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Double-crested Cormorant

Phalacrocorax auritus

Order:
Suliformes
Family:
Phalacrocoracidae
Sections

Behavior

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Fig. 3. Double-crested Cormorants dive for their prey.

Double-crested Cormorant swimming underwater. By N. John Schmitt, adapted from Johnsgard 1993.

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Fig. 4. Right foot of Double-crested Cormorant showing totipalmate webbing.

Drawing by N. John Schmitt.

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Fig. 5. Double-crested Cormorant spread wings to dry their feathers.

Wettable plumage reduces bouyancy, an advantage in underwater diving. Photo © Brian E. Small.

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Figure 6. Male advertising nest site (Wing-Waving display).

Drawing by N. John Schmitt, adapted from Johnsgard 1993, after Van Tets 1959.

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Adult breeding (Alternate) Double-crested Cormorant in flight; Oceanville, Atlantic Co., NJ; April.

Adults have black or dark-brown plumage, with a dull greenish or bronze gloss that may be absent from worn feathers. The orange-yellow skin of face and throat (gular region) is distinctive throughout year. The “double crest” is a poor field mark; these feathers are variable and are fully developed for only a short time early in year. Distinctive in flight with long neck and slender, hooked bill; holds head up, neck slightly bent. Taken 17 April, 2014 at Oceanville, New Jersey. The following is a link to this contributor's image via Birdshare: Brian Kushner.

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Second-winter Double-crested Cormorant; Moss Landing, CA; October.

Second-winter Double-crested Cormorant with wings open. Spread wing posture may facilitate drying of plumage; cormorants are thought to have wettable plumage that facilitates underwater pursuit of prey. Taken 9 October, 2010 at Moss Landing, California by Brian Sullivan.

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Double-crested Cormorant colony; Cape Cod, Barnstable Co., MA; July.

Taken 30 July, 2013 at Cape Cod, Massachusetts. The following is a link to this contributor's image via Birdshare: Nature.Catcher.

Locomotion

Walking, Hopping, Climbing, Etc.

At rest, holds body nearly vertical, with neck in S-shape and bill tilted slightly upward. On water, similarly tilts bill slightly upward. Clumsy on land. Posture upright, legs relatively short; walks with waddling gait and frequently makes two-legged hops. May use hooked bill to aid in clambering around on rocks and branches (especially young birds). Perches readily on branches, navigational structures and sometimes mast or rigging of boats, or cables and transmission lines.

Flight

Compared to many other seabirds, cormorants have short wings of low aspect ratio, resulting in high cruising speed and low load-lifting ability; individuals are constrained to forage at relatively short ranges and bring small loads of food for their young (Pennycuick 1991). Wing shape is broad, rounded, with ulna and humerus about equal in length and outer part of wing relatively short. In closed wing, secondaries almost equal in length to primaries; P8 and 9 longest, P10 and 7 slightly shorter. Outer vanes of P7–9 and inner vanes of P8–10 emarginate.

This species, unlike some other cormorants, maneuvers sufficiently at slow flight speeds to land on trees or wires. Flies with regular wingbeats, gliding only briefly. Holds neck outstretched or slightly folded. The following data (mostly from Florida) may include flight performance of larger migrants (auritus) as well as local residents (floridanus): Wingspan 1.16 m, aspect ratio 7.52, wing-loading 7.73 N/m2, wing beat frequency 5.03 Hz ± 0.138 SD (n = 172), airspeed 14.5 m/s ± 1.8 SD (n = 189), or 52 km/h (Pennycuick Pennycuick 1989, Pennycuick 1991). Pectoral muscles account for 12.6% of carcass weight (n = 11; Hartman 1961).

Taking flight from perch, initially loses altitude; takeoff from level land requires several hops combined with vigorous flapping. On water, makes similar 2-footed thrusts, synchronized with wing beats; most other waterbirds paddle their feet alternately for takeoff. If no wind, and after fishing, takeoff may require >10 m (Lewis 1929).

When over water, usually flies close to surface; over land, flies much higher. During long foraging flights or on migration, flocks may travel in shallow Vs or echelons. Soaring infrequent, except slope-soaring along cliffs, but occasionally soars in thermals. Landing on water is preceded by gliding descent into wind, then spreading of feet and tail. Tail makes first contact, and bird skids along surface.

