and white crown and cheek patches seem to typify the elfin character of the cypress gloom.”

Although no cypress trees loom over Thalia, the loblolly canopy has the same darkening effect. As Watts wrote in the life-history series The Birds of North America: “Although occasionally breeding on coastal islands, this species most often inhabits forested wetlands, swamps, and bayous of the deep south where poor lighting seems to be the most reliable characteristic of its breeding sites.”

“Poor lighting” couldn’t be more welcome. It’s 7 a.m. in late July, and already temperatures are in the 90s. Watts and I make a loop around Thalia in his truck. We easily find heron nests. The splatters of whitewash below give them away. With more than 30 nests, Thalia boasts the largest colony of Yellow-crowned Night-Herons in Virginia.

Some residents take issue with the herons’ presence, “mostly due to the fouling of roofs and anything else that’s below the nests,” says Watts, “but most enjoy watching the birds raise their young.”

The night-herons frequently build nests in wooded neighborhoods with parklike appearances and open understories such as those beneath loblolly pines, Watts has found. In one study, 95 percent of 257 nests were in loblollies. The birds have a preference for pines with a limb configuration in which the lowest limbs of the crown are horizontal. “That usually translates to 40- to 70-year-old trees,” says Watts.

In Virginia, Yellow-crowned Night-Heron colonies in residential areas make up more than 80 percent of the birds’ population. “Pairs seem to prefer to set up housekeeping over rooftops, driveways, and roads,” says Watts, leading to some interesting heron-human interactions. Cars parked beneath nests, for example, may be covered with shells from the birds’ fiddler crab meals.

In large trees, heron nests are usually placed away from the trunk on the outermost fork of a limb. I look straight up at a bundle of sticks above the corner of two roads and wonder why it doesn’t come crashing down. The nest’s three young herons, their bodies still more fuzz than feather, seem oblivious to the 50-foot plunge awaiting the merest misstep. Flimsy as it is, the same “structure” may be used for years. Nests may last for an indefinite period without maintenance. “The human houses below should be so lucky,” says Watts.

The nesting habitat of choice in Virginia, however, is unusual in the wider world of night-herons. In most locations, the birds nest away from people — on islands and in swamps, forested wetlands, uplands near water bodies, mangroves, and similar habitats. And their widespread cousin, Black-crowned Night-Heron, is not known to nest near residential areas, instead forming large colonies over water.

Look for Yellow-crowned across much of the Southeast, in scattered spots around the Midwest, along coasts and islands of the Caribbean, Mexico and Central America, and along the Pacific and Atlantic coasts of Ecuador, Colombia, Venezuela, Brazil, and other countries in South America.

A Yellow-crowned Night-Heron fluffs after preening its feathers. Photo by Kim Caruso

Early arrivals

In 2015, the first year of Watts’ study, the night-herons arrived about 20 days earlier than pairs recorded in the same areas in the 1960s. The trend has continued: The herons returned a full week earlier in 2016 than in 2015. In spring 2017, the birds’ arrival date was about the same as in 2015. Last year (2017) had a cool, wet spring, which may have slightly delayed the herons.

In the 1960s, the earliest egg-incubation date recorded was April 14. In 2015, it was March 23, and in 2016 a week before that. The 2017 timing was similar to that of the 2015 season. “Average egg-laying now is approximately 2.5 weeks earlier than during the 1960s,” says Watts. “The birds are proving to be sensitive barometers of shifts in regional temperatures.”

What’s driving the change in timing? There’s a clue in the Thalia neighborhood’s name.

Once upon a time, the locals say, Thalia was swampland. Indeed, the marsh plant Thalia, a genus of six species found in aquatic habitats from Illinois to Argentina, may have given the community its name.

Today, the mucky marshlands that surround Thalia are home to innumerable fiddler crabs that roam over the mudflats, the males each waving a large claw in a fiddling motion to attract a mate, keep an intruder at bay, or ward off a predator. Fiddler crabs live in burrows in the mud, moving onto the flats to find food — bits of algae or decaying marsh plants — when the tide is low. In winter, fiddlers stay deep below the frost line in burrows, then emerge in spring by the thousands — to the sight of long night-heron legs slowly stalking them across the mudflats.

Throughout their range, Yellow- crowned Night-Herons are crab-eaters first and foremost, feasting on crabs adapted to specific locales.

Along the Atlantic Coast, says Watts, “the life of a Yellow-crowned Night-
Heron is spent in pursuit of one thing: fiddler crabs.” The birds’ hunting times are scheduled around low tide when the crabs are accessible. The herons stalk fiddlers in salt marshes, says Watts, “running them down on the mudflats. Females gorge on fiddlers to build energy to produce eggs, and breeding pairs feed the crabs to their young.”

The Yellow-crowned is primarily a tropical species, according to Watts. Four of the five living subspecies are sedentary and confined to tropical latitudes. The Virginia birds are among the group that migrates north and south with the warm weather each year. The herons’ return in spring is tuned to the emergence of their crab prey.

