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Double-crested Cormorant

Phalacrocorax auritus

Order:
Suliformes
Family:
Phalacrocoracidae
Sections

Breeding

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Figure 7. Annual cycle, Ontario.

Annual cycle of breeding, migration, and molt of Double-crested Cormorants from northern populations (Ontario). Thick lines show peak activity; thin lines, off-peak.

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Figure 8. Double-crested Cormorant nest become built-up after years of use.

Drawing by N. John Schmitt.

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Juvenile and adult Double-crested Cormorants in nesting tree; Moonglow Dairy, CA; June.

Taken 30 June, 2013, at Moonglow Dairy, California by Brian Sullivan.

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Adult Double-crested Cormorants with juveniles at nest; Almond Marsh, Lake Co., IL; June.

Taken 23 June, 2012, at Almond Marsh, Lake County Forest Preserve District, Lake County, Illinois. The following is a link to this contributor's image via Birdshare: JanetandPhil.

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Double-crested Cormorant nest, Baja Mexico.

Scammon's Lagoon, Baja Calif., Mexico. 16 June 1932. Ruler is 50 cm.

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Double-crested Cormorant clutch, Florida

Monroe Co., Florida. January.

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Double-crested Cormorants nesting in tree; Wheat Ridge, Jefferson Co., CO; April.

Taken 29 April, 2009, at the Clear Creek Trail, Wheatridge Green Belt, Jefferson County, Colorado. The following is a link to this contributor's image via Birdshare: Ross Michaels.

Phenology

Most data presented here are for migratory populations from the Great Lakes and prairies (Figure 7); in s. Florida, the species is known to nest year-round (Kushlan and McEwan 1982), but there is no information for individuals. In the n. Bahamas, most eggs are laid Nov–Dec (Watson et al. 1991). Chronologies differ among sympatric species of cormorants; on Mandarte I., British Columbia, and the Farallon Is., CA, Double-crested Cormorants are first to nest (Ainley and Boekelheide 1990); in Nova Scotia, they nest later than Great Cormorants (Ross 1974b).

Pair Formation

Males arrive first, unpaired. Advertising displays by males and courtship are evident at nest site shortly after arrival (Lewis 1929, Mendall 1936a, Van Tets 1959; see Behavior: sexual behavior, above). In Alberta, 6 out of 18 individuals paired within 11 d (Brechtel 1983).

Nest-Building

Both members of pair build a new nest, or repair an old one. Nest often is sufficiently complete within 2–4 d for egg-laying (Mendall 1936a). Adults continue to bring material until chicks leave (Van Tets 1959). Occasional unmated males, sometimes in subadult plumages, build complete nest and occupy it through portion of breeding season (Van Der Veen 1973).

First Brood Per Season

First eggs laid 2–4 wk after arrival: 2 wk in Alberta (Vermeer 1969e), 3 wk in Utah (Mitchell 1977), and 3–4 wk in Ontario (DVW). On Farallon Is., CA, egg-laying generally starts late Mar (Ainley and Boekelheide 1990); occurs Apr–Sep in British Columbia (Campbell et al. 1990b); late Mar–late May in Utah (Mitchell 1977); late Apr–Aug in Ontario (Peck and James 1983); from about 10 May to Jul in St. Lawrence River estuary (Lewis 1929). Most eggs in the colony are laid within 2–3 wk of first clutch initiation, but timing varies greatly among adjacent colonies (older colonies are generally 2 or 3 wk ahead of new ones) and within individual colonies. Will readily re-lay if first clutch is destroyed. Not unusual to have fresh eggs at the time of banding earliest young, mid- to late Jun on Great Lakes (DVW).

First young appear approximately 30 d after first eggs are laid, and young leave nests to form creches and roam over ground colony when 3–4 wk old. At tree or cliff nests, young may remain in nest until able to fly (6–7 wk old; Mendall 1936a), or may be dislodged earlier by disturbance.

Second Brood Per Season

Second broods are rare. On Mandarte I., British Columbia, 1 second clutch (containing 3 eggs) was observed after the first brood was raised (Drent et al. 1964); Bédard et al. (Bédard et al. 1995a) suggested that some pairs raised 2 broods in Quebec. Not known if there are double-brooded pairs in Florida, where cormorants breed year-round (Kushlan and McEwan 1982).

