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Double-crested Cormorant

Phalacrocorax auritus

Order:
Suliformes
Family:
Phalacrocoracidae
Sections

Demography and Populations

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Figure 10. Relative abundance of the Double-crested Cormorant during the breeding season, based on BBS data.

Based on data from the Breeding Bird Survey, 2007-2011. From Sauer et al. 2014, see text for details.

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Figure 11. Regional trends in Double-crested Cormorant populations, 1966-2011; data from the Breeding Bird Survey.

Data show best estimates of population change for the species over its range; from Sauer et al. 2014, which provides details.

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Figure 12. Early winter density of the Double-crested Cormorant, as determined by data from the Christmas Bird Count, 1980-2006.

Numbers show the number of individuals counted per 100 party hours in each region with CBC count circles.

Compared to other widespread colonial waterbirds, the population dynamics of Double-crested Cormorants have been poorly studied; no comprehensive life table has been constructed. Key demographic parameters have been established in a few colonies: in a 3-yr study of P. a. albociliatus on Mandarte I., BC, numbers were increasing by 8.4%/yr (Van Der Veen 1973). Chapdelaine and Bédard (1995) reported an 8% annual growth rate for colonies in the St. Lawrence River estuary 1978-1990. At a cormorant colony on East Sand I., WA, the average annual growth rate (λ) was 1.15 over 11 yr (1991-2002; Anderson et al. 2004). Weseloh et al. (2002) reported growth rates throughout the Great Lakes from 1990-2000; annual rate of change from -22% to > +47% for individual sites. Large demographic differences are likely between populations that are resident or migratory, expanding or stable, but such differences at large scales have not been extensively examined.

Measures of Breeding Activity

Age At First Breeding; Intervals Between Breeding

In British Columbia, mean age at first breeding 2.74 yr (n = 127 birds color-banded as chicks); van der Veen (Van Der Veen 1973) observed 1 female in 1-yr-old plumage nesting, and calculated that 4.7% first bred at 1 yr, 16.5% at 2 yr, and 78.8% at 3 yr; i.e., most birds first breed at beginning of fourth year. No data on interval between breeding attempts. Breeding percentages for Lake Champlain colonies: Young Island, 97.2% for adults and 26.0% for year 2 subadults and Four Brothers Islands (aggregate) 96.3% of adults and 30.0% of year 2 subadults breed (Duerr 2006).

Clutch

Mean clutch size 2.7–4.1 eggs (mode 4; see Appendix 2 and Breeding: eggs, above, for details). Eggs are readily lost during incubation. Failed clutches are replaced, but only 1 brood is raised per season.

Annual And Life Time Reproductive Success

For studies on annual reproductive success, see Appendix 2 . Hatching success typically 50–75%; fledging success 1.2–2.4 young/nest, or 74–95%. On Young I., Lake Champlain, 2.54 juveniles fledged per nest (Fowle et al. 1999). On Four Brothers, 1.64, 1.46, 1.08, and 1.22 juveniles fledged per nest on Islands A, B, C, and D, respectively (Duerr 2006). The average fledge rate in Lake of the Woods, Ontario, was 1.34 chicks, with a fecundity estimate of 0.67 female chicks fledged per nest (Chastant et al. 2014). In the Eastern Basin of Lake Ontario, the average fledge rate was 2.04 chicks per nest and a fecundity estimate of 1.02 female chicks fledged per nest (Chastant et al. 2014).

Reports often do not distinguish between eggs lost from nests before incubation is complete and those that were incubated to term but failed to hatch. Chick loss from hatching to fledging is often low; e.g., 5% in coastal British Columbia (Drent et al. 1964). All studies may be subject to observer effect and subsequent predation (see Conservation and Management: effects of human activity, below), and all figures are much lower for DDE-contaminated populations (see Gress et al. 1973, Weseloh et al. 1983). In the St. Lawrence River estuary, reproductive success parameters also are lower for late-nesting cormorants (June) compared to early nesters (May; McNeil and Léger 1987). For Mandarte I., BC, average lifetime production calculated as 3.28 young (Van Der Veen 1973).