Swimming And Diving

Figure 3.  Swims readily and for long distances, but usually leaves water at end of fishing bout. Actively pursues prey underwater. Wings too large to be used as principal means of underwater propulsion, which comes from feet. Morphological adaptations include low buoyancy (Lovvorn and Jones 1991a), partly attributable to plumage that is wettable and traps relatively little air (Wilson et al. 1992b). Cormorants submerged briefly gain water equivalent to 1–3% of body mass (Mahoney 1984). Anatomy of synsacrum and hindlimbs place the large totipalmate feet (medialink) directly behind body when swimming; leg muscles make up 5% of body mass (Owre 1967a). External nares (nostrils) are permanently closed in adults.

When swimming on surface, strokes feet alternately. Dives from surface by simultaneous action of both feet, which may push bird almost out of water before it submerges, or it may slip smoothly underwater. The “leap” may be associated with deeper dives. During submerged swimming, holds wings against sides and propels itself by simultaneous strokes of both feet (Lewis 1929, Van Tets 1959). Underwater speeds not reported (but about 1.5 m/s for other cormorant species).

Diving performance little studied: at one site in Nova Scotia, mean depth 4.7 m (maximum 7.9; Ross 1974b), shallow compared to dives of some other cormorant species. Freely diving captives readily feed at 12 m (M. Enstipp unpubl.). Dive duration usually 20–25 s; average 19.3–22 s in water about 3 m deep, and 22.5–27.7 s in water 4.5 m deep (Mendall 1936a). Dive:rest ratio 2.43: dives 25.1 s, rests 10.3 s (Ross 1974b). On lakes in NY, average individual dive durations ranged from 17 – 34 s; overall average was 22 s (Coleman 2009). Average total amount of time cormorants spent diving (underwater) ranged from 62 – 70 min, and the average was 66 min (Coleman 2009). Cormorants on Oneida Lake and Lake Ontario frequently dove 10 – 15 m; birds on Lake Champlain never exceeded 8.6 m and most were < 2 m. The deepest dive recorded was from Lake Ontario, at 25.8 m (Coleman 2009). Only 7.3 % and 14.5% of dives exceeded 10 m, on Oneida Lake and Lake Ontario, respectively (Coleman 2009). For wintering birds in Mississippi (where many were feeding at shallow catfish ponds, average depth 1.4 m), average dive time 11.9 s, and rate 1.6 dives/min (King et al. 1995a). Maximum durations reported: 41 s (Mendall 1936a), 45 s (King et al. 1995a), and 70 s (Lewis 1929).

Heart rate changes during diving reported by Kanwisher et al. (Kanwisher et al. 1981): in resting cormorants, 100–120 beats/min, which doubled in response to disturbance. During moderate activity, both surface swimming and voluntary diving, rate increased to 170–230 beats/min. Between dives 50% tachycardia to 280–340 beats/min. Forced dives are accompanied by bradycardia; heart rate may fall to 50 beats/min depending on oxygen concentration, but mechanism is unknown (Jones and Larigakis 1988).

Self-Maintenance

Stretching, Preening, Bathing

Descriptions from van Tets Van Tets 1959, Van Tets 1965, except as noted otherwise. Shaking movements include backward shaking of foot and lateral shaking of tail. Wing-flapping often follows swimming or bathing and precedes Wing-Spreading (see below). May stretch wing and leg together (“one-sided stretch”) or both wings (“upward wing stretch”). When bathing, “the body rolls back and forth in a rocking-horse motion, while the wings splash loudly on the water, throwing a spray onto the back of the bird” (Van Tets 1959: 28). When oiling plumage, uses both bill and back of head to spread preen-gland secretions. Preening includes extensive nibbling, not only of plumage but also of legs and feet. One instance of a molted secondary feather used as a brushlike tool in spreading preen-gland secretion to wings (Meyerreicks 1972). Allopreening (of mate and young) is common. Head-scratching is direct (without lowering wing). Use of pectinate (comblike) third claw needs further study.

Wing-Spreading

Figure 5. In this conspicuous activity, holds both wings out from body and ruffles feathers (especially wing- and tail-coverts and back feathers). Functions have been frequently discussed (e.g., in Elowson 1984, Hennemann III 1988, Sellers 1995); feather-drying generally thought to be primary, and there is scant evidence for other proposed functions (balancing, signaling, or as an aid to swallowing prey). Wettable feathers are interpreted as an adaptation for diving and not merely a consequence of inadequate oil glands. However, this behavior is not shown by all species of cormorants, and there is no evidence that wet wings interfere with flight. Drying may reduce heat loss, but wing-spreading does not serve a major role in thermoregulation, in contrast to Anhinga (Hennemann III 1988). Wing-Spreading often follows directly after swimming, and may be accompanied by wing-fanning and interspersed with bouts of preening. Usually, bird is perched in diurnal loafing area or near nest, but occasionally a bird on water will show this behavior. During Wing-Spreading, individual faces into wind and, at low wind speeds, faces away from sun (Hennemann III 1984). Wing-Spreading is not confined to sunny conditions and has even been observed in light rain (JJH). Physiological mechanisms that may contribute to spread-wing posture include slow-twitch and slow-tonic fibers identified in forelimb muscles (Meyers 1997).