Nest Site

Selection

Individuals are thought to show high fidelity to colony site but data are limited. Duerr (2006) reported colony fidelity rates of between 44 and 87% on several islands in Lake Champlain, VT; not known if they return to the same nest. Experiential basis for preference (e.g., tree versus ground) is unknown. Males choose the nest site and display or advertise for female. In ground colonies, tall nests remaining from previous seasons are first occupied, then sites adjacent to these, and finally peripheral sites (Brechtel 1983, Siegel-Causey and Hunt 1986). For arboreal colonies, central location is more important than characteristics of the tree (Léger and McNeil 1987b). Within a large colony site, nest-building in spring often spreads from several nuclei, where eventual densities are highest. Late nests usually are more widely spaced at the edges of such sub-colonies.

Microhabitat

May nest on ground or cliffs, in trees or shrubs (Chapdelaine and Bédard 1995), or on artificial nest structures and transmission line towers; also on abandoned wharves or on bridges (Stenzel et al. 1995). Ground-nesting sites are characteristically on low-lying rock islands or reefs, away from or with little or no vegetation; less often on sandy substrates, or on mats of emergent vegetation in marshes. Arboreal sites include various evergreen and deciduous trees; these are initially alive but nesting activity often kills trees within 3–10 yr (Lemmon et al. 1994).

Site Characteristics

Natural substrates include limestone and granite bedrock, large boulders or fallen trees, gravel, cobbles, beach ridges, soil, or standing trees. Where both are available, prefers rocks to soil (JJH). On Great Lakes, ground nests are placed > 0.5 m above the high-water mark (DVW); in coastal British Columbia, ground nests are 4–17 m above high-tide line (Campbell et al. 1990b). In Massachusetts, rare for nests to be washed out by spring tides in absence of storms (JJH). Arboreal nests are 0.5–30 m above ground in forks of branches or adjacent to trunks; may usurp attended or unoccupied nests of Great Blue Herons (Ardea herodias) and Great Egrets (Ardea alba; Thompson 1981).

Nest

Construction Process

Male gathers nest material when alone, but nest-building is ineffective until the female guards against thieves. Both adults take part; male brings most material and female constructs. Materials from nearby nests are rapidly stolen if undefended; all traces of such nests may disappear. Occasional newly laid eggs are seen in late nests consisting of no more than a ring of seaweed (JJH); these nests are likely to be attended by inexperienced parents, inept at building or defense.

Structure And Composition Matter

Nest composition depends on materials available near colony, but unlike nests of other cormorants seen on the Pacific Coast, Double-crested nests characteristically include finger-sized sticks and other bulky items collected from diverse locations—some floating, others taken from the tide or drift line or by diving, and some twigs and leafy stems broken from living plants (Lewis 1929); some materials are carried several kilometers (JJH). Along seacoasts, base of nest is usually seaweed (e.g., Fucus). Diverse flotsam and jetsam often are incorporated: rope, fishnet, plastic buoys, deflated balloons, even a broiler grill (JJH). In the Gulf of Maine in 1987–1988, 37% of 497 nests contained plastic debris (Podolsky and Kress 1989). Parts of dead birds are commonly incorporated. Lining is most often of grass, rootlets, and similar materials. As the season progresses, nest also receives pebbles and bones from pellets, and a liberal coating of guano on the outside that cements the whole together. Resulting nest is substantial and, if spared by winter storms, receives additions each year and eventually becomes a tall turretlike structure (Figure 8), especially in areas of low rainfall—e.g., Mandarte I., British Columbia.

Dimensions

For ground nests in n. Lake Huron, average outside nest diameter 45–47 cm, nest height 17–21 cm; as season progressed, size of newly built nests decreased (Blomme 1981). Outside diameter 55–91 cm; average height 10–43 cm (Bent 1922, Lewis 1929). Nests in trees usually are much smaller but have deeper inner cavities (Lewis 1929). Ground nests that are used repeatedly may reach heights of >2 m (Siegel-Causey and Hunt 1986; Figure 8). Average inside diameter at 2 sites 25–28 cm (Blomme 1981). Inside diameter 23–30 cm; nest cup depth of 1.3–3.8 cm (Bent 1922, Lewis 1929).