When the thermometer rises above 59 degrees Fahrenheit, fiddlers emerge from their burrows and begin to scuttle across the mudflats. “The date in spring when the temperature passes the 59-degree threshold is getting earlier,” says Watts, “extending the season of fiddler availability. Yellow-crowneds appear to be adjusting to that shift in season.

“We have no idea, however, how the birds are aware when the fiddlers are emerging. It’s a total mystery.”

The birds are so crab-dependent that more northerly populations, including herons in the Virginia Beach area, depart in the fall when fiddlers return to their burrows for the winter. That happens when the temperature drops below 59 degrees. Then the herons fly to subtropical and tropical latitudes where crabs are active year-round. North American Yellow-crowneds winter from North Carolina to south Florida, west to the Texas coast, and south to Costa Rica and islands in the Caribbean.

Whether in Virginia, Florida, Brazil, or anywhere else, the night-heron “hunts using slow stalking movements,” wrote Watts in The Birds of North America. When it catches a large crab, he stated, “the bird methodically dismembers and eats it, body first.”

In the 1980s, Watts and colleagues conducted a study of the Yellow- crowneds’ preferred meals. The scientists collected and identified more than 2,000 crab claws under nests and found that three species — the mud fiddler, red-jointed fiddler, and white-fingered mud crab — made up 94 percent of the herons’ diet. The sand fiddler, ghost crab, blue crab, mole crab, toad crab, and common mud crab accounted for the other 6 percent.

Night-heron
An adult night-heron flies with a stick to add to its nest. Photo by Ray Hennessy/Shutterstock

Doyenne of the night-herons

 The three main fiddler crab species are found in the shallows of salt marshes and mudflats, a fact well-known to Virginia’s former first lady. Constance Darden was passionate about Yellow-crowned Night-Herons and their prey. A keen observer of the herons’ habits, she kept detailed records in the 1940s and again in the 1960s of their spring arrivals and fall departures in the Norfolk area.

Darden, who married Colgate W. Darden, Jr., governor of Virginia from 1942 to 1946, carefully watched the herons from the couple’s Norfolk residence in Algonquin Park along Crab Creek, a tributary of the Lafayette River. In 1946, the first nest appeared on their property. Later, a colony of the birds nested there.

For Connie, as she was known, the heronry became the site of hundreds of hours of birdwatching. “She took it up while her three children were still young,” remembers her daughter, Irene Darden-Field of Courtland, Virginia. “She’d often go out before dawn to check on the herons, then be back in time to have breakfast on the table.”

Her careful recording of the birds’ habits, especially observations she made in the 1960s, gave Watts the basis for his current study. “Her invaluable dataset is now housed at the Center for Conservation Biology,” he says.

In the May-June 1947 issue of The Raven, the journal of the Virginia Society of Ornithology, Darden wrote that “this heron remains in our coastal section from late March to early October. The first nest known to us was one found in our yard in Norfolk in the early stages of building on May 22, 1946, about 60 feet up in a [loblolly] pine tree a few paces from our porch. All books describe the Yellow-crowned Night-Heron as shy and retiring, but I have heard of four of our neighbors who have nests in their yards.”

Three of those nests, she wrote, were about 60 feet high, and all were built of sticks approximately 18 or 20 inches across. One nest, noted Darden, “was found in my neighbor’s yard which embraces the small growth of pines bordering a cove of Crab Creek. This creek is well-named for it contains an abundance of swimming and fiddler crabs, the latter making up a large part of the diet of these herons.”

The late ornithologist Witmer Stone of the Academy of Natural Sciences in Philadelphia had discovered that a quart of fiddler crab shells was often found beneath a Yellow-crowned nest. But Darden made her own deductions. “My guess would be that at least three times this amount lay under our tree,” she reported.

Then Darden left Norfolk for more than a decade. She returned in 1960 and began tracking herons again. “Not until April 14th did I think to keep a nesting record,” she wrote. “By that time there were already five occupied nests in three trees. The following day I found four more nests in progress; and the next, two more; likewise, two more on the 18th. That meant 13 nests established in four days. Later other nests were added, so a total of 19 were built during a month.”

Throughout the remainder of the ’60s, Darden kept careful notes.

“In closing,” she wrote, “I shall report what I saw the afternoon of October 17th, 1960. A number of constant ‘quock’ cries brought me running out on our point. Several Yellow-crowned Night-Herons were circling the water round and round and one or two more joined them until there were six adult and two immature birds in the group. A Laughing Gull chased one of the birds and it flew into the marsh, but the others left together heading down the creek to the south. Apparently the start of fall migration.”

More than 50 years later, Bryan Watts has picked up the story.

 

Cheryl Lyn Dybas is an ecologist and science journalist who also writes for National Geographic, National Wildlife, BioScience, BBC Wildlife, and many other publications. In past issues, she has written about Harlequin Duck, Northern Saw-whet Owl, and Painted Bunting.