The double-crested cormorant typically forages in shallow waters, usually less than 30 kilometres from the coast, either alone or in large feeding flocks (5). After a little dive clear of the water’s surface, its outer feathers quickly become soaked, reducing buoyancy, but the inner feathers are waterproofed and insulate against the cold waters. Like other cormorants, this sleek, efficient predator skilfully catches its prey in its long, hooked beak after an underwater pursuit, whereby alternate strokes of the webbed feet propel and steer the streamlined body forwards (2) (8). Each dive rarely lasts longer than 30 seconds between rest intervals of around 20 seconds at the surface, and upon returning to land, the double-crested cormorant assumes a characteristic posture and stands upright with the wings extended outwards, exposing itself to the sun to dry (5) (8). This cormorant feeds almost exclusively on fish, such as sandeels, flatfish and perch, although some crustaceans may also be eaten (2) (3).
During the breeding season, this sociable bird gathers into huge colonies of several thousands, often with other cormorants, gulls, auks, herons and ducks (2). Males arrive at nesting sites first, and immediately begin unusual displays to attract a mate, waving their wings while the tail is cocked upward and forward. Upon pairing up, mating birds set about constructing a flimsy nest of sticks and seaweed on the ground, in a tree or even on a pylon (2) (3). Breeding pairs regularly change nesting sites between seasons, but competition for the best location is always fierce and breeding birds will engage in aggressive territorial behaviour (3) (5). Typically this rarely goes beyond threatening postures, but on occasions when it does, fighting may ensue, with bill-grappling, or birds seizing the opponent by the neck and wings (3) (5) (8). Both the male and female take turns in incubating the clutch of usually 3 or 4 eggs for around 25 to 29 days (2) (3) (5). The chicks hatch naked and helpless, and parental care even extends to sheltering the newly hatched young from the sun, with adults observed standing with their wings outstretched with their backs to the sun (3) (5). The chicks grow quickly on an energy-rich fish diet and will fledge after around 42 days in the nest, before becoming fully independent after a further 30 days (2) (3). The double-breasted cormorant may first breed in its third year and live to over 15 years of age (3).