Geoscience Reference
In-Depth Information
square meter and averages an impressive 20,000. Shorebirds forage in greater numbers on mudflats with the
highest densities of Corophium and avoid mudflats where it is scarce, even though other prey, such as saltwa-
ter worms, may be extremely numerous— proof that they are critically dependent on the mud shrimp as their
prime source of caloric energy for their impending flight to South America.
The key to the successful functioning of this prey-predator relationship is the reproductive cycle of Corophi-
um, which undergoes a population explosion just in time for the arrival of the sandpipers from their Arctic
breeding grounds. In Europe, Corophium produces only one generation of young, whereas in Fundy, two gen-
erations are produced annually. Corophium live but a year, and in Fundy, ice-scouring drastically reduces the
overwintering population. The survivors produce their first young, which look and act like miniature adults, in
late May. The progeny grow and mature quickly, releasing more young in mid-July—just when the shorebirds
are beginning to arrive.
The females are the first arrivals. During the short Arctic breeding season, they produce a clutch of three or
four eggs. The precocial chicks are capable of taking care of themselves at birth, and the females leave shortly
thereafter, first flying from their central and eastern Arctic breeding grounds to the west coast of James Bay,
where extensive marshes and mudflats provide an ideal, food-rich staging area for their 1,500-kilometer
(932-mile) flight to the Maritimes. The females arrive in Fundy with a fat reserve and, unhampered by the
more aggressive males, immediately begin topping up for the last leg of their epic journey by selectively re-
moving the larger, overwintering mud shrimp.
By late July the males and juveniles, who appear to travel together, begin arriving, in time to take advantage
of the second generation of mud shrimp production. The birds consume between 9,600 and 23,000 mud shrimp
during a single 12.5-hour tidal cycle. To maximize their food intake, the birds must follow the retreating tide
very closely. As the tide falls the mud shrimp emerge from their burrows to feed on algae or to seek a pro-
spective mate. They do so for only about twenty minutes, when the mudflats begin to dry out, forcing the mud
shrimp to retreat to their burrows. The ever-vigilant peeps, feeding with an almost maniacal diligence, are able
to double their weight, from 20 to 40 grams, in ten to fourteen days of feeding. These are the highest weight
gains for shorebirds recorded on the Atlantic coast, suggesting that there is a selective advantage for birds us-
ing Fundy, which shorebird biologist Peter Hicklin has cogently described as a “fat station.”
The shorebirds need every bit of this caloric capital for their 4,000-kilometer (2,500-mile) journey over
open water. They time their departure in advance of a cold front, signaled by a northwest wind, which carries
them in the direction of Africa, seemingly off course. But over the Caribbean, they encounter the northeasterly
trade winds, which push them back toward their desired destination of the north coast of South America.
