Cormorants are monogamous and breed in colonies ranging from several pairs to a few 1,000 pairs. Double-crested cormorants have some site fidelity, meaning they return to the same site to breed year after year. Young cormorants often return to colony sites where they hatched or to nearby areas to breed.
In April, the pair begins construction of an elevated platform nest composed of twigs, branches, and other plant materials. These nests often reach a height of 12 to 20 inches and may be re-used in subsequent years.
Like other colonial-nesting birds such as great blue herons (Ardea herodias), cattle egrets (Bubulcus ibis), great egrets (Ardea alba), black-crowned night herons (Nycticorax nycticorax), gulls (Larus spp.), and terns (Sterna spp.), cormorants prefer islands with sparse vegetation. Cormorants nest either in trees, cliffs, or on the ground. After many years of repeated nesting, their guano will kill trees and other vegetation, forcing cormorants to nest on the ground.
Cormorants tend to be attracted to nesting sites of other colonial water birds. Occupying similar habitat may affect other colonial water-bird species such as gulls, terns, egrets, herons, and black-crowned night herons, as well as some waterfowl. Cormorants may directly compete for nesting sites, or alter nesting habitat. For example, cormorant guano deposited under nest trees can kill understory vegetation important for nesting black-crowned night herons. At West Sister Island National Wildlife Refuge in Lake Erie, which supports one of the largest heron colonies in the Great Lakes, great blue heron numbers have declined annually since the double-crested cormorant arrived in 1992, presumably due to a combination of nest site competition, loss of nesting sites, and an increase in human activity.
Cormorants are expert divers, with webbed feet, streamlined bodies, and feathers that hold water and reduce buoyancy. They are believed to dive to depths of 8 to 25 feet. After feeding, cormorants characteristically dry their feathers by perching with their wings outstretched.
Double-crested cormorants of the Atlantic coast and interior populations are seasonal migrants. They leave the northeast in September, migrating south along coastlines and river valleys. The two primary migration routes are down the Atlantic coast and through the Mississippi and Missouri Valleys to the Gulf Coast. Cormorants return to their northern breeding grounds in late March or April.
During the breeding season, double-crested cormorants inhabit lakes, ponds, slow-moving rivers, lagoons, estuaries, and open coastlines. They need suitable nesting sites with feeding areas nearby. Cormorants may nest in trees or on the ground, on steep cliffs, or rocky or sandy islands. They also may use artificial sites such as bridges, wrecks, abandoned docks, or towers. Nesting trees and structures are usually located in or near the water on islands, in swamps, or along tree-lined lakes. Cormorants choose live evergreen or deciduous trees for nesting, though the trees often die within 3 to 10 years because of the significant accumulation of guano deposited on them. They prefer to nest in trees when available, rather than nesting on the ground.
Outside of the breeding season, cormorants use a variety of habitats including marine islands and coastal bays, in addition to those habitats used during the breeding season. Cormorants need places with nighttime roosts and daytime resting or loafing areas during all seasons. They roost on sandbars, rocky shoals, cliffs and offshore rocks, utility poles, fishing piers, high-tension wires, channel markers, pilings, and trees near their fishing grounds.
Like most colonial waterbirds, double-crested cormorants can have a significant impact on vegetation at breeding and roosting sites through normal nesting activities. Their guano is acidic and can change soil chemistry, killing ground vegetation and irreversibly damaging nest trees. Cormorants also destroy vegetation directly by stripping leaves and small branches from trees for nesting material. At times, the weight of the birds and their nests can even break branches. Loss of trees can lead to increased erosion, particularly on sand spits and barrier beaches.
In one example on Little Galloo Island in Lake Ontario, all of the trees have died over time due to a combination of defoliation and guano. Damage to vegetation can occur relatively quickly after cormorants move into an area. For instance, in the St. Lawrence estuary, cormorants on several islands caused irreversible damage to trees in less than three years. After cormorants started nesting on Young Island in 1982 at Lake Champlain, all but one nesting tree was killed by 1996.
In some cases, cormorant colonies have significantly affected rare plant communities. For example, the islands in western Lake Erie are home to rare Carolinian woodlands with stands of Kentucky coffeetree (Gymnocladus dioicus). Large cormorant colonies there could threaten the continued existence of these plants. The interactions between colonial water-birds and vegetation are natural occurrences that have taken place throughout history. However, in human-altered ecosystems where alternative habitat is limited or unavailable, cormorants can affect the persistence of plant communities and other wildlife species that rely on these habitats.
Double-crested cormorants feed almost exclusively on fish (Figure 6), primarily bottom-dwelling or schooling “forage” fish. These birds are adaptable, opportunistic feeders that prey on many species of small fish (less than 6 inches) usually foraging on those that are most abundant and easiest to catch. This includes fish such as alewife (Alosa pseudoharengus), gizzard shad (Dorosoma cepedianum), yellow perch (Perca flavescens), sculpins (Cottus spp.), and sticklebacks (Pungitius pungitius). Because a cormorant’s ability to catch a particular species of fish depends on a number of factors (distribution, relative abundance, behavior, habitat), the composition of a cormorant’s diet can vary quite a bit from site to site and throughout the year, and can reflect the number and types of fish present in a given area at a given time.
Typically, cormorants feed during the day in shallow water (less than 25 feet) within a few miles of the shore and the breeding colony. To capture fish, cormorants dive below the surface and pursue prey underwater. Dives may last from 20 to 25 seconds or more, and between dives the birds sometimes swim with their heads submerged, searching for prey.
They grasp their prey in their bills and sometimes swallow fish underwater. Cormorants swallow large fish, or those that are difficult to handle (e.g., eels or spiny fish), at the surface. At times, they may throw their prey into the air, catch it, and swallow it head first. Cormorants typically forage individually, but also may gather into feeding flocks of 10s to 100s of birds, especially when preying on small schooling fish.
Adult cormorants feed regurgitated food to their nestlings. For very young chicks, an adult will arch its neck, take the head of the chick into its mouth, and regurgitate a semi-liquid food. Older nestlings will thrust their heads into the adult’s throat and remove whole fish regurgitated into the neck pouch
Overall, double-crested cormorants are not major consumers of commercial and sport fish species. However, exceptions have been documented at specific sites. Cormorants often congregate where there are high concentrations of fish such as stocking release sites, aquaculture ponds, dams, and other areas. In these instances, as well as in some open water situations, they can have significant local impacts.