This article originally published in the September 2024 issue of Natural History Magazine and is republished here with permission. Copyright © Natural History Magazine, Inc., 2024. Story and photographs By Pete Myers.
Atmospheric river after atmospheric river—nine in just a few weeks—drenched California from late December 2022 to mid-January 2023. After the barrage, at Point Reyes National Seashore, forty-five miles north of San Francisco, a large concentration of shorebirds gathered at dawn, having been flushed out from nearby mudflats onto Limantour Beach, which had been ravaged by high waves and high tides. The impact was so severe that for several miles the contour of the beach, and presumably the distribution of the invertebrates upon which shorebirds feed, had been massively changed by high surf cutting into the dunes and sometimes washing over them.
To see more of Pete Myers’ photography, visit Calidris.Photography.
The beach, however, which was alive with birds at dawn, was desolate a few hours later, but teeming again in twenty five hours. A week later, the beach was void of birds at sunrise, but bristling with them in the afternoon. For anyone who has watched shorebirds often enough, this behavior was not a one-off due to the storms but is an ongoing pattern. The comings and goings of shorebirds seem inexplicable. Why are flocks of birds streaming from the local estuaries onto the beach as the tide rushes in? Why, when the birds reach the wave-washed zones, does bedlam break out among sanderlings (Calidris alba) and long-billed curlews (Numenius americanus) joined by Heermann’s gulls (Larus heermanni), black-bellied plovers (Pluvialis squatarola), willets (Tringa semipalmata), marbled godwits (Limosa fedoa), even ring-billed gulls (Larus delawarensis) and surf scoters (Melanitta perspicillata). And why the dramatic changes in composition of bird flocks from August through January and April through June?
It might seem like chaos, yet these are nature’s rhythms and they drive every aspect of shorebird behavior.
Limantour Spit lies on the south edge of Point Reyes. The spit is a straight, narrow sliver of sandy beach— Limantour Beach—and dunes are a few meters high, at most. On the north side of the spit, across a narrow estuary, are tall cliffs where peregrine falcons (Falco peregrinus) roost between hunts. The north inland side of the spit includes two substantial esteros (marshy estuaries) with extensive mudflats. When the tide is out, the open, often gooey flats lure thousands of shorebirds to feed on an array of invertebrates—various worms, decapod crabs, ghost shrimp, and different forms of mollusks big and small. As the tide rises and covers the flats for major parts of most days, the shorebirds head for the beach. The tide dictates the feeding schedule. Migration patterns determine the composition of the flocks. As the tide comes in, sanderlings are typically the first species to leave the mudflats. While their numbers fluctuate year-to-year, up to 500 or so individuals call Limantour their winter home. They begin to arrive from Arctic breeding grounds in late July and stay in the Point Reyes area until April, some until early May.
As the tide rises, sanderlings in small flocks of ten to twenty or more fly in from the estero on the other side of the dunes and begin feeding almost immediately. As soon as a wave starts to withdraw, they chase it down to its base, probing rapidly as they run. They are hunting for the Pacific mole, or sand, crab (Emerita analoga)—a keystone species in this tumultuous habitat where waves crash and pull back every five to fifteen seconds.
Mole crabs, members of the decapod family Hippidae, differ markedly from the brachyuran crabs that scurry along the beaches or swim and crawl on the ocean floor. Mole crabs have no claws and instead of digging burrows they rapidly bury themselves in wet sand by vibrating their tail. This produces a thixotropic effect, liquefying the sand, allowing the crabs to push their rear end downward into the sand by using a pair of modified legs that act as paddles. As they move deeper, the displaced sand settles on and around them, leaving exposed only mouth parts, eyestalks, antennae, and the uppermost part of their carapace. Female mole crabs vary in size from less than a quarter of an inch long, when first settling out of the plankton onto the beach in spring, to just under two inches long as mature crabs. By late fall they are carrying a red egg mass beneath their flap-like tail. Adult males are smaller, less than an inch long.