Sleeping And Roosting

Except when incubating or brooding, adults generally stand when sleeping, on land or in trees. May rest neck down the back and tuck bill underwing (No subsequent evidence for Lewis's [Lewis 1929] suggestion that cormorants might sleep at sea). May tuck one foot into flank-feathers in cold weather.

Daily Time Budget

Spends only a small proportion of day actively foraging and much time at diurnal loafing sites. Inactive at night (Mendall 1936a). Marked individuals from a 2-yr radiotelemetric study of 59 wintering birds in Mississippi; spent 57% of daylight hours loafing (relatively less in colder year), 18% actively foraging, 18% on water, but not actively fishing, and 8% flying (King et al. 1995a). Nesting birds on Lake Champlain, VT, during the egg and nestling phases, respectively, spent average of 52% of daylight hours resting, 15 to 19% flying, 20 to 18% swimming, and 12 to 11% diving (Fowle 1997). Activities of 15 radio-tagged adults on Oneida Lake, New York, from July through September, indicated a bimodal daily activity pattern with peaks at 09.00 h and 15.00 h Eastern daylight time. During post-breeding, a less synchronous pattern with greater morning activity peaking at 09.30 h (Coleman and Richmond 2007).

Agonistic Behavior

Physical Interactions

Fighting reported only at nest sites where nest and pair bonds are not yet firmly established. Use bill to grab opponent by neck, wing, or frequently bill and then shake with sideways movement. Some pecking observed as individuals join roosts. Adults occasionally attack (but rarely kill) strange chicks; killing of their own chicks has been reported (Siegel-Causey 1980). Direct agonistic behavior including egg-dumping has been described with a number other ground nesting species including Herring Gulls (Larus argentatus), American White Pelicans (Pelecanus erythrorhynchos), Ring-billed Gull (L. delawarensis), and Caspian Tern (Hydroprogne caspia; see Somers et al. 2011). Impacts to Herring Gull nest success have also been documented (Somers et al. 2007).

Communicative Interactions

At lower intensities of aggression, threatens opponent (or predator) by stretching neck and shaking wide-open mouth, while giving hissing call — eh-hr eh-hr eh-hr. Ritualized agonistic displays are associated with takeoff and landing in both sexes. Before takeoff, individual stretches neck in direction it wishes to go, inflates head and neck and gives t-t-t-t-t call through almost-closed bill. Before landing, often calls urgurgurg and gives Kink-Throat Display, which is given also during working of nest material; lowers hyoid apparatus, making orange pouch conspicuous. Immediately after landing, gives characteristic postlanding display in which it holds head horizontally and slightly below arched and inflated neck. These displays also precede and follow a hop, which functions as symbolic or reduced flight, and occurs in various social contexts (van Tets Van Tets 1959, Van Tets 1965).

Spacing

Territoriality

Reported only at nest, where small area within beak range is defended. Average length of full stretch 0.38 m (Siegel-Causey and Hunt 1986).

Individual Distance

Variable, but low. At roost reached by flight, perched birds are spaced about a wingspan apart (1 m), but at loafing sites reached by climbing out of water, birds appear to be closer together (JJH). Within nesting colony, nests are regularly spaced at about 0.6–2.0 m.

Sexual Behavior

Information from Van Tets 1959.

Mating System And Sex Ratio

Apparently monogamous. Infrequent large clutches (> 5 eggs) may be laid by > 1 female, but nest attendants not known. No information on sex ratio.

Pair Bond

Evident only at nest. After finding nest site, male advertises repeatedly with conspicuous Wing-Waving Display: Stands with breast down, tail cocked upward and forward, and bill pointed upward (Figure 6). This posture emphasizes the brightly colored and species-specific patterns of head and neck: crests, gular pouch, and eye. Raises and lowers wing-tips about 1.7 times/s, with synchronized head movements, pulsations of cloaca, and loud ugh-ugh-ugh sounds. Display ceases as soon as mate (or potential mate) arrives. Next, male gives recognition display (Gape), when standing or sitting. This display can be seen throughout year, given by either sex, and resembles threat, except that movements are slow and stylized. With mouth wide open, revealing the bright blue lining, stretches neck and waves head slowly forward and obliquely upward while giving the call ah-r-r-r-r-t-t.

