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.

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« 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.