Life Span and Survivorship

Commonly live for > 8 yr in the wild; oldest banded bird 22 yr 6 mo Klimkiewicz and Futcher 1989) (Lutmerding and Love 2011); wear and loss of aluminum bands, however, likely lead to underestimates of survival based on recoveries. No analyses of band-wear published. From subsequent sightings of >548 banded fledglings on Mandarte I., British Columbia, van der Veen (Van Der Veen 1973) estimated first-year survival of 0.48, second-year of 0.74, and subsequent annual survival of 0.85; mean adult life expectancy 6.1 yr. Birds banded as adults at the Spider I., WI, colony had an average annual survival rate of 0.696; for birds banded as nestlings, survival rates were: 0.305- first year, 0.774 - second and third year, and 0.633 – adults (Stromborg et al. 2012).

Survival estimates for birds in Eastern Lake Ontario were 0.45 for young-of-the-year, 0.63 for age 1, 0.87 age 2, and 0.84 age >3 yr (Chastant et al. 2014). Survival estimates for birds in Lake of the Woods, Ontario, CA were 0.19, 0.78, 0.83, and 0.83 for young-of-the-year, age 1, age 2, and ages >3 yr, respectively (Chastant et al. 2014). On Lake Champlain, VT, the nonbreeder/breeder (subadult/adult) ratio was 0.23 (n = 71 flocks; Fowle 1997). These values are similar to those of other species in the family (Johnsgard 1993).

Disease and Body Parasites

Diseases

Information limited. Newcastle disease causes wing and leg paralysis and killed several thousand cormorants in the interior in the 1990s (Glaser et al. 1999). Some strains of this paramyxovirus cause massive mortality and pose a threat to poultry, which must be destroyed if infected (Banerjee et al. 1994, Kuiken 1999). Newcastle disease spread from cormorants to domestic turkeys in N. Dakota (Heckert et al. 1996). In Saskatchewan in 1995, Newcastle disease affected only young cormorants and was said to have killed 21% of hatched chicks (Kuiken et al. 1999). Prevalence of antibodies to virulent avian paramyxovirus serotype 1 (APMV-1 causative agent of Newcastle disease) in after–hatch year birds sampled from locations in the Great Lakes and Southeastern states was high, ranging from 47% to 91.6% (Farley et al. 2001, Cross et al. 2013). Antibody prevalence in chicks was much lower: 1.7% to 16.4% (Cross et al. 2013). Few cormorants were found to be positive for avian influenza virus (AIV) and are unlikely to be involved in the circulation of AIV (Cross et al. 2013). Type E Botulism may also be an important source of mortality for cormorants, particularly on the lower Great Lakes (Shutt et al. in press).

Body Parasites

Ectoparasites include feather lice (Mallophaga: Eidmanniella kuwani, Pectinopygus faralloni, P. gyrocornis, Piagetiella incomposita; Malcolmson 1960); fleas (Siphonaptera: Ceratophyllus niger [hen flea]; Easton 1982). In Florida, Threlfall (Threlfall 1982a) found 3 mallophagans, 1 tick, and 2 mites. Endoparasites include roundworms (Nematoda); Contracaecum spiculigerum is found in large numbers in the stomach (proventriculus) where they burrow into the wall and into ingested fish (Huizinga 1971), also Syncuaria squamata (Wong and Anderson 1987a), and Digenean trematode Amphimerus elongatus (Pense and Childs 1972). In Texas, a helminth community of 17 species included 6 species per individual cormorant (n = 134; Fedynich et al. 1997). In Florida, birds from west coast significantly more infected with endoparasites (18 species, 6 per infected cormorant), than those from east coast (12 species, 3 per bird; Threlfall 1982b). Ameboid parasites include Edwardsiella ictaluri (Taylor 1992d).