Spacing

Nearest-neighbor distance for nests in Massachusetts was 76 cm ± 5.1 SD between centers (range 66–85, n = 15; JJH); for 3 colonies in Lake Ontario, mean 72.1–73.6 cm ± 5.7–8.0 SD (n = 15 nests/colony; DVW). In British Columbia, spacing varied from 60 to 220 cm; high density was >1 nest/m2 (Siegel-Causey and Hunt 1981).

Microclimate

On low islands, ground nests are often completely exposed to the elements; they are occasionally damaged by summer storms and frequently destroyed by ice in winter. Nests may be preferentially built along cracks, fissures, crags, or overhangs that afford some protection (DVW); cliff nests are more protected. No information on orientation of nests or their insulative value.

Maintenance Or Reuse Of Nests

Both sexes add new material to nest throughout incubation and while chicks are small. After changeovers during incubation, the relieved bird returns with miscellaneous nest materials and presents them to the sitting bird before leaving to go fishing (Van Tets 1959).

Nonbreeding Nests

At some very small nests, most built late in season, preliminaries of breeding may occur but no eggs are laid. These structures are ephemeral, lasting only about 2 d, and are more likely to be built near to loafing areas than to active nests (Siegel-Causey and Hunt 1986), and by immature birds.

Eggs

Data from Palmer 1962a unless otherwise noted.

Shape

Elongate ovate to cylindrical ovate (Bent 1922).

Size

See Table 1 . Within clutches from s. Alberta, no size differences among eggs 1–4, but eggs 5 and 6 are significantly smaller (Brechtel 1983).

Color And Surface Texture

Shell pale blue, unmarked, but pigmented layer often obscured by variable outer calcite cover (20–80 µm) that is initially white and very porous (Van Scheik 1985), later stained brown from feces and dirt. Calcite layer is chalky in texture, giving irregular surface, and is excluded from the “true” thickness presented in Table 1 .

Mass

For P. a. auritus: average 46.5 g (n = 50; Lewis 1929), 46.1 g (n = 20; Mitchell 1977), or 44.9 g (n = 448; Brechtel 1983). This amount is 2.7% of adult mass, which is small compared to other seabirds (modal clutch weighs 10.7% of adult).

Eggshell Thickness

Eggshells are very susceptible to contaminants. Pre-1947 egg measurements are given in Table 1 ; not known to vary significantly over the geographic range.

Additional pre-1947 data set and role of DDE in eggshell-thinning were discussed by Anderson and Hickey (Anderson and Hickey 1972). Double-crested Cormorant was one of the first species to show DDE-induced eggshell-thinning after commercial introduction of DDT at end of World War II. Cormorant eggs (from museums) showed decline in eggshell thickness index as follows: 1948, Illinois—3%; 1949, California—7%; 1955, Wisconsin—10%; 1959, Ontario—27% (Anderson and Hickey 1972).

Eggshell-thinning >20% leads to extensive egg breakage during normal incubation and results in reproductive failure. Thinning of 20% associated with 9–14 ppm DDE in eggs (Pearce et al. 1989), contra the 36 ppm suggested by Keith and Gruchy (Keith and Gruchy 1972). During 1969–1972, eggshell-thinning of up to 37% occurred in cormorant eggs from s. California (Gress et al. 1973) and Ontario (Postupalsky 1978b), sites of high DDE contamination. Eggs from the prairies, Maine, and Atlantic Canada showed less thinning (7–10%), and lower contaminant levels (Anderson et al. 1969b, Kury 1969, Zitko 1976). By the late 1980s, with declining contaminant levels, most populations showed normal or near-normal eggshell thickness (Weseloh et al. 1995b, T. Custer unpubl.). See Conservation and Management: effects of human activity, below.

Clutch Size

Modal clutch 4 (range 1–7). In Ontario, 1,022 clutches were distributed as follows: 1 egg, 12.2%; 2 eggs, 13.6%; 3 eggs, 30.1%; 4 eggs, 39.8%; 5 eggs, 3.5%; 6 eggs, 0.7%; and 7 eggs, 0.1% (Peck and James 1983); similar distribution for 497 clutches in s. Alberta (Brechtel 1983) and for 76 clutches in Utah (Mitchell 1977). Some eggs are lost during incubation, especially during disturbances, so reported numbers likely are low. Large pebbles (mean 4 cm, range 0.5–10) occasionally found in ground nests, treated like eggs (Hobson 1989).

