September 20, 2026: The Great Migration

We spent this week walking on the bits of the White Mountains that found their way some 200 miles south to Cape Cod.  As the glacier that covered New Hampshire 20,000 years ago slowly worked its way southward, it ground huge amounts of rock into fine sand and gravel which was pushed southward by its leading edge.  When the glacier eventually melted, it deposited that pile of material off present day Massachusetts, forming the basis of Cape Cod.  Then as the ice to the north melted, the resulting rivers washed massive amounts of sand southward, which along with ocean currents, formed the unique shape of Cape Cod - the long arm out to Chatham and up and around to Provincetown.

After the glacier melted, Nantucket and Martha's Vineyard were connected to the mainland.

The sand and mud that collected in Cape Cod Bay and Nantucket Sound is now one of the most productive set of marshes and mudflats in the world.  There is an excellent display at the Cape Cod Museum of Natural History in Barnstable that describes the formation of the cape over the millennia, including what the future holds for this fragile land mass as climate changes accelerate.  Today, it's a huge feeding stop for millions of migrating birds as they make their long journey south for the winter.  One example is this flock of Tree Swallows that was circling over the marsh grasses while feeding on insects that hatch this time of year.

Tree Swallows

At times they would all align and fly in formation.

I did find one lost soul that was trying to catch up with the others.

I wonder if any of the tree swallows from Lake Wicwas visit the cape on their 4,500 mile trip to the Gulf Coast or the Caribbean.

Farther out on the mudflats in Cape Cod Bay was an unfathomable number of terns - mostly Common Terns, but probably some Roseate Terns mixed in. 

They sat preening on the mud for a long time.

Then suddenly the whole flock took flight in a long wave, maybe when a scout spotted a school of fish ready for picking out of the shallow water as the tide came in.

Terns need to consume a lot of food to fuel their 4,500 mile flight to South America.

In contrast to the huge flocks of terns and swallows, was this solitary Black-bellied Plover working the mudflats (my first time seeing this this bird).

Black-bellied Plover

It was calmly plucking worms, small crustaceans, bivalves, crabs, and any other living creatures from the mud with its long beak. 

This plover has already made a long trek from the artic where they breed at the very northern edge of Canada.  It may spend the winter here on the cape, or continue all the way down to South America, a trip of 12,000 miles!

On the other side of the cape, at the Monomoy Wildlife Refuge in Chatham, there were long lines of Cormorants resting on the sand bar.  

Cormorants on a sandbar ar Morris Island.

We have cormorants on New Hampshire lakes during the summer, and some of these may stop at Cape Cod on their way to the Gulf Coast, Mexico, or the Caribbean.  Cormorants travel less distance than many migratory birds.

Also at the Monomoy Wildlife Refuge was another solitary bird I'd never seen before, this one a raptor soaring over the narrow spit of land, on the hunt for small mammals rather than fish. 

Norther Harrier

A northern harrier can be identified by the clear demarcation of white belly and dark brown head at the neckline.

Farther west toward the mainland is the the Audubon Great Marsh Sanctuary in Barnstable, where we saw Great Egrets far out in the marsh, as well as some small, brightly colored crabs on the damp mud at low tide.

Look for tiny white specs in the marsh that are egrets.

Atlantic Marsh Fiddler Crab.

We only saw one interesting mammal species all week, but it was a treat:  Gray Seals.

A gray seal pokes its head up to check its surroundings.

Playing in the surf.

Weighing between 400 and 800 pounds, gray seals are much larger than harbor seals.  We saw several swimming north  at Nauset Beach along the eastern-most coast of the cape.  When we arrived at the beach there were large warnings about sharks.  When we saw the seals, we knew why!

Cape Cod is a gold mine of nature with widely varied habitats including open ocean, sand dunes, marshes, mudflats, and cedar forests. 

Nauset Beach

Monomoy Wildlife Refuge at Chatham.

Sunrise over Nantucket Sound

We always learn something new on our visits to Cape Cod, all while enjoying the beauty the cape offers.  Next week we'll be back checking on New Hampshire's progress towards autumn.