Cormorants may spread fish diseases or parasites, but importance has yet to be established.

Causes of Mortality

Disturbance of breeding colonies can lead to extensive mortality of hatchlings from exposure, and of eggs and young (up to age 3 wk) by predation (particularly by gulls and corvids; see Conservation and Management: effects of human activity, below). Adults and large chicks are taken by Bald Eagles (see Behavior: predation, above), but no quantitative information on predation mortality is available. Of 295 band recoveries in Texas (1971–1985), 41% found dead, 28% entangled in fishing gear, and 17% shot (Thompson et al. 1995a). For birds banded on the Great Lakes (1928–1995; n = 2,393), recoveries in the same categories were 56%, 13%, and 9% (E. Woodsworth pers. comm.). Fishing gear is a major cause of death, but also affects encounter probability.

Range

Very few observations of individually marked birds, and only limited examination of available banding data, so this topic is necessarily conjectural and in need of more careful study.

Initial Dispersal From Natal Site

In stable populations, natal philopatry is probably high; many young first breed where they were hatched. New colonies are thought to be formed by young birds, often at sites they have used as roosts or loafing areas, which may be the closest suitable habitat to the natal colony. An expanding group of colonies in Lake Huron, MI, included individuals from most breeding sites within 230 km (Belyea et al. 1997).

Dispersal From Breeding Site Or Colony

See Breeding: immature stage, above. Postbreeding dispersal is not directional. Young may move shorter distances than adults; for birds banded on the Great Lakes, recoveries in Jul–Sep of 154 immatures at 175–260 km from colony, 82 adults at 500–550 km (E. Woodsworth in litt.).

Fidelity To Breeding Site And Winter Home Range

New colony sites may be abandoned within a few years, but once well established, they are likely to persist. Philopatry to proximity of the natal colony is suggested by recoveries in Jun of banded birds at least 3 yr old: median distance only 25 km (Dolbeer 1991). On Lake of the Woods, Ontario, age-specific inter-annual breeding site fidelity 0.68 for young of year, 0.80 age 1 and 2, and 0.70 age > 3 (Chastant et al. 2014). No information on inter-annual fidelity to wintering areas.

Home Range

Individuals forage far from the colony or roost, but localization within such a large potential area has not been examined. Birds followed by airplane in Wisconsin flew on average < 3 km (range < 1 to 40 km) from their breeding colony to first foraging site (Custer and Bunck 1992). In Massachusetts, a few individuals return to breeding colonies from 30 km, but most feed much closer (JJH). Birds from the Farallon Is., CA, do not feed near the islands, but regularly travel to estuarine habitats along the mainland, a round-trip of at least 70 km (Ainley and Boekelheide 1990). Dorr et al. (2012) reported breeding season 95% kernel home ranges of cormorants marked on Little Galloo Island, NY and subject to egg-oiling varied from 19 km2 to 130 km2. Wintering cormorants in Mississippi repeatedly visited some catfish ponds and flew past others (King et al. 1995a). Basis for this selectivity not reported. Most (81%) cormorants return to the same winter roost each night (Tobin et al. 2002).

Reported winter home range sizes vary greatly, ranging from an average of 81 km2 to 17,490 km2 (Scherr et al. 2012, King et al. 2012). The effect of aquaculture farms on home range size in the southeast is mixed, with some research showing larger home ranges in aquaculture producing areas (Scherr et al. 2010) and some showing no effect (King et al. 2012.) King et al. (2012) also found no effect of age class or body mass of marked cormorants on winter home range size. See also Habitat, above.

Population Status

Numbers

From Hatch 1995a, U.S. Fish and Wildlife Service 1999a. Total in about 1990 estimated to be 1–2 million individuals (approximately 350,000 breeding pairs), but there is considerable uncertainty about coverage and especially the non-breeder fraction.

