Beach Birds

Geology’s Link to Avian Abundance

By Todd McLeish


One of the rarest breeding birds in the Northeast finds the beaches of Rhode Island particularly appealing in summer—not for swimming and sunbathing, of course, but for nesting and feeding.

Piping plovers, sparrow-sized pale shorebirds classified as threatened on the U.S. endangered species list, breed more densely in Rhode Island than anywhere else in their Atlantic coastal range.

Image courtesy of National Audubon Society

About 90 pairs of the birds—up from just 10 in the 1980s—can be found laying eggs and raising their chicks on a half dozen beaches in the state, from Goosewing Beach in Little Compton to Moonstone Beach in South Kingstown and Napatree Point in Westerly.

They choose those particular beaches for reasons of geology as much as for any other characteristic. According to ornithologist Peter Paton, they seek ocean-facing beaches where crashing waves create a sizable wrack line of seaweed and other debris for feeding, wide unvegetated beaches that give them plenty of visibility to watch for approaching predators, and sand that frequently blows over the dunes to create washover fans for nesting habitat.

“Piping plovers are particularly adapted to open beach areas created by storms,” says Paton, a professor of natural resources science at the University of Rhode Island. “When Hurricane Sandy came through and pushed sand back into the dunes behind the beaches and created big sand fans, that provided them with prime nesting habitat.”

“But if the storms and sea level rise push beaches farther and farther into vegetated upland areas,” he adds, “or if there’s no place for the beaches to move, it could be a serious issue for them.”

The plover/geology connection doesn’t stop there, however. The plumage of the birds is sand colored, allowing them to more easily blend in with their environment when they are wandering the beach looking for food or when incubating their eggs. And their nest is generally placed in an area of sand with scattered small cobblestones because the eggs are camouflaged to look like the stones.

Least terns, gull-like birds that are also on the endangered list, choose the same beaches as the plovers for nesting, and for similar reasons. American oystercatchers do, too, though their preferred nesting habitat isn’t nearly as narrowly defined. The large black-and-white shorebird with a bright orange beak seeks open, undisturbed, sandy beaches near shellfish beds for feeding.

Several other bird species, like spotted sandpipers and savannah sparrows, raise their young in the vegetated dunes adjacent to beaches, and even more birds prefer nearby salt marshes.

The link between birds and geology isn’t one that many birders or geologists spend much time thinking about, but the link exists nonetheless. What the birds seek in the geology of their preferred beach varies from species to species. The grain size of the sand matters to some birds, as does the slope of the beach, whether it has an adjacent coastal pond, and how protected it is from waves and storm surge.

Luckily, Rhode Island’s beaches are highly variable, providing many of the elements required by a wide variety of birds for feeding and nesting. Unlike the shoreline of New Jersey or the Outer Banks of North Carolina, where the beaches are much the same for hundreds of miles, a short walk on an Ocean State beach often turns up considerable geologic diversity. And that diversity is the result of the region’s glacial history.

“One characteristic of our glacial shorelines is the heterogeneous sediment types and landforms that intersect the shoreline,” says Bryan Oakley, a coastal geologist and assistant professor at Eastern Connecticut State University, who has studied the Rhode Island coastline for nearly a decade.

Piping Plovers choose to nest near ocean-facing beaches where crashing waves create a sizable wrack line of seaweed and other debris for feeding, wide unvegetated beaches that give them plenty of visibility to watch for approaching predators, and sand that frequently blows over the dunes to create washover fans for nesting habitat. Photo courtesy of Janice Sassi

He noted that the region is dominated by two types of sediment: till, which consists of what he calls “a poorly sorted mixture of everything from clay to gravel deposited directly by the ice sheet,” and stratified deposits of sand, gravel, and silt from the rivers and lakes formed by the melting glacier.

“Till produced things like Green Hill and Quonochontaug and Point Judith, boulders without a lot of sand in front of them,” Oakley explains. “The Matunuck Headlands from Cards Pond to East Matunuck is comprised of stratified deposits. The beaches themselves might not look too different—you’ve got a pile of sand with a dune behind it—but what you find further down differs from place to place. There are differences in grain size that relate to the underlying glacial geology.”

Considerable variability exists among the beaches in Narragansett Bay as well. According to Janet Freedman, a geologist with the Rhode Island Coastal Resources Management Council, the sand and sediment in bay-facing beaches are often a bit muddier than those on the south shore due to a division in the glacial ice sheet that deposited finer grain sediment, mica schists, and sandstone on beaches in the bay.“Along the south shore, the glacial material is more granitic, and that is more of a coarser grain size,” she says.

“A beach is just a pile of sediment,” adds Oakley. “If you have a source of sand and gravel suitable for the wave and tidal energy at the site, you’ll have a beach. The difference in the bay is that there are lots of shoreline protection structures, so the beaches are armored to some degree, more so than we see on the south shore.”

From a bird’s perspective, all that geologic variability means a wide variety of food is likely hidden in the sediments, waiting to be eaten. While just a handful of bird species nest on the state’s beaches, dozens more visit throughout the year—especially during migration—to bulk up on the tiny invertebrates that fill nearly every nook and cranny of the coastline.

