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.

Connections: Birds and Crabs, Part IX

Another small shorebird - ranging in length from five to seven inches – is the Semi-palmated Sandpiper (Palmation refers to the webbing between an animal’s toes.  Thus, a semi-palmated bird is one that has very short webbing between its toes.  Again, you would have to be really really close to this bird [eye to toes] to notice this particular feature).  The adults have moderately long black legs and a moderately long bill that may droop slightly at the tip.  Their body is gray-brown on top and white underneath with a streaked breast.  One of their most distinctive habits is their ability to sleep on one leg with their bill tucked into their back (Unless you are into some serious yoga, this is not something I would recommend you try – no, not even in the privacy of your own bedroom).
            The Semi-palmated Sandpiper’s non-breeding winter range extends along the coastline of the northern edges of South America – from the southern tip of Peru all the way around to southern Brazil.  On the other hand, their summer breeding range traces a primarily aquatic territory from northwestern Alaska, across northern Canada, and up into the Arctic Circle.
            Like many migratory birds, Semi-palmated Sandpipers nest on the ground.  The male makes several shallow scrapes on the ground.  During this process, the female patiently waits and then carefully selects one of the scrapes (in much the same manner, and in approximately the same amount of time, as one’s daughter might select a wedding dress) – adding grass and other plant material to line the nest.  The female will lay about four eggs in the nest and afterwards, the male assists with the incubation process.  Again, the females, like Ruddy Turnstone females, leave the young with the male after just a few short days.  Soon thereafter, the young begin feeding for themselves.
            Semi-palmated Sandpipers migrate in flocks which can number in the hundreds of thousands.  Flocks of these birds, flying in formation, are quite a sight, particularly as they sweep over their favored feeding locations along the Delaware Bay.  For the most part, these birds will congregate on mudflats.  When not dining on horseshoe crabs eggs, they will often forage on aquatic insects and small crustaceans.

Connections: Birds and Crabs, Part VIII

            Another predominant shorebird to visit the shores of Delaware Bay is the small (7-8 inches in length), plump Sanderling.  In summer its back, head and neck is a bright rusty-red with splatterings of black.  It also sports a white belly, black legs and bill.  In flight they show a strong white wingbar.  The winter bird, on the other hand, is very pale, almost white apart from a dark shoulder patch.  This is the source of its species name, alba, which is the Latin for "white."  Sanderlings are distinguished from other wading birds by the absence of a hind toe (Note to potential birders: You have to be really, really close to a Sanderling to see that it doesn’t have this appendage.).  This species includes some of the smallest of all shorebirds, with a weight range of 1.4 to 3.5 ounces.«
            Like Red Knots, Sanderlings are circumpolar breeders wintering primarily in the southern regions of South America, Africa and Australia.  Known as a highly gregarious bird, it will often form enormous flocks on sandy beaches or coastal mudflats.  Its diet consists mainly of small crabs and other tiny invertebrates.  They are also distinguished by a unique “bicycling” motion as they scamper across the beach, stopping frequently to pick up small food items.  A flock of these birds scampering across the beach looks very much like the peloton in the Tour de France as it winds its way around deep valleys and across rich vineyards throughout the three-week race.
            After departing the feast on the Delaware Bay, these birds head for the breeding grounds of the high Arctic region.  There, the female will lay a clutch of three to four eggs in a ground scrape.  Their diet changes considerably while in these polar regions – everything from insects to various kinds of plant material.

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« A sanderling weighing 50 grams (1.7 ounces, or the weight of ten nickels) can eat one horseshoe egg every five seconds for 14 hours a day (that’s 10,080 eggs a day).