The Horseshoe Crabs of Delaware Bay
Many shorebirds are long distance migrants that complete
trips between their wintering and breeding grounds in stages.
Rather than stopping and feeding frequently during their journeys,
they fly directly between a few widely separated stopover areas
where they feed for extended periods.
Given an abundant food source, shorebirds have the ability to
quickly store the fat they need to fuel their long distance flights.
The stopover areas provide seasonally abundant food sources that are
critical for the next leg of their trips.
Stored fat may also be needed to survive once they reach their breeding grounds.
Many shorebirds breed in the artic or sub-artic and arrive before it has warmed enough
for food to be available. Under these conditions, shorebirds must continue
to live off their fat reserves for the first part of the short artic
breeding season.
Surveys have shown that the Delaware Bay is the 2nd largest stopover location
in the Western Hemisphere for northward bound shorebirds (only Copper River Delta
on the Alaskan coast hosts more birds).
Estimates of anywhere from 300,000 to more than a million shorebirds stop at
the Delaware Bay each year on their way north.
What makes the Delaware Bay so attractive to the shorebirds is the eggs of
the largest concentration of spawning horseshoe crabs
along the Atlantic coast.
The eggs of horseshoe crabs are buried in nests at a depth of 15-20cm
and are inaccessible to most shorebirds.
However with the high concentration of horseshoe crabs that occur
along the Delaware Bay, previously laid nests are dug up in the
process of laying new nests.
The eggs from these disturbed nests accumulate on the surface and provide
most of the eggs that the shorebirds feed on (image).
Without a high concentration of horseshoe crabs,
the abundant food source would essentially vanish.
There are 4 main species of shorebirds that use the Delaware Bay as a stopover
area during their northward migration: Red Knot, Semipalmated Sandpiper,
Ruddy Turnstone, and Sanderlings (see Delaware Bay Shorebird Gallery).
For some of these species, significant
proportions of the hemispheric populations can concentrate along the bay
in May and June (Red Knot:80% and Sanderlings:30%).
The migratory flight of the Red Knot is an example of the extreme distances shorebirds travel and the dependence they have on stopover areas such as Delaware Bay. Red Knots arrive at the Delaware Bay in late May below their fat-free weight after flying 7,000 miles from southern Brazil. They double their weight during their 2-3 week stay (graph) before flying 1,000 miles in early June to their breeding grounds in the low Canadian Artic. One estimate of the number of eggs that a knot must eat to double its weight is 135,000 (Harrington, 1996).