More recent data (thru 2005) show a Continental population, including four subspecies, estimated at between 1,080,800 and 2,163,600 (Wetlands International 2006), suggesting little overall change from earlier estimates. Systematic censusing covers only a minority of the species, and some of the largest populations are least well enumerated (e.g., Manitoba, Mexico). Numbers nesting in Florida are poorly known because the breeding season is long, colonies are inaccessible, and the cormorants are surrounded by many other nesting birds. Numbers of breeding pairs in regional populations are estimated as follows: interior, 220,000; Atlantic Coast, 96,000 (these 2 make up subspecies auritus); Alaska (cincinatus), 20,000; West Coast (albociliatus), 22,000; Florida (floridanus), 12,000; Bahamas (heuretus), 212.

Trends

From Hatch 1995a. Updated results (to 2011) at: http://www.mbr-pwrc.usgs.gov/cgi-bin/atlasa11.pl?01200&1&11 and Figure 10.

Numbers of Double-crested Cormorants have increased significantly since about 1975. Conjectural explanations invoke lowered mortality from pesticides (especially recovery of eggshell thickness) and from the direct killing that was characteristic of earlier decades, increased food in breeding season from changes in fish communities (some resulting from overfishing, others from introductions of nonnative fish or other changes), and enhanced overwintering survival of adults and young.

Great decreases in numbers occurred in the 19th and early 20th centuries (see Distribution: historical changes, above), probably throughout the range and resulting from persecution, although information is more limited from the Pacific (especially Baja California) and from Florida and the Bahamas. For example, a huge colony that formerly existed on Isla San Martín, nw. Baja California, is now much reduced, probably the result of persecution and introduction of domestic animals (Carter et al. 1995b). Numbers increased from the 1920s until the 1950s, when pesticide impacts reached serious levels (see Conservation and Management: effects of human activity, below). All northern areas studied have shown pattern of growth interrupted in mid-century. Interior populations reached low points about 1970, and the Atlantic population ceased growing. During this period, the Double-crested Cormorant was recognized as of special concern in several states; it was on the Audubon “Blue List” from 1972 to 1981 (Tate and Tate 1982).

Increases in 1975–1995 were explosive in migratory populations of the n. Great Plains, Great Lakes, and Atlantic Coast; in many large areas, doubling times were < 5 yr for decades. However, growth of northern coastal populations (Nova Scotia to Massachusetts) may have ceased by 1990. Population models show that some of the annual increases on Lake Ontario must have resulted from immigration (Price and Weseloh 1986). Individual colonies have grown rapidly; e.g., on Little Galloo I. in e. Lake Ontario, numbers increased by 31%/yr for 18 yr from 1974 (Weseloh and Ewins 1994), and on Lake Champlain by 21%/yr (Fowle 1997). On the upper Mississippi River, increases were slower, and numbers nesting had not returned to historical levels by 1993 (Kirsch 1997). Numbers of breeding birds on the West Coast also continued to grow, but did not reach pre-DDT levels in s. California (Small 1994). Trends in Alaska and in Florida are less clear. For more details, see papers in Nettleship and Duffy (Nettleship and Duffy 1995).

Analyses of Breeding Bird Surveys from 2001-2011 suggest large survey-wide increases in many portions of the range, particularly the Southeast, but declining populations in s. Florida, the Canadian Prairies, n. New England and the Pacific Northwest (http://www.mbr-pwrc.usgs.gov/cgi-bin/atlasa11.pl?01200&1&11; Figure 10). Other data, however, suggest that currently (2014) the Great Lakes meta-population may be stable or declining (Guillaumet et al. 2014), having reached carrying capacity in some areas of that range (Ridgway et al. 2006). Numbers in the Great Lakes may be affected by widespread management activities on the breeding and wintering grounds (Guillaumet 2014). Great Lakes-wide, the number of nesting DCCOs declined 5.2% from 114,507 breeding pairs in 2005 to 108,592 pairs in 2009. This decline was more pronounced in the U.S. Great Lakes where more management occurs, relative to the Canadian Great Lakes (7.3 and 3.2 %, respectively; USFWS Draft Double-crested Cormorant Environmental Assessment 2014). Great Lakes counts in 2009 still exceed those from the mid 1990's.