Egg-Laying

Little information available on time of laying; most eggs are laid during the first 3–4 h after sunrise (Lewis 1929), occasionally in afternoon (Mendall 1936a). Eggs are laid daily (Lewis 1929, Mitchell 1977) or on alternate days (T. Custer pers. comm). For other cormorant species, intervals reported as 2–3 d (Johnsgard 1993). Re-laying is frequent following loss of a complete clutch, occurring within 19 d (Mitchell 1977). At Mandarte I., British Columbia, re-laying reported at 29 of 33 failed nests, with maximum of 11 eggs laid at 1 site (Van Tets 1959). In Manitoba, 30-50% of lost clutches replaced (Mcleod and Bondar 1953). In 1972, when eggshells were thin, Weseloh et al. (Weseloh et al. 1983) inferred from broken shells a rate of egg production of 4.1/nest (maximum 10), when nests contained average of 2.0 eggs.

No information on adult behavior during this period, including sexual guarding.

Intraspecific egg-dumping not known to occur systematically, but occasional clutches of up to 7 eggs are likely to have been laid by > 1 female.

Incubation

Onset Of Broodiness And Incubation In Relation To Laying

May be quite variable; starting with first egg according to Mitchell (Mitchell 1977), with third egg according to Lewis (Lewis 1929), or with any egg (Mendall 1936a). Some of these apparent discrepancies were resolved by measuring egg temperature and video recording in a nest with transparent bottom. Incubation begins gradually and reaches maximum intensity when third egg is laid (Van Scheik 1985).

Incubation Patch

None. Eggs rest on top of warm webs of parent's feet; parent lowers abdomen and breast onto them. Temperature of incubated eggs in 3- or 4-egg clutches is about 36°C (Van Scheik 1985).

Incubation Period

Variation from 25 to 28 d may reflect unspecified methods of determining period, or real regional differences. Period of 25 d reported by Lewis (Lewis 1929) and Mendall (Mendall 1936a), and 28 d by van Tets (Van Tets 1959) and Mitchell (Mitchell 1977). For 22 eggs from Manitoba incubated artificially, 10 (45%) pipped at 26 d, others at 27 and 28 d (Hanbidge and Fox 1996). Some infertile eggs are incubated for > 80 d (Van Tets 1959). Hatching is asynchronous, but spans fewer days than laying. Artificial incubation is discussed by Powell et al. (Powell et al. 1997b). Eggs that float in fresh water have been incubated for 18 d (Brechtel 1983).

Parental Behavior

Both sexes incubate, female more during the first half of the incubation period. Pair change over at intervals of 1–3 h (Mendall 1936a). Do not retrieve eggs that fall out of nest.

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

Young embryos are very cold-tolerant: 90% survived internal egg temperature of 0.5°C. Thermal stress above normal incubation temperature is more rapidly lethal. Internal temperature of 41.5°C caused 33% mortality; 48.5°C, 100% mortality. Eggs exposed to sun heated as fast as 0.36°C/min (Van Scheik 1985). Fresh cormorant eggs that were stored at room temperature for 2–3 d before artificial incubation hatched normally (76%), but those stored for 5 d hatched poorly (13%; Powell et al. 1997b). No records of nocturnal desertion during incubation.

Embryonic Development

Landmarks in relation to days of incubation: at 7 d, wing and tail buds; at 13 d, alula and egg tooth; at 16 d, claws; and at 19 d, scales on legs and feet (Hanbidge and Fox 1996). Jones et al. (2010) indicate that specific fish diets can be important to egg development and declines in anadromous fish may impact reproductive capacity.

Hatching

Preliminary Events And Vocalizations

No information.

Shell-Breaking And Emergence

No information on time of day.

Intervals Between Hatching Of Eggs

Typically < 1 d, so a set of 4 eggs hatches in 2–3 d (Lewis 1929); from a few hours to 5 d (possibly resulting from loss of intermediate eggs; Léger and McNeil 1987a). This asynchrony facilitates brood reduction when food is scarce (Hunt and Evans 1997).

Parental Assistance And Disposal Of Eggshells

Adult provides no assistance to hatchling; throws eggshells from nest (Van Tets 1959).

