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 listed as threatened on the U.S. endangered species list, breed
more densely in Rhode Island than anywhere else in their Atlantic coastal
range.
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.
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| Piping plover nest at Napatree Point (Peter Paton) |
“Piping
plovers are particularly adapted to open beach areas created by storms,” said
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 added, “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 raise their
young in the vegetated dunes adjacent to beaches, like spotted sandpipers and
savannah sparrows, 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 land forms that intersect
the shoreline,” said Bryan Oakley, a coastal geologist and assistant professor
at Eastern Connecticut State University, who has studied the Rhode Island coastline
for nearly a decade. 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 explained. “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 relates 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 said. “A beach is
just a pile of sediment,” added 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 of
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 said. “They compare it to a desert, but that’s far from what it is.”
Simmons said 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 said. “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 polychaete
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 perfect
habitat for some species in the summer are rocky and full of boulders in winter
with nowhere for the creatures to hide,” Simmons said. “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 their 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 said. “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
out the peninsula on an ATV, Janice Sassi, manager of the Napatree Point
Conservation Area, pointed out the wide sandy beach on the ocean-facing side,
where waves crash loudly and 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,” said 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 said 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 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 said. “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.”
This article first appeared in the Winter 2018 issue of the magazine 41 North.
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