Demography and Populations

Compared to other widespread colonial waterbirds, population dynamics of Double-crested Cormorant poorly studied; no comprehensive life table constructed. Key demographic parameters have been established at only 1 colony, in a 3-yr study of P. a. albociliatus on Mandarte I., British Columbia, where numbers were increasing by 8.4%/yr (van der Veen 1973). Large demographic differences are likely between populations that are resident or migratory, expanding or stable, but such differences have not been examined.

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 (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.

Clutch

Mean clutch size 2.7–4.1 eggs (mode 4; see Appendix 2 and Breeding: eggs, above, for details). Eggs readily lost during incubation. Failed clutches are replaced, but only 1 brood 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%. 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 often low; e.g., 5% in coastal British Columbia (Drent et al. 1964). All studies probably are 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 St. Lawrence River estuary, reproductive success parameters also are lower for late-nesting cormorants (Jun) compared to early nesters (May; McNeil and Léger 1987). For Mandarte I., British Columbia, average lifetime production calculated as 3.28 young (van der Veen 1973).

Oldest banded bird 17 yr 9 mo (n = 5,589 encounters from 96,651 banded; Klimkiewicz and Futcher 1989); 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 (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. 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).

Diseases

Information limited. Newcastle disease causes wing and leg paralysis and killed several thousand cormorants in the interior in 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). 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).

Body Parasites

Ectoparasites include feather lice (Mallophaga: Eidmanniella kuwani, Pectinopygus faralloni, P. gyrocornis, Piagetiella incomposita; Malcomson 1960); fleas (Siphonaptera: Ceratophyllus niger [hen flea]; Easton 1982). In Florida, 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 1987), 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 1992).

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

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 available. Of 295 band recoveries in Texas (1971–1985), 41% found dead, 28% entangled in fishing gear, and 17% shot (Thompson et al. 1995). For birds banded on Great Lakes (1928–1995; n = 2,393), recoveries in same categories were 56%, 13%, and 9% (E. Woodsworth pers. comm.). Fishing gear is a major cause of death, but also affects encounter probability.

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 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. in press).

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 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 natal colony is suggested by recoveries in Jun of banded birds at least 3 yr old: median distance only 25 km (Dolbeer 1991). No information on interannual fidelity to wintering areas.

Home Range

Individuals forage far from colony or roost, but localization within such a large potential area has not been examined. Birds followed by airplane in Wisconsin flew average of <3 km (range <1 to 40 km) from 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 Farallon Is., CA, do not feed near islands, but regularly travel to estuarine habitats of mainland, a roundtrip of at least 70 km (Ainley and Boekelheide 1990). Wintering cormorants in Mississippi repeatedly visited some catfish ponds and flew past others (King et al. 1995). Basis for this selectivity not reported. Most cormorants return to the same roost each night (M. Tobin pers. comm.). See also Habitat, above.

Numbers

From Hatch 1995, USFWS 1999 . Total in about 1990 estimated to be 1–2 million individuals (approximately 350,000 breeding pairs), but considerable uncertainty about coverage and especially nonbreeder fraction. 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 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 1995 . Numbers of Double-crested Cormorants have been increasing 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 nineteenth and early twentieth centuries (see Distribution: historical changes, above), probably throughout 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 as result of persecution and introduction of domestic animals (Carter et al. 1995). Numbers increased from 1920s until 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 midcentury. Interior populations reached low points about 1970, and Atlantic population ceased growing. During this period, 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 have been explosive in migratory populations of n. Great Plains, Great Lakes, and Atlantic Coast; in many large areas, doubling times have been <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 upper Mississippi River, increases have been slower, and numbers nesting had not returned to historical levels by 1993 (Kirsch 1997). Numbers of breeding birds on West Coast also continue to grow, but have not reached 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 (1995). Analyses of Breeding Bird Surveys from 1966 to 1996 suggest surveywide increase of 6.8%/yr (Sauer et al. 1997). (However, such methods are not well suited to colonial species nor to coastal habitats). Estimates of breeding pairs obtained in 1996 or 1997 suggest that overall growth since about 1990 had slowed to 2.6%/yr, but was still high (22%/yr) in Ontario and States bordering the Great Lakes (Tyson et al. in press).

Wintering inland in 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). Numbers wintering inland are only a small fraction of total population. Analyses of Christmas Bird Counts, 1959–1988, suggest significant increase in Mississippi (18.7%/yr) and an increase of 7.3%/yr for all areas combined (Sauer et al. 1996).

Cormorant populations are influenced by some factors that limit numbers, and others that act in a density-dependent way to regulate them (Cairns 1992). 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 1991), but incidence of nonbreeding events has not been examined.

Demographic parameters likely to respond to density and result in regulation of numbers are age of first breeding, occurrence of nonbreeding, and abandonment of whole clutches. Effects on growth of young, asymptotic mass, and fledging success may be detectable. 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 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. (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, 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. 1995, Hatch 1995). 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 1990). 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 1973–1974). 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 incubation stage (Ewins et al. 1995).