Geoscience Reference
In-Depth Information
rock. Many eggs do roll off their ledges, which happens more frequently in the first few days after they are
laid. Murres have been observed to gather a few pebbles together— building a primitive nest—and the pebbles
become cemented to the substrate by sediment and excrement, providing some stability to the egg. This prac-
tice is more the exception than the rule, however. It appears that the pear shape of the egg makes it more likely
to rotate rather than to roll off of the smooth surface of the ledge.
The shape of the egg and the accumulation of sediment and excrement do enhance the egg's stability, but the
most important factor is the shift in weight distribution as the egg develops. During incubation, the small end
of the egg becomes heavier, causing the large end to rise, which reduces the radius of the curve described by
the egg when it is disturbed. The mechanism for this phenomenon is described in Leslie M. Tuck's classic
monograph “The Murres”: “. . . at the beginning of incubation the egg's center of gravity, because of the small
size of the air cell, is close to the large end. Gradually the embryo develops and the size of the air cell in-
creases, causing the movement of the center of gravity toward the narrow end of the egg and a change in the
position of the egg on the ledge.” Tuck points out that this increase in stability is very important because the
possibility of the birds brooding a second egg diminishes dramatically during incubation.
If the egg survives on its precarious perch, the time comes when the chick must take the plunge to the sea.
This “sea-going” of the murre chicks is a dramatic and death-defying event, and usually happens during the
twilight hours to thwart marauding gulls. Adult birds gather at the base of the cliffs and call excitedly to the
chick, which then walks off its ledge, calling shrilly as it plummets downward, furiously beating its still-devel-
oping wings. Invariably, it is accompanied by an adult, which seems to hover over the chick with what Tuck
describes as a “butterfly-like” flight. Once in the water the chick is mobbed by several adults, jostling and
hounding it. Eventually, the crowd disperses and the chick is accompanied out to sea by one or two adults, pre-
sumably its parents.
Alcids are also susceptible to a variety of large-scale mortalities from human sources. Some 450,000 murres
(or “turres” as they are known to Newfoundlanders) are harvested annually for food in the traditional hunt in
the waters off Newfoundland and Labrador. In the past, monofilament drift nets set for both cod and Atlantic
salmon exacted a heavy toll on alcids, especially murres and puffins, near their breeding colonies, drowning
tens of thousands. The decline in the fishery has been matched by a decrease in this seabird by-catch.
Oil pollution, however, remains a major source of seabird mortality off Newfoundland and especially affects
the alcids, which spend their lives far out at sea when not at their breeding colonies. This oil is not from cata-
strophic spills but from smaller but equally deadly dumping of oily bilge water by ships that follow the great
circle route through Newfoundland waters. This illegal but largely uncontrolled activity is equal to an Exxon
Valdez spill every year— accounting for the deaths of some 300,000 seabirds annually. Sea ducks, such as ei-
ders, and the auks are especially affected. The dumping seems to be more prevalent in winter, when oil slicks
are less susceptible to detection because of rough seas and storms. Unfortunately, the winter is also when mil-
lions of thick-billed murres that breed largely in Arctic waters gather on the Grand Banks, where they meet
their sad fate. The loss of these birds in Newfoundland waters has likely led to the substantial declines recently
observed in the Icelandic and Greenland breeding populations.
Down on the Labrador
Straightened out, the Labrador coast would stretch for 20,000 kilometers (12,400 miles). The northern
coast—north of Okak Bay—is deeply indented with fiords, and in the far north, the Torngat Mountains rise
dramatically and precipitously from the sea to a height of nearly 1,500 meters (4,900 feet), making them the
highest mountains in Canada east of the Rockies. This northern region was not covered by the Wisconsinan
glacier, but because the whole coast is composed of Precambrian Shield rocks, which generally resisted glacial
erosion, today, most of the coastline is characterized by high relief. Moreover, it is still rising as it rebounds
from the relatively recent removal of the glaciers' great weight.