Although few research studies have been conducted about the creatures that live in the sand on New England beaches, Tim Simmons has become a local expert on the topic. A retired biologist who worked for many years for the Massachusetts Natural Heritage and Endangered Species Program, he has spent much of his career studying beach tiger beetles and other associated species in Massachusetts. He calls beach sand “an incredible ecological niche” for tiny amphipods, beetles, flies, and other invertebrates.

“People don’t see how alive a beach is,” he says. “They compare it to a desert, but that’s far from what it is.”

Simmons says that some amphipods and beetles choose to live in the fine sands because that’s where they find it easiest to burrow beneath the surface. Others choose coarser sands where there are larger spaces between the grains, while still others prefer the saturated sand closest to the waterline.

“There are a whole lot of things in there that you can’t really even see unless you’re wearing polarized sunglasses,” he says. “I remember watching shorebirds feasting on something, but I couldn’t see what. Every time a wave came up and went back down, the birds ran in to grab something. Turns out it was the larvae of a polychaeta worm. The birds could see them, but I couldn’t until I put on my sunglasses.”

Because the composition of the sand on many beaches changes from season to season and storm to storm, many of the invertebrates that live in the sand must be prepared to move when the conditions change or when their food supply shifts.

“Some beaches that are the perfect habitat for some species in the summer are rocky and full of boulders in winter with nowhere for the creatures to hide,” Simmons says. “In the mud at inlets to coastal ponds, you get a different suite of species than in the sand. But then when washover fans carry beach sand over the mud, the mud creatures have to pack up and move.”

Those that can’t move fast enough get washed away or become food for something else. The predatory tiger beetle that Simmons studies is capable of rapid movements and can often detect vibrations caused by the eroding beach in time to emerge from its burrow and escape to a safer location. They move from season to season, too, traveling to the dunes in winter and back to the sand near the wrack line in summer.

“There’s also a grasshopper that’s almost completely bound to dune blow-outs, where a storm has blown out the vegetated face of a dune and created a bowl-shaped depression,” he says. “As juveniles, they’re flightless and hop around, and birds often forage there for them in the early morning.”

The wrack line is another beach ecosystem that changes daily and where another unique suite of creatures can be found.

A 2004 study by a URI graduate student recorded an abundance of species that spend at least part of their lives in the wrack line. Some eat the decomposing seaweed or use it as cover from predators and extreme temperatures, others lay their eggs there, and wolf spiders make daily migrations from the dune grass to the wrack to prey upon amphipods.

Shorebirds of many varieties—from piping plovers and ruddy turnstones to whimbrels and semipalmated sandpipers—eat anything and everything they can find in the wrack.

A visit to Napatree Point further illustrates this link between birds and geology. The mile-long sandy peninsula extending southwest from Watch Hill in Westerly has experienced dramatic changes in the last 100 years—from being the site of 39 homes prior to the 1938 hurricane to a somewhat pristine barrier beach today that shifts with almost every storm.

Riding down the peninsula on an all-terrain vehicle, Janice Sassi, manager of the Napatree Point Conservation Area, points out the wide sandy beach on the ocean-facing side, where piping plovers are often observed feeding on flies and amphipods in the wrack line during the breeding season. It’s here that wintering and migrating sanderlings adroitly chase the receding waves to quickly grab larval crabs that briefly expose themselves to filter feed, then the birds dash shoreward to avoid getting their ankles wet as the next wave approaches. Closer to the point, where seaweed-covered boulders divide the shrubby dunes from the sea, purple sandpipers forage for arthropods and mollusks amid the plant material on the waist-high rocks.

On the opposite side of the dunes, just 75 yards away, a narrow beach faces the quiet Little Narragansett Bay, where thousands of migrating shorebirds feed on horseshoe crab eggs deposited beneath the sand at the high tide line in May and June, where the region’s highest concentration of oystercatchers forage in the adjacent mussel beds, and where about 40 other species regularly search for a meal among the rocks, seaweed, and sand during migration, including northern harriers that hunt for mice and voles in the dunes in winter and nesting osprey that hunt for fish wherever they can find them in summer.

“There are so many birds here in the summer and during migration that it’s like you’re at the mall during the holidays,” says Sassi, noting that she and her volunteers work hard to ensure that the hordes of boaters and beach-goers that also visit Napatree in summer do not disturb the birds.

Bryan Oakley says that while the sand is not likely much different on one side of the peninsula or the other, the wave energy is certainly lower on the bay side, making for a better habitat for many species of birds and their prey. The calmer water on the bay side is also home to abundant minnows of several varieties, the perfect meal for the terns that nest nearby.

“As often as I’m here, it’s different every single time,” Sassi says. “The light is different, I see something different, and yet the birds are always here. Where else can you go where you have bay on one side, ocean on the other, you have a lagoon, the dunes are allowed to migrate as they’re supposed to, and you have shrublands and all kinds of wildlife? To realize that the geology plays such a role in attracting the birds is amazing.”