Double-crested Cormorant (Phalacrocorax auritus)
When DDT was introduced in the late 1950s, double-crested cormorant populations in North America suffered. Throughout the 1960s and 1970s, an 86% decrease in population size occurred, and some colonies disappeared altogether. It was determined that large numbers of abnormally thin-shelled eggs, a result of DDE (a metabolite of DDT) toxicity, were crushed by incubating parents. Once DDT was banned in North America, the cormorant population gradually began to increase, and today it is more common than ever before! However, PCBs still take their toll on cormorants, causing such deformities as crossed bills, abnormal foot development, and eye and skeletal malformations.
At up to 130 cm in length, and almost 2 kg, these large birds are not seasonally or sexually dimorphic. Its body is iridescent black with a large orange-red throat pouch. It has a long, slender, hooked beak and the double crest of two whitish tufts on its head is what gives this bird its name.
The double-crested cormorant breeds throughout the St. Lawrence seaway, the Great Lakes, and across North America. It also maintains year-round populations in Alaska, and on the southwest and southeast coasts of the United States. It is found throughout the Atlantic provinces from northern Labrador and south. Its wintering range includes the western and southern United States coastlines, and various islands in the Gulf of Mexico.
Double-crested cormorants are colonial nesters, and require undisturbed nest sites with a good food supply. The nests are constructed mostly from sticks, and are usually built on the ground, although they may also build on cliffs, or in trees and shrubs. Three or four blue eggs are laid, and are incubated for around 26 days. The young are altricial, and become independent at 8 to 10 weeks of age. The male and female share both incubation and parental duties. Colonies are not synchronized with regard to nesting, so eggs, young, and fledglings may all be seen at the same time in a single colony.
Double-crested cormorants are gregarious year-round, and it is not uncommon to see large flocks of them fishing together. Group defense of nesting colonies from avian predators, such as gulls and crows, is typically aggressive. Flocks often fly in V-formations, much like geese do.
Double-crested cormorants are often observed standing on land, with their wings spread wide. Cormorant contour feathers are designed to decrease buoyancy, making pursuing fish underwater easier. Because of their design, these feathers are not waterproof, and they must be dried when the cormorant emerges from the water. Other functions of this spread-wing posture in cormorants may include the elimination of parasites and the realignment of feathers.
When the daytime temperature is high, both adult and nestling cormorants often hold their mouths open, fluttering the throat pouch rapidly. This behaviour is called gular fluttering, and is used to increase evaporation, thus increasing evaporative cooling, and reducing body temperature.
In the 1940s and 1950s, cormorants were considered pests and control measures were implemented to prevent their alleged competition with commercial and sport fishermen. During the 1990s, the fishing industry once again began to persecute cormorants, claiming that they consume large quantities of fish (salmon and lake trout), and the prey of these fish. However, research has shown that, in the Great Lakes, the double-crested cormorant eats few sport fish, and concentrates instead on "junk fish" such as alewife and smelt. It has been calculated that cormorants eat only 0.5% of the fish that are eaten by salmon and lake trout.
Human disturbance can adversely affect nesting colonies of double-crested cormorants. In a study conducted in the St. Lawrence Estuary, it was found that frequent human visits to colonies increased the likelihood of nest abandonment and gull predation, and appeared to deter late arrivals to the colony from nesting.
In addition to predation by gulls of various species, other predators common at double-crested cormorant colonies include crows, ravens, and herons. If terrestrial mammals such as raccoons or foxes gain access to a colony, they too become active predators of eggs and nestlings.