Habitat

Occupies diverse aquatic habitats in all seasons; nonbreeding birds are distributed more widely. Most inland breeders winter in coastal areas; increasing numbers winter inland near aquaculture sites. In all seasons, requires, in addition to feeding habitats, suitable places for daytime resting or loafing and nighttime roosts. Between bouts of fishing, cormorants spend much time perching on exposed sites such as rocks or sandbars, pilings, ship wrecks, high-tension wires, or trees near favored fishing sites. Such daytime resting places (loafing areas) may also be nighttime roosts for some individuals, but roosts are often more remote and used by larger numbers. Most individuals forage in shallow water (<8 m deep), typically <30 km from colony or roost, within sight of land; rarely seen offshore. Numbers increase rapidly wherever prey are readily accessible, and predictable seasonal patterns may result. Proximity of preferred roosts and nesting colonies (which may be limited in number) to foraging areas greatly influences daily flight distances. Some birds nesting (or roosting) on coastal islands fly inland to feed at freshwater sites (JJH).

Occurs on ponds, lakes, artificial impoundments, slow-moving rivers, lagoons, estuaries, and open coastlines. Individuals forage up to 62 km from colony or roost (see Demography and populations: range, below). Colonies are established at sites safe from ground predators and close to feeding areas (typically <10 km away); generally these sites have been used as roosts and/or loafing areas for several years. Selects small rocky or sandy islands, where available, and may use artificial sites such as bridges, shipwrecks, abandoned docks, or nesting-towers (Meier 1981). Also nests on mats of emergent vegetation in marshes (Gilligan et al. 1994). When nesting (or roosting) in trees, these trees are usually standing in or near water, on islands, in swamps, or at tree-lined lakes. Ground-nesting may be both the ancestral and the preferred habit for this species, nesting in trees being a response to predators (Lewis 1929). May shift from tree- to ground-nesting at a colony site as trees die and colony grows, but where predators are present, depends for nesting on flooded snags or live riparian trees.

Before and after breeding season, likely to occur farther from colony sites and to feed and roost in new areas. Seasonal patterns illustrated for Penobscot Bay, ME, where cormorants shifted between river and bay (Blackwell and Krohn 1997). Spring arrivals (Apr) fed largely in fresh water, concentrating at river dams and roosting nearby. In May, they consumed many salmon smolts; in Jun, nestlings on islands in the bay were fed marine fish (see Food habits: diet, below).

Largest numbers occur along southern coasts, where habitat use is poorly known, but, as in breeding habitat, birds require feeding, loafing, and roosting sites. May assemble in tens of thousands on sandbars in coastal inlets. Many cormorants wintering near catfish (Ictalurus sp.) farms in Mississippi roost in isolated cypress swamps; morning flights from such roosts to foraging areas average 15.7 km (range 4–62, n = 33; King et al. 1995).

Sex differences in range or habitat use have not been examined, but this possibility was suggested for birds wintering in Texas: King et al. (1987) found 27 females among 29 cormorants collected in Houston Ship Channel. For birds wintering inland, males predominated in samples shot at roosts in Mississippi; in catfish production areas, the proportion of males was 0.78 (n = 284), and near Mississippi River, 0.66 (n = 177; Glahn et al. 1995); in Alabama, 0.48 (n = 77; Glahn et al. in press).