Geoscience Reference
In-Depth Information
abandoned, and only 22.3 percent of nests produced one or more young. On Île aux Pommes I had watched as
a newly hatched chick fell behind its mother when it was briefly rebuffed by waves lapping the shore. With
great exertion it struggled to the top of a beach stone; then, as it paused momentarily, a herring gull swooped
in, plucked the duckling cleanly from the rock, and with one devastatingly efficient toss of the head swallowed
it whole.
Such a scene calls into question why eiders choose to nest among predatory gulls. Female eiders are large
enough to defend themselves against gulls and rarely leave their nests; therefore predation on eggs is low. In
some instances, the wary gulls might perform a positive function by warning eiders of a potential threat, such
as a fox or a human. The study on Île aux Pommes showed that eiders nesting within 5 meters (16 feet) of her-
ring gull nests actually had higher success, indicating that territorial defense by herring gulls does confer an
advantage on the eiders.
Once in the water, the adults and ducklings form extended families called crèches—literally nurseries.
Chick-crèches are typical of colonial nesters like eiders, whose eggs hatch roughly at the same time. Such
groupings range in size, from a single duckling with an adult female other than its own mother, to huge creches
of fifty or more ducklings and several accompanying females, often nonbreeders whimsically referred to as
“aunts.”
In the past, it was thought that crèches constituted a deliberate cooperative system to care for the young or
were an evolved behavioral trait. But recent research indicates that crèches form accidentally, when broods run
into each other on the crowded islands, or spontaneously, in the face of gull attacks—the duck equivalent of
circling the wagons. Regardless, the larger the crèche is, the lower the duckling mortality, a reflection of the
dilution principle whereby the fledgling loses itself in the crowd and lowers its individual risk of becoming a
victim of predation.
These protective extended family units stay together for the entire rearing period of ten weeks. Once the fe-
males guide the ducklings to shallower, more protected nearshore waters, the young are able to feed them-
selves by diving for molluscs and crustaceans. Following the nesting season, the eiders move downstream,
males and nonbreeding females converging at sites in the Lower Estuary where their chief prey item, the blue
mussel, is most abundant. Ducklings and the adult females accompanying them gather along the south shore of
the St. Lawrence, during July and August, in areas where the brood has access to its principal prey, the peri-
winkle. Eiders swallow their prey whole, shell and all, to be ground up in the bird's muscular gizzard.
The St. Lawrence eiders begin to move south in mid-September, flying low over the water in a long-line
formation, stringing out impressively along the horizon and appearing to skim and dip between the waves.
They show up along the northeast coast of New Brunswick from October through November, when they are
forced out of the gulf by ice formation. Some eiders, however, remain in the gulf, where ice conditions allow
access to food. Aerial surveys have shown that roughly 30 percent of the eider population over-wintering along
the coasts of eastern North America is found in the Gulf of St. Lawrence, with 95 percent of that total concen-
trated along Quebec's North Shore, between Anticosti and the Mingan Islands.
Those that do migrate take two routes: many pass overland across the Chignecto Isthmus from the Northum-
berland Strait to the Bay of Fundy, and the balance migrate through the Strait of Canso. While migrating over
land, they assume a higher flying pattern—perhaps to allow a view of water on the other side—with the front
ranks massed in a semi-circle and followers in a long wavy line behind. Many are destined to spend the winter
in Massachusetts Bay, though smaller numbers overwinter in ice-free areas along Nova Scotia's south shore,
forming large rafts in winter, with as many as a thousand individuals.
BESIDES ITS BREEDING
islands and food-rich fringing marshes, the St. Lawrence River itself is a wildlife thor-
oughfare. While on the eider islands, I witnessed a remarkable parade—a large pod of beluga whales, perhaps
as many as a hundred animals, passing by on their way upstream. Like synchronized swimmers, their start-