The crabs—brownish gray, sometimes with a hint of red and shaped almost like an olive with a somewhat pointed rump—feed as waves wash up and down the beach. They use their two large antennae, which have fronds resembling those of a giant moth, to filter out bits of kelp, detritus, and plankton—their prey. Once a wave has passed, they retract their antennae, vigorously whisk the sand with their tail to liquify it and bury themselves until the next wave crashes over them.
Prime feeding moments for the mole crabs occur as the waves recede. But that same phase of the wave cycle is also prime hunting time for shorebirds that rush down toward the base of the wave. Millions of mole crabs, with all eyes trained on the shore and the advancing predators, must quickly grab their last bite before burying themselves.
On the mudflats of Limantour Estero, other species follow the sanderlings to the beach in a fairly regular order as the tide rises. Small numbers of black-bellied plovers spread out one-by-one, rarely in flocks, to defend feeding territories, chasing away other plovers, often calling out in protest if another black-bellied plover merely flies by its territory. They too seek mole crabs, but their running style is more stilted than the continuous blur of sanderling legs and they usually feed only part way down the wave-washed zone instead of pursuing all the way to the base.
The higher the tide, the more shorebirds are forced from the mudflats to the beach. Dunlins (Calidris alpina), least sandpipers (Calidris minutilla), western sandpipers (Calidris mauri), and semipalmated plovers (Charadrius semipalmatus) follow this flight plan with every tidal cycle, day after day from mid-summer when they begin to return from northern breeding areas. These birds seldom feed on the beach, but instead roost above the high tide line in flocks that number hundreds to thousands of birds. Heermann’s gulls join the hunt— dense flocks of twenty to forty gulls at the end of summer and into the fall, having spent most of the year in the Gulf of Mexico.
At the lowest point of the wave cycle, those mole crabs that were near the top of the receding wave have buried themselves completely out of reach of sanderlings but not marbled godwits or long-billed curlews. These birds, with monstrous beaks way out of proportion to the rest of the bodies, sometimes thrust their mandibles deep into the sand. For a longbilled curlew, these bills can be over eight inches, and four inches for a marbled godwit, as opposed to about one inch for a sanderling.
The beak of a godwit, curlew, or sandpiper has a special adaptation ideal for grabbing prey deep beneath the surface. The technical term is rhynchokinesis. Muscles and tendons run from the tip of their upper beak back to the skull where they are attached. When the muscle pulls, the beak is pulled up and back at the tip, opening the tip wide. This allows the birds to thrust their closed beak into hard sand, and if they encounter prey, they can open their bill deep in the sand and grab it.
At the lowest point of the wave cycle, those mole crabs that were near the top of the receding wave have buried themselves completely out of reach of sanderlings but not marbled godwits or long-billed curlews. These birds, with monstrous beaks way out of proportion to the rest of the bodies, sometimes thrust their mandibles deep into the sand. For a longbilled curlew, these bills can be over eight inches, and four inches for a marbled godwit, as opposed to about one inch for a sanderling.
The beak of a godwit, curlew, or sandpiper has a special adaptation ideal for grabbing prey deep beneath the surface. The technical term is rhynchokinesis. Muscles and tendons run from the tip of their upper beak back to the skull where they are attached. When the muscle pulls, the beak is pulled up and back at the tip, opening the tip wide. This allows the birds to thrust their closed beak into hard sand, and if they encounter prey, they can open their bill deep in the sand and grab it.
Shorebirds scurry and sometimes fly along the beach to avoid being knocked over by a wave, a rare occurrence. But quickly, water above the mole crabs becomes too deep for most bird species to continue the search. That is when fish come in to hunt the crabs. Surf scoters, a diving duck species, join them. Both fish and scoters scoop the mole crabs out of the underwater sand. Thus, predation is virtually constant. The conversion by mole crabs of kelp, detritus, and plankton to oily lipid deposits in their compact bodies has made them the invaluable food source for birds and fish.
