Connections: Birds and Crabs, Part X

More than a million shorebirds stop over to gorge themselves on horseshoe crab eggs before continuing their northward migration.  During their stopover, the six most abundant shorebird species will consume approximately 539 metric tons (that’s 1,188,279.4 pounds)« of horseshoe crab eggs.  While this may seem like an awfully large number to you and me, it’s simply because of the low metabolic efficiencies of the birds.  As the birds eat the eggs and as the eggs are passing through the birds’ gastrointestinal tracts the cuticles of those eggs are often resistant to both chemical and enzymatic digestion.  As a result, very few of the eggs are broken down and available as nutrition for the hungry birds.  But, birds being birds (does the term “birdbrain” come to mind?) they continue to stuff their bellies with as many possible eggs as they can cram into their gullets – irrespective of any potential nutritional value (Life in a fraternity house is what I’m imaging now – how about you?).

            There have been some scientific calculations which estimate that at least 1.8 million female horseshoe crabs must spawn on the shores of Delaware Bay to provide this food surplus.  For example, nearly 50% of the world’s red knot population stops each spring in the Delaware Bay area.  When they arrive they are literally down to bone, skin and feather, having used up all their fat reserves and much of their muscle in flight.  They need the eggs to survive. 
            Numerous surveys have shown significant declines in the numbers of shorebirds.  The sanderling population alone has decreased an alarming 80 percent over the course of the last several decades.  Suspected causes for the decline are the use of pesticides in the winter grounds, loss of coastal wetlands along migration routes, competition with humans over prime coastline areas, and a possible decrease in available food sources.
            Biologists and birders are constantly tracking shorebird movements and population trends.  This information may help clarify a multitude of factors affecting bird populations.  Obviously, it will only be the fat, healthy birds who survive the long distance migration to arctic feeding grounds.  The short arctic summers mean time to raise only one small brood.  Consequently, low reproductive rates make it harder for any species of shorebirds to sustain healthy populations.
            The relationship between shorebirds and horseshoe crabs is both necessary and critical.«  We know the two are intimately related; however, current evidence suggests that shorebird feeding has no impact on the horseshoe crab population.  It remains to be seen what environmental factors are currently impacting the birds and whether the birds will be able to survive conditions over which they have no control; but rather which control them.

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« In case you were wondering, 1,188,279.4 pounds (of horseshoe crab eggs) is equivalent to the weight of 2,495,346 Big Macs from McDonald’s, 4,659,917 orders of medium french fries from Burger King, or 3,046,869 small Frosty’s from Wendy’s.  Yum!

« On average, it has been estimated that migratory shorebirds will consume approximately seven billion (that’s 7,000,000,000) horseshoe crabs eggs in a two-week period.