Connections: Birds and Crabs, Part V

Although upwards of 30 different species of birds may crowd the beaches of Delaware Bay, there are four primary shorebirds that use the Bay as a stopover area during their northward migration.  Red Knots, Semi-palmated Sandpipers, Ruddy Turnstones, and Sanderlings make up approximately 97 percent of the birds that visit these shores.  Let’s take a look at each of these distinctive species.

            Red knots have an average wingspan of about twenty inches (this is slightly more than the distance from the tip of your middle finger to your elbow) and an overall length of approximately nine inches (a distance slightly longer than that from the tip of your middle finger to your wrist).  Their backs look like a psychedelic checkerboard – a crazy assembly of mottled buff, streaks of black, and bits of white in a seemingly haphazard pattern.  Their distinctive name comes partly from the fact that much of their head and all of their belly is robin-red.
            Red knots winter along the southern coasts of South America, primarily in Tierra del Fuego (Spanish for “land of fire,” and discovered by Ferdinand Magellan in 1520).  During its breeding season the Red Knot inhabits the mainland and islands scattered across the Arctic Circle.  Take a look at a globe and you will quickly appreciate the incredible distances it has to fly each year - 9,300 miles from south to north every spring and 9,300 miles from north to south every autumn (that’s an annual journey of 18,600 miles – a distance equivalent to three-and-a-half round trips, by car, between Los Angeles and Washington, DC).  As you might imagine, this bird is regarded as one of the longest-distance migrants in the animal kingdom.«
            Red knots migrate in enormous flocks – gatherings that are considerably larger than most other shorebirds.  Most migrating birds tend to cover enormous distances in one fell swoop (pardon the pun); however, red knots tend to segment their journeys into sections of about 1,500 miles at a time.  As a result, they tend to having “staging areas” – specific landing spots along the entire Atlantic coast.  They will use the spots regularly (just like you might travel to the same vacation destination year after year) – stopping in the same places at approximately the same times year after year.  These favorite stopping off points are familiar territory for the red knots and they can anticipate the available of food at each location.  On the downside, however, these sites make the bird susceptible to poaching, severe habitat change, and endemic diseases and toxins.  As an example, red knots were heavily hunted in the early 20th century, and have never fully recovered in parts of eastern Canada. 

---

« A red knot banded in May 1987 was identified again on Delaware Bay in May 2000. During the intervening thirteen years, it was estimated that this single bird flew approximately 242,350 miles, a distance greater than that between the earth and the moon (the average distance between these two celestial objects is 238,855 miles).

Connections: Birds and Crabs, Part IV

As you might imagine, eggs that lay buried in the sand have the greatest likelihood for survival.  Eggs driven closer to the surface have the least likelihood for survival due to any number of environmental factors (lack of moisture, lack of adherence), but also because they become more readily available for shorebird consumption.  There’s also another factor at play here – the length of the shorebirds’ bills.  For example, red knots (whom you will meet shortly), one of the most prevalent of the migrating shorebirds, have some of the longest bird bills – on average 1.3 inches long.  Thus, they can forage in moist sand and extract any eggs within 1.3 inches of the surface.  Obviously, eggs buried deeper are beyond their reach and thus considerably safer and considerably more viable.
            You might think that the length of the red knot’s bill would deny it access to any of the deep-buried eggs.  Not so.  For, if nothing else, the red knot are opportunistic feeders – in short, they allow others to do all their work for them.  In this case the workers are the other horseshoe crabs who dig and scrape and mess around the nests of previous females – disturbing and excavating previously-laid eggs as they try to lay their own eggs.  Being the first to lay one’s eggs isn’t always a good thing – it may subject those eggs to the “digs” of females who follow.  Archeologists sometimes experience something similar  when they discover an ancient city that has been built on the ruins of a previous ancient city which has been built on the ruins of yet another ancient city.  A succession of spawning females – all nesting at approximately the same spot on the beach – has the potential to release thousands and thousands of eggs from each of the previous nests and onto the beach.  The more eggs released, the more eggs available for consumption – even by birds with short bills.

Connections: Birds and Crabs, Part III

          It may be surprising to learn (as it was for me) that the relationship of horseshoe crabs with the spring migration of migrating shorebirds has only received extended scientific attention since the early 1980s.  It was then that the New Jersey Audubon Society’s initiated shorebird surveys of Delaware Bay beaches.
            The shorebirds are lured here by an incredible banquet of little green eggs.  Approximately thirty different species of shorebirds bent on doubling their body weights cram the Delaware and New Jersey shorelines.  According to Mark Botton, a professor of biology at Fordham University and one of the leading experts on the American horseshoe crab, these “staging areas serve as [vital] stopover points for the birds to feed before continuing their migration.  An estimated 425,000 to 1,000,000 birds stop in Delaware Bay…during May and June, as they travel from their South American wintering grounds to their Arctic breeding grounds.” 
            Most of the activity tends to occur during the third or fourth week in May.  Hundreds of thousands of migrating shorebirds arrive just as hundreds of thousands of horseshoe crabs emerge from the depths of Delaware Bay to lay their eggs.  Intensive studies have revealed that each female crab will deposit up to twenty egg clusters during her spawning period.  Each of those clusters will contain between 3,000 and 4,000 eggs.  Thus, each female has the capacity for laying up to 80,000 eggs during a single spawning season.«  Multiply that 80,000 by up to a million or more crabs and you can quickly see that the banquet offering for birds has the potential to exceed their wildest dreams (if, indeed, birds could dream).
            Most female crabs will deposit their eggs approximately 4-6 inches below the surface of the sand.  Although this depth is beyond the reach of most shorebirds, constant wave action and the burrowing of other spawning crabs (female crabs will deposit their eggs wherever they want – irrespective of whether another female has deposited her eggs in a particular location or not.  In short, every square inch of the beach is “fair game” as a potential egg deposit area.) move some of the eggs toward the sand’s surface.  It is these “disturbed” eggs that the voracious shorebirds seek.