But how can a single species support the energetic needs of so many predators? The answer is because there is almost an inexhaustible food supply of kelp, detritus, and plankton. The mole crab population is replenished each spring as last year’s larvae settle onto the beach to become adults. A single mile of sandy beach can provide a wave-washed home to millions of mole crabs.
For shorebirds hunting mole crabs, crab size can be everything. But it is not just bigger is better. When sanderlings, for example, catch a small mole crab, they can swallow it immediately and keep on probing, without a pause. This is especially the case in April and May when mole crab larvae settle out of the ocean onto the beach by the billions. At that stage in life, they are translucent and about the size of a small pea. Hundreds of thousands of them can be in as little as a square meter in peak densities on the beach. But the amount of energy they contain is also small.
As young mole crabs grow through the summer, they can become too big to swallow immediately, and captors risk a fight with others for the prey. Fighting for possession of a captured crab can be intense. Wings flare and hammer. Beaks thrust, sometimes wide open. And often it won’t be just two birds struggling. Three or four may join the chase. Sometimes the frenzy is so intense that the possessor drops the crab in the surf, and if the crab is quick enough, it burrows down and is lost to its captors, who continue to fight even though their prey has disappeared.
Some sanderlings are lucky enough to catch one of the larger mole crabs and quickly pivot up and above the wavewashed zone, frantically trying to escape other sanderlings as well as ring-billed gulls. Flight chases unfold frenetically. The gull usually wins when the flying sanderling drops its prey and the gull swoops in.
At a good mole crab patch beset by a flock of sanderlings, bedlam goes on incessantly. It can get complicated when, in addition to the sanderling flock, some of the sanderlings have claimed territories. This overlay of a territorial array along the beach, with individuals defending linear territories five to thirty yards in length, combined with marauding flocks flowing through the territorial array, adds bedlam to bedlam.
The territory’s boundaries are relatively stable through multiple tidal cycles, though neighbors will fight over the boundaries for days in a row. But suddenly a flock arrives. The territorial bird lowers its wings to reveal a black patch at the bird’s wrist while lowering its tails and raising its back feathers as if hunchbacked. It runs at the intruders, squeaking loudly, and they respond by running on to the next territory. But when one or more pulls a mole crab out of the sand, the result is chaos. Sometimes the territorial bird arches its back feathers to such an extreme that each feather appears separate from all others. The net effect is to make the territorial bird appear much larger than anyone else around. If there are too many intruders, the territorial bird temporarily gives up defense, although even then it will occasionally go into a territorial posture and run half-heartedly after an intruder.
In my research on sanderling beach territoriality with Peter Connors and Frank Pitelka at the University of California Davis-Bodega Marine Laboratory, in Bodega Bay, we were able to resolve, at least for sanderlings, two competing hypotheses about territory size. Do sanderlings adjust their territory size to match their energy needs? Or do they defend a territory as big as they can, limited by how many intruders attempt to feed in their territory? More intruders mean more energy and time expended in defense. Our data rejected the first hypothesis. More intruders attempting to gain access to the best feeding places make defense of a large territory impossible. Thus, territories in areas of high food density are smaller. And if the intruders arrive in a large flock, territorial defense ceases, at least until marauding birds have departed.
The western snowy plover (Charadrius nivosus nivosus)—a US Fish and Wildlife Service listed Threatened species—is the one shorebird that remains on Limantour year-round. It nests on open, sandy beaches, especially in some of the wider expanses of Limantour. Toward the western end of the spit, roosting plovers hunker down in horse or human footprints or behind micro-topographic features created by waves moving over sand. On any given walk along the western spit in winter, I have encountered as many as 100 snowy plovers, many color-banded by researchers from the park and from Point Blue, an ecological research institute that has evolved significantly since its start as the Point Reyes Bird Observatory. The birds were banded as chicks on Limantour and will likely remain resident for the rest of their lives.