Figure 11 shows relative density of early winter populations in the US, based on Christmas Bird Count data, with concentrations in the Southeast (southern portions), and along the Pacific coast. Wintering inland in the s.-central U.S. is not a new phenomenon but increases in numbers since late 1970s coincided both with resurgence of breeding populations to the north and with development of catfish-farming and other aquacultural ventures (Jackson and Jackson 1995).

Population Regulation

Cormorant populations are influenced by some factors that limit numbers, and others that act in a density-dependent way to regulate them (Cairns 1992b). The relatively large clutch size of cormorants, compared to other seabirds, is thought to be an adaptation to widely fluctuating food supplies and suggests the importance of stochastic variation and of catastrophes in limiting numbers. However, appropriate evidence is lacking. Numerical declines in El Niño years are described for Washington State (Wilson 1991b), but the incidence of non-breeding events has not been examined.

Demographic parameters likely to respond to density and result in regulation of numbers are age of first breeding, non-breeding numbers, and abandonment of whole clutches. Effects on growth of young, asymptotic mass, and fledging success may be detectable. Dorr et al. (in press) indicate that density-dependent affects may influence primary and secondary sex ratios of breeding birds. Overwinter survival of both young and adults is likely to have major impacts on numbers but no evidence is yet available for density-dependent (regulatory) action.

Colony Size And Turnover

Local density of breeders may be affected by the availability of colony sites as well as the distribution of prey in space and time. In many areas where Double-crested Cormorants now occur, the numbers breeding are not limited by available nest sites, although competition for the best sites may be fierce. Individuals occupy new colony sites before the original appears to be filled. Where several potential colony sites exist close together, initial colony growth may occur only at one site; subsequent occupation of additional sites as the local population grows is followed by a decline in numbers at the original site (JJH). New sites have often been used as roosts for some time.

However, factors influencing colony size and colony formation or movement, including overlaps of colony feeding-ranges, have not been examined in this species. Depletion of prey within foraging range of the colony, proposed as an important factor for many seabirds, is difficult to measure. Some direct evidence for prey depletion was obtained from “scuba” transects by Birt et al. (Birt et al. 1987), who compared densities of relatively sedentary fish in bays of Prince Edward I. at different distances from cormorant colonies. Although there is little evidence that cormorants have major impacts on prey populations (see Conservation and Management: conflicts, below), more information is needed about such local depletion.

Numbers nesting together vary over a wide range (1–8,400 pairs in recent surveys). However, the largest recent colonies (on Little Galloo I. in Lake Ontario and on Prince Edward I.). are dwarfed by earlier reports from Baja California where the colony on Isla San Martin may have exceeded 350,000 pairs early in the twentieth century (Carter et al. 1995b, Hatch 1995a). In 1989–1990 the median colony size on the U.S. Great Lakes was 85 nests (36 colonies, 11,099 nests; mean 308, range 3–4,072 nests/colony) (Scharf and Shugart 1998). Nests on Little Galloo I. peaked at 8,400 in 1996 (DVW). In the Gulf of Maine in 1984 the median colony size was 178 nests (131 colonies, 31,622 nests; mean 241 ± 246 SD [range 1–1,077 nests/colony]) (Andrews 1990b). However, such numbers are of uncertain value if colonies are distinguished merely by physical separation rather than as social units.

Colonies generally stay at the same site, but may move, or shift back and forth between sites, especially when numbers are small and in response to disturbance (Drury 1973). Movement of a large colony on Prince Edward I. (4,500 pairs, 3 km) was associated with human disturbance (Cairns et al. 1998). Such movements illustrate the importance of completing censuses in a single year. Nest counts during the nestling stage yield more reliable population estimates than counts during the incubation stage (Ewins et al. 1995b).

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