---

« For those who wish to compare horseshoe crabs with humans, here are some relevant facts:  A human female typically has about 400,000 potential eggs, all formed before birth.  Only about 480 of those eggs will actually be released during her reproductive years.  And, in case you were about to ask - your average chicken will lay about 300 eggs in her lifetime.

Connections: Birds and Crabs, Part II

           These shorebirds are travelers with a purpose – like a family on a cross country summer drive.  Except that the winged travelers in this geographical location have come all the way from the southern tips of South America and will eventually travel all the way to distant and necessary nesting grounds near the Arctic Circle.  This is an annual ritual - a long and exhausting journey each spring that provides these birds a few precious stops along the way…a few precious places to feed and rest.  Delaware Bay just happens to be one of those stops and the timing of their visit is critical.  It is vital that the birds reach the Arctic while the snow is melting to ensure their eggs will hatch in time for the annual insect hatch.  Those insects constitute the primary diet of the young shorebirds.  If the birds arrive too early the insects are not there.  If they arrive too late the newly hatched young have nothing to eat.  This is similar, in many respects, to the feeding habits of teenage boys (“There’s nothing in the refrigerator!,” he yells.  “Sure there is, I just went shopping,” his mother yells back.  “There’s nothing GOOD in the refrigerator!,” he yells even louder.).
            The Delaware Bay is a critical stopover point in the long-distance journey to Arctic regions.  It is vitally important that the hungry birds find plenty of food here or they will not have sufficient energy to complete their arduous journey.  Most have traveled night and day and day and night to arrive here.  After departing from near the bottom of the world they make a single pit stop on the southeast coast of Brazil.  After a brief rest they take off for an unbelievable nonstop journey of 5,000 miles over vast oceanic distances and occasional land forms for the shores of Delaware Bay.  Imagine traveling for 5,000 miles hoping, just hoping, that there will be some food at your destination.  If there is, you live!  If not,….
            The Delaware Bay is not haphazard – for it is here that one of the great massings of creatures takes place each year.  It is here that one of the great mysteries of nature is about to unfold – the arrival of these birds in concert with the arrival of horseshoe crabs upon these beaches.  Consider, if you will, the incredible biological coincidences of these two species arriving almost at the same place at almost the same time – all within a very narrow window of opportunity.  It is though a great script has been written and the players (birds and crabs) are merely fulfilling their ecological obligations – obligations that have been taking place for thousands of years.

Connections: Birds and Crabs, Part I

          It is early morning as I walk out on a very deserted beach along the western edge of Delaware Bay.  The sun is just edging through last night’s cloud cover and it sprinkles the sand with a few errant rays of sunshine.  As I gaze out over the beach I can see clusters…no, hoards…no, armies of birds swopping and swarming and dancing and flittering across the vast sandy stretch.

          I grew up a mere wing flap from San Juan Capistrano in southern California.  Each year, on St. Joseph’s Day (March 19) the swallows would return to San Juan and begin rebuilding their mud nests, each of which would cling to the ruins of the old stone church.  The arches (which are ideal for nest-building) of the two story, high vaulted Chapel were left bare and exposed, as the roof collapsed during the earthquake of 1812.  The town of San Juan Capistrano always takes on a festive air as scores of (human) visitors from all parts of the world gather to witness the "miracle" of the swallow’s return.

            What I am now witnessing on a stretch of Delaware beachfront reminds me of the hordes of migrating birds I saw in my youth.  Here, however, the birds are of every stripe, color, and description.  There is a constant blur of wings, a cacophony of shills, shrieks, and calls, and the “crissing” and crossing of flight paths as winged travelers dart through the pale blue sky and dance their way over the consistency of waves.  With shrill squeals and whistles, flock after flock and battalion after battalion of birds loop, glide, descend and fight.  They swoop and swirl and swarm over long lengths of sand – claiming territory for a few moments and giving it up just as easily with each new arrival.  The constant noise and never-ending activity makes the beach seem like an avian insane asylum.  This is craziness – bird craziness - to the tenth power!
            Crowded onto a far-reaching strip of land that bisects the waters of Delaware Bay and the lazy dunes of sand behind me is one of the largest gatherings of birds in the world.  These birds are on a quest – they are determined, they are pushy, but most of all they are hungry.  Their numbers are a million or more and they have been flying almost nonstop from their winter homes thousands of miles to the south.  From remote beaches in Patagonia, Tierra del Fuego, and the furtherest and most isolated reaches of South America a seemingly unending multitude of starving shorebirds have come to Delaware Bay.
            The gulls are bullies – they crowd together pushing and shoving against the great mass of their long-distance cousins.  Long-billed dowitchers poke and root beneath the wet sand at the water’s edge.  Yellowlegs wade in the shallows searching for food.  Red knots sweep across the sand like large flapping curtains randomly tossed over the landscape.  Sandpipers dance in and out of the gentle waves playing tag with each other.  Ruddy turnstones, sanderlings, plovers, dowitchers, dunlins, and willets all arrive – wave after unending wave after unending wave.  This does not have the gentility of San Juan Capistrano (tiny swallows are, oh, so much more polite) – the birds here are ON a mission, not IN a mission.
            They are here for the eggs…a feast of horseshoe crab eggs.