Copulation occurs on the nest, preceded by female giving modified recognition display. Male holds female by the neck with his bill. No distinctive post-copulation display. Occurrence of reverse mounting has not been examined. Pair bond persists through breeding season; not known if mate is retained in succeeding years. Nest reliefs during incubation are often accompanied by much caressing (Mendall 1936a). Yearlings are known to form pair bonds and to build and guard nests, but not to lay fertile eggs.

Extra-Pair Copulations

No information.

Social and Interspecific Behavior

Degree Of Sociality

Generally very gregarious throughout the year; typically forms dense nocturnal roosts, diurnal loafing areas, and breeding colonies (hundreds to thousands). Solitary nests are rare (Mendall 1936a). Often forages individually, but readily gathers to form feeding flocks (tens to hundreds). By day often travels singly, or in small parties; travels in larger flocks (may be in the thousands) when approaching or leaving nocturnal roosts, and notably on migration.

Nonpredatory Interspecific Interactions

Frequently nests in mixed colonies, but interactions have not been described and behavioral means of any segregation not examined. On the Pacific Coast, Double-crested often nests at the top of cliffs and selects nest sites that are more open and gently sloping than those of Pelagic and Brandt's cormorants. In Nova Scotia, Great Cormorant is more likely to be found nesting on cliffs, whereas Double-crested occurs in trees; both may occupy level ground (Ross 1974b). In Maine, Great Cormorant is more likely to be found on offshore islands, Double-crested inshore (see Breeding; nest site, below).

Readily nests on the same island as other species of colonial waterbirds, on the ground or in trees. In the interior and on Great Lakes, usually nests in subcolonies on the ground within or adjacent to nesting areas of gulls (DVW). In New England, 95% of Double-crested Cormorant colony-sites included gulls (Andrews 1990b). These gulls benefit from food regurgitated by cormorants, and eat pellets cast by the cormorants as well as their eggs and young made vulnerable by disturbances. Also nests with American White Pelicans (Pelecanus erythrorhynchos) where breeding ranges overlap.

In trees and shrubs, constructs new nests among active nests of herons and egrets and may take over active heron nests (by unknown means). At 1 site in Florida, Double-crested Cormorant nested with 13 other species of colonial birds (Girard and Taylor 1979). Expanding colonies may usurp space occupied by gulls, terns, or herons, and modification of vegetation by cormorants may alter the range of nest sites available (see Conservation and Management: conflicts, below).

Sympatric cormorants, often in mixed flocks, feed opportunistically on similar prey, showing substantial ecological overlap but some segregation by habitat (Ainley et al. 1981a). Mixed foraging groups of Double-crested Cormorants and gulls or terns are sometimes seen, often feeding on shoaling fish such as herring; each species takes cues from the actions of the other. Gulls also attempt to steal or pirate fish from cormorants. Kleptoparasitism by American White Pelicans reported (O'Malley and Evans 1983). Known to follow porpoises (Bent 1922: 254).

Predation

Kinds Of Predators

Predation of eggs and young is widespread, but most extensive during periods when a colony has been disturbed (see Conservation and Management: effects of human activity, below). Several species of gulls and corvids, and probably grackles, are serious predators at this stage. When accessible to mammalian predators, young cormorants are taken by coyotes, foxes, and raccoons (DVW). Bald Eagles (Haliaeetus leucocephalus) are important predators in some areas (Todd et al. 1982, Hobson et al. 1989), especially on large chicks, but also on adults; their arrival causes panic in a cormorant colony. No other major predators reported, but full-grown birds are taken occasionally; e.g., Great Horned Owl (Bubo virginianus; Morse 1971b), caiman (Caiman crocodylus; Stevenson and Anderson 1994b), and 1 in pouch of a dead Brown Pelican (Pelecanus occidentalis).

Response To Predators

Responds to individual crow or gull with threats and pecks; may stand and vomit fish. When large predator (or human) appears, adults leave colony and circle overhead; then most settle on water nearby until intruder has left. Adults that remain on their nests successfully threaten Great Black-backed Gulls (Larus marinus) seeking regurgitated fish or eggs. In British Columbia, nests on steeper sites and at the center of colonies suffered less predation from crows and gulls than those on flatter peripheries (Siegel-Causey and Hunt 1981). Responses to mammalian predators include nesting in trees rather than on the ground, but reactions to presence of predator (other than human) not reported. Colonies move if subjected to repeated predation (see Conservation and Management: effects of human activity, below).

Recommended Citation

Dorr, Brian S., Jeremy J. Hatch and D. V. Weseloh. 2014. Double-crested Cormorant (Phalacrocorax auritus), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA. https://doi.org/10.2173/bna.441