Young Birds

Condition At Hatching

Altricial hatchling is feeble, barely able to move limbs stiffly, lift head and move it from side to side; naked, with shiny, brown translucent skin; eyes closed and egg tooth present. Mean mass 27.6–34.7 g (n = 96; Mendall 1936a, Dunn 1975b, DesGranges 1982, Léger and McNeil 1987a). Size: culmen, average 9 mm ± 1.1 SD; tarsus, 16 mm ± 0.9 SD; ulna, 18 mm ± 2.2 SD (Dunn 1975b).

Growth And Development

Rapid compared to other seabirds of similar size, but only reported from expanding populations; about 24 d between 10 and 90% of chick body mass. Linear growth approximately 63 g/d. Rate of attaining asymptotic weight, KG (from Gompertz) 0.116, KL (from logistic) 0.191 (Léger and McNeil 1987a). Growth of internal organs may not parallel weight gain. Gut growth initially is very rapid; intestine length triples during first week (Dunn 1975b). Asymptotic mass is less than adult mass by 7% (Dunn 1975b), or by 2% (Léger and McNeil 1987a). Fledglings being lighter than adults is unusual in seabirds and may relate to post-fledging parental feeding. Asymptotic mass of 1,584–1,942 g reached in 40–50 d (DesGranges 1982, Léger and McNeil 1987a). For smaller Florida birds, mass of 1,150–1,550 g reached in similar time (Cummings 1987).

Most mortality occurs in first few days, and growth rate is independent of hatch order (Robertson 1971a, DesGranges 1982, Léger and McNeil 1987a). Brood size affected growth rates only in supernormal broods (> 6 chicks; Robertson 1971a). Colony differences in expanding population may reflect age and experience of parents; fastest growth occurred at colonies with highest reproductive success, and greatest asymptotic weights occurred at largest colonies (DesGranges 1982). Growth of young in stable or declining populations not reported.

Eyes open by 3–4 d; iris black; egg tooth drops off between 4 and 7 d. Dense short down appears by day 6 or 7. Whole body covered with thick black down by age 2 wk; bird appears to be covered by “black wool” (Lewis 1929); head and neck are last to be covered. Thermo-regulatory ability is absent at hatching, partly developed by 8 d, complete by 14 d (Dunn 1976a). Contour feathers grow from different sets of follicles than the down. Primary sheaths are visible by 13–14 d; they erupt by 16–19 d, when sheaths of tail-coverts and feathers of sides and back have appeared and throat-pouch takes on yellowish color. By 21–23 d, wing feathers are 2.5 cm long; development then slows, and by 28 d wing feathers are 6 cm long. At 35 d, feathers on back, breast, lower underparts, and forelimbs are well grown, but none on head and neck. By 42 d, few feathers on head, greenish gloss of dorsal feathers evident; neck still thickly down-covered. Neck-feathers complete by 49–58 d, but few data.

Behavior develops as follows (Lewis 1929, Mendall 1936a). On days 0–3, nestlings eat and sleep only; beg by raising head and waving it from side to side while giving feeble high-pitched peep, peep, peep. Eyes open by 3–4 d, when young can crawl on floor of nest. Food-begging now involves waving head back and forth with bill closed, sometimes hitting parent's gular pouch. In periods of warm, dry weather, nestlings will beg for water by waving wide-open bill upward (Van Tets 1959). Begin to preen at or before 10 d; sit erect and flap wings by 10–11 d. Sibling relationships are harmonious except for competition at feeding time; older young outcompete youngest for food and will stampede over small young and nests when frightened. Young birds walk from ground nests as early as 21 d when approached (by predators or humans) and will enter water or swim; if undisturbed, will remain in nest until about 28 d. Use bill to aid climbing over and among nests.

Parental Care

Both parents tend young equally (Van Tets 1959, Léger and McNeil 1985).

Brooding

Almost continuous for first 12 d after hatching; ceases by 14–15 d, coinciding closely with appearance of down and effective thermoregulation (Dunn 1976a). Adults attending nest after this may shade young on sunny days, but often visit only to feed and are increasingly absent at night (Mendall 1936a). In St. Lawrence River estuary, nest attendance by day was close to 100% during first month following hatching; mean length of nest attendance periods was 126–134 min, and mean number of nest reliefs/day for the brooding period was 6.8 (Léger and McNeil 1985).

