Stirrings in the Muck: Fiddler Crabs, Yellow-Crowned Night Herons Locked in Climate Change Dance

Yellow-crowned night heron in salt marsh looks for a crab meal.A yellow-crowned night heron stalks fiddler crabs in a salt marsh near Norfolk, Virginia.(Photograph: Bryan Watts)

Thalia, it’s called, this upscale neighborhood in Virginia Beach that’s 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 human homeowners seeking proximity to water.

It’s also become prime real estate for salt marsh-loving yellow-crowned night herons.

Yellow-crowned night heron in Norfolk tree.Yellow-crowned night heron in Norfolk, Virginia, just after arriving for the nesting season. (Photograph: Bryan Watts)

Herons in the ‘hood

Like other waterfront spots in the Virginia Beach-Newport News region, “almost every house here has a yellow-crowned night heron nest in its loblolly pines,” says Bryan Watts, director of the Center for Conservation Biology, an organization under the aegis of the College of William & Mary in Williamsburg and Virginia Commonwealth University in Richmond.

Watts is in the midst of a breeding season census of the herons. The resulting data are the linchpin in a study of the herons’ response to the earlier start of spring. 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 [1904-2002], 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.

Spring is coming sooner not only to the Virginia coast, but to the entire North American Coastal Plain, a region that stretches along the sea’s edge from Texas to Massachusetts. The North Atlantic Coastal Plain was recently named the world’s 36th biodiversity hotspot.

In one corner of that hotspot, Virginia Beach’s Thalia, the loblollies shoot skyward, shading houses – and yellow-crowned 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 A. Sprunt, Jr., in a 1954 edition of Florida Birdlife. “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 states in a chapter on yellow-crowned night herons in the 2011 publication 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.”

Yellow-crowned night heron nest.Thalia and other neighborhoods in Norfolk host families of yellow-crowned night herons. (Photograph: Bryan Watts)

Homeowners, human and avian

“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 Thalia residents take issue with the herons’ presence “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.”

Yellow-crowned night herons frequently build nests in wooded neighborhoods with parklike appearances and open understories such as those beneath loblolly pines. In Virginia, colonies in residential areas make up more than 80 percent of the yellow-crowned night heron population.

“Pairs seem to prefer to set up housekeeping over rooftops, driveways and roads,” says Watts, leading to some interesting bird-human interactions. Cars parked beneath nests, for example, may be covered with shells from the birds’ fiddler crab meals.

A heron nest is usually placed away from a tree trunk on the outermost fork of a limb. I look straight up at a bundle of sticks above the corner of Dale and Thalia 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 an indefinite period without maintenance. “The human houses should be as lucky,” quips Watts.

Yellow-crowned night heron carrying stick.A parent bird does some nest remodeling, adding a stick here, a branch there. (Photograph: Bryan Watts)

Early arrival of spring – and herons

In 2015, the first year of Watts’ study, yellow-crowned night herons arrived and laid eggs more than 20 days earlier than pairs Darden recorded in the same area in the 1960s. In 2016 and 2017, the trend continued. Each year, the herons arrived and laid eggs a full week earlier, on average, than in 2015. Watts is analyzing this year’s data; the direction, he says, is likely to be the same.

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

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 area its name.

Mud fiddler crab in a salt marsh.A male mud fiddler crab defends his salt marsh territory. (Photograph: Bart Paxton)

Lure of the fiddler crab

Today the mucky marshlands that surround Thalia are home to fiddler crabs that roam over the mudflats, the males waving large claws in a fiddling motion to attract mates, keep intruders at bay or ward off predators. 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 mud-covered burrows, then in spring emerge by the thousands….

…to the sight of yellow-crowned night heron legs slowly stalking them across the mudflats.

All yellow-crowned night herons are crab-eaters first and foremost, feasting on crabs adapted to their 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 at low tide when the crabs are accessible. The herons stalk fiddlers in salt marshes, “running them down on the mudflats,” says Watts. “Females gorge on fiddlers for energy to produce eggs, and breeding pairs feed the crabs to their young.”

The yellow-crowned night heron is primarily an equatorial species, “with four of the five living forms confined to tropical latitudes,” Watts says. The Virginia yellow-crowneds are in the group that migrates north and south with the warm weather each year. The herons’ return in spring is tuned to their crab prey.

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

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

Yellow-crowned night heron with blue crab.Yellow-crowned night herons “live for crabs,” scientists say. (Photograph: Bryan Watts)

Crab-dependent

Yellow-crowned night herons 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 the 59-degree threshold. Then the herons fly to subtropical and tropical latitudes where crabs are active year-round.

For reasons that aren’t clear, yellow-crowneds extended their range northward in about 1925. Although the expansion seems to have leveled off in 1960, the yellow-crowned night heron is rated a species of Least Concern by the International Union for Conservation of Nature (IUCN).

Could climate change affect the herons and their prey enough to change that designation? It’s too early to tell, Watts says.

One indication: where crab populations are faring well, yellow-crowned night herons may be, too.

In the 1980s, Watts and colleagues conducted a study of Virginia 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 six percent.

Yellow-crowned night heron waiting for a meal.There must be a fiddler crab here somewhere. A yellow-crowned night heron waits for an unwary crab. (Photograph: Bryan Watts)

Doyenne of the night herons

The crabs are found in the shallows of salt marshes and mudflats, a fact well-known to Constance Darden. She was passionate about yellow-crowned night herons and their prey. A keen observer of the herons’ habits, Darden kept detailed records in the 1940s and again in the 1960s of their spring arrivals and fall departures in the Norfolk area. She wondered whether the birds might be arriving earlier each spring, and leaving later each fall.

Darden carefully watched the herons from her Norfolk residence in Algonquin Park along Crab Creek, a tributary of the Lafayette River. In 1946, the first yellow-crowned nest appeared on her 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.

Crab claws and more crab claws.Leftovers from yellow-crowned night heron meals collect beneath the birds’ nests. (Photograph: Bryan Watts)

Basis for new research

Darden’s careful recording of the birds’ habits 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 found in our yard in Norfolk in a [loblolly] pine tree a few paces from our porch.”

Another 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 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 that amount lay under our tree,” she reported.

Soon after, Darden left Norfolk for more than a decade. She returned in the early 1960s and picked up her story of the herons. Throughout the 1960s, Darden kept careful track of yellow-crowneds’ comings and goings.

“In closing,” she wrote in The Raven, “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 hopes to answer Darden’s long-standing questions. The clues, he says, are buried in the muck.

Fiddler crab defending his turf.Crab vs. heron: which will win? (Photograph: NPS)

Versions of this article appear in BirdWatching and Oceanography magazines.

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