Among the other shorebirds on the spit, willets and long-billed curlew are the closest nesters, flying to interior marshes in Nevada, Montana, Alberta, and Saskatchewan. Marbled godwits go a bit farther, breeding no closer than Montana and the Canadian Prairies. But others continue northward until they get to the true Arctic. And sanderlings travel as far as the Canadian islands in the high Arctic.
Like the other shorebirds, snowy plovers move from beach to estero as the tide falls. On the mudflats they often feed far back from the edge of the water, especially in areas where the mudflats are a mixture of mud and sand. On the beach, they go for surface invertebrates, such as flies and beach hoppers, never for mole crabs. Their running cadence differs dramatically from sanderlings, and is much more similar to black-bellied and semipalmated plovers. Instead of almost continuous forward motion, snowy plovers run several steps, then stop and look for prey. Then repeat. Rarely do snowy plovers participate in any of the beach bedlam. Sometimes they squabble over a resting spot, but, usually, when that happens, the aggressor drops down into the sheltered beach pocket and the loser moves on.
If snowy plovers are without drama, peregrine falcons wreak bedlam. They hunt throughout the day, appearing out of nowhere to swoop down on shorebird flocks on the beach, on the mudflat, or in mid-air. Resident merlins (Falco columbarius) have similar predatory behavior, but are seen less often than peregrines, which appear almost every day.
A rolling wave of shorebirds, taking flight frantically and seaward along the beach, low over the water, and in tight flocks, is usually the first indicator of a peregrine attack. The flocks burst seaward, almost skimming the breaking surf, but then swerve upward. They rapidly change directions and almost never fly in a straight line. The curvature in the flight often means most of them are showing either their white belly or dark backs, and when they change directions the overall color of the flock flashes from dark to light and back again.
The other main group of sandy beach birds— vultures, ravens and several gull species—seek out the dead to eat. They cruise back and forth along the length of the spit looking for carrion. It comes in many forms: dead birds, crabs, fish, seals, even dead whales. In the past, California condors (Gymnogyps californianus) were at the top of California’s beach carrion food chain. They may return to Limantour if condor recovery plans continue to be successful.
Birds die at sea for many reasons, and then wash onto the beach. Sometimes the deaths are so widespread that they are likely to be related to failure in the food supply. Often it is a seemingly random individual, cast on shore by waves, dead or dying upon arrival and finished off by the carrion eaters. While not the frenetic bedlam that erupts when shorebirds battle over mole crabs, discovery of a carcass can also be violent. Common ravens (Corvus corax) are very aggressive and will chase gulls away from a body, pouncing down from the sky. But turkey vultures (Cathartes aura) and ravens often commingle, almost seeming to take turns.
The biggest prizes aren’t dead birds, however. They are the rotting, hulking bodies of pinnipeds, including northern elephant seals (Mirounga angustirostris) and harbor seals (Phoca vitulina). Turkey vultures reign supreme around these discoveries, and the food supply can last for days if not weeks. Ravens will usually hang back while vultures tear into the flesh. But the ravens will leap in if given the chance, only to fly away when the vultures close ranks.
I have returned most years to Limantour Beach since my first visit in 1976. The landscape, despite big fires in nearby forests, remains largely the same, even if now threatened by drought and disrupted by serial atmospheric rivers.
But there have been shifts in the bird community out on the beaches, most notably the virtual disappearance of willets and the decline in marbled godwits. Mole crab numbers, however, still appear abundant.
What remains the same is the intense energy and chaos of birds on these beaches, living and dying, hunting and hunted, and my gratitude for the continued existence of this place of wild solitude and for the mole crabs, without whose presence the wave-washed zone of Limantour would be infinitely poorer. For more shorebird images, visit https://bit.ly/Liman tour Bedlam.