Feeding

From Mendall 1936a. At hatching, adult forces tiny particles of partly digested food into mouths of the blind and helpless nestling. By 3 d, adult slowly and gently places its open mouth over entire head of young, which take food directly from adult. Feeding becomes more frenzied as young grow; young clamor to reach arriving adult first, vigorously insert head into adult's open mouth and pouch to seize whole fishes.

Mean number of feedings/chick/d changed from 3 at 0–5 d of age, to 6 at 16–20 d, to 2–3 at 56–60 d. The maximum number of feedings/chick/d was 5–7 for broods of 1–4 chicks, 16–30 d. Number of feeding periods/brood/d was 4–5 during first 5 d after hatching, and reached peak of 9–10 when chicks were 16–25 d old. Efforts of parents to feed young increased with brood size, but were significantly greater only when 3- and 4-chick broods were compared with 1- and 2-chick broods between 21 and 25 d of age. Weights of meals fed to chicks varied from 5 to 125 g for a 2-chick brood aged 7–18 d (summarized from Figure 4 in Léger and McNeil 1987a).

Largest, most vigorously demanding young usually are fed first; smaller young are fed after activities of older young birds subside. Smaller birds in brood may starve or be trampled by vigorous older young, but such mortality is usually confined to first few days. In Manitoba, brood reduction apparently is facultative in small clutches, grading to obligate in large clutches (≥ 4 young), in which last-hatched chick never survived in presence of 3 older siblings (Hunt and Evans 1997). Severity of brood reduction varies, probably depending on local food supplies (DesGranges 1982, Stenzel et al. 1995). Roaming young return to nest to be fed; adults feed only their own young (Mendall 1936a).

On hot days, parents bring water to nestlings in response to water-begging. After a changeover, relieved parent fetches water (instead of nest material) and pours it into open mouths of chicks (Van Tets 1959).

Nest Sanitation

By 3 wk of age, young void over edge of nest. Remains of fish usually are removed from nest but not eaten. Dead young are not removed, but flattened into nest structure.

Invertebrate Associates

Flies are abundant in cormorant colonies, and nests may harbor large numbers of hen fleas (Ceratophyllus niger).

Carrying Of Young

Not reported.

Cooperative Breeding

Not reported, except Michael (Michael 1935b) described 2 cases of 3 birds building nest.

Brood Parasitism

Not reported; eggs of associated nesting species—e.g., Herring Gull (Larus argentatus) and Ring-billed Gull (Larus delawarensis)—are sometimes found in cormorant nests and vice versa, but no reports of these eggs hatching (DVW).

Fledgling Stage

Departure From Nest

From Van Tets 1959. Young leave ground nests at 3–4 wk, when they are well feathered, but 2–3 wk before flying. At nests in trees or on cliffs, young may remain until able to fly (6–8 wk), although disturbance during this period will cause them to leave prematurely.

Growth

No information.

Association With Parents Or Other Young

After leaving ground nests, young form creches with other young and move within the colony, returning to the nest site to be fed (see Food Habits: feeding, above). After fledging, feeding may take place anywhere in the colony (Mendall 1936a).

Ability To Get Around, Feed, And Care For Self

From Van Tets 1959. Young are very mobile and social after 4 wk and roam throughout colony. Invasion of other territories usually is tolerated by resident adults; if not, young are only slightly rebuffed. From third week, can swim and dive if chased into water, but do not do this voluntarily until 6–7 wk, when plumage is complete. First real attempts at flight occur at about 6 wk, when young flap to water from colony or cliff, swim ashore, climb back to sufficient height, and try again. Diving ability develops at about 6–7 wk; by 7 wk, young can take off from water, and they begin to accompany adults on flying and swimming trips. Young are completely independent of parents at 10 wk (Mendall 1936a).

Immature Stage

Autumn premigratory dispersal from colony is not well known. On Lake Ontario, juveniles and adults roost together at breeding sites, but by July, additional nocturnal roosts form on islands 2–3 km away; origins of these birds are unknown (DVW). Immature birds (1- and 2-yr-olds) are present on breeding colonies by June; some build incomplete nests. Breeding by some 2-yr-olds reported on Mandarte I., British Columbia (Van Der Veen 1973).

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