Geoscience Reference
In-Depth Information
Delaware Bay. Feeding sites such as Delaware Bay are critical links in a chain that allows shorebirds to com-
plete their life cycle—which, for red knots, spans some 25,000 kilometers (15,500 miles) from wintering
grounds in the Southern Hemisphere to breeding grounds in the Northern Hemisphere. Shorebirds follow what
Brian Harrington of the Manomet Observatory for Conservation Sciences, in Massachusetts, has described as a
“moveable feast.” They concentrate at far-flung sites where food is uniquely abundant to fuel the next leg of
their journey. This tendency for significant proportions of a whole population to concentrate at a single site
makes shorebirds particularly vulnerable, since it breaks the normal link between the abundance of a species
and its immunity to extinction.
Migrant Trap at Land's End
The Cape May Peninsula forms the northern shore of Delaware Bay and is a concentration point for migrating
birds, especially in the fall. Vast numbers of songbirds stop over here on their southerly migrations, pausing
before making the flight over the mouth of Delaware Bay to Cape Henlopen on the other side. In 2001, it is es-
timated that 1.25 million American robins, 300,000 red-winged blackbirds, 75,000 house finches, 2,500 east-
ern bluebirds, 2,000 rusty blackbirds, as well as thousands of sparrows, juncos, and yellow-rumped warblers
converged on Cape May.
These birds were following what in the past has been referred to as the “Atlantic flyway,” a term that is now
debated if not defunct in ornithological circles. Birds, and other aerial migrants like butterflies and dragonflies
(millions of monarch butterflies and dragonflies join this aerial parade), may follow certain lengthy geographic
features such as mountain ranges or rivers. Migrants are attracted to these so-called leading lines for the ad-
vantages they provide, such as updrafts or ample food supply. By contrast, the Atlantic coast is thought to act
as a diversion line along which migrants such as passerines and hawks concentrate because they want to avoid
flying over the waters of Delaware Bay and the Atlantic beyond. Shorebirds fly great distances over the open
Atlantic, but most birds, from hawks to songbirds, avoid flying over water if they can and so follow the coast-
line south. It is thought that the predominant northwest winds of autumn push most birds east of the Rocky
Mountains toward the east coast, where they encounter the forbidding Atlantic and then head south along the
coast, describing a dogleg migration.
Dragonflies (left) join millions of birds that use the leading line of the Atlantic coast as a migratory route; the brackish
marshes of Delaware and Chesapeake Bays are also home to amphibians and reptiles such as this rare bog turtle
(right).
