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Main Foods Taken
At most sites almost entirely fish, generally slow-moving or schooling species; size range 3–40 cm, but commonly <15 cm. Less frequently other aquatic animals, including insects, crustaceans, and amphibians (Palmer 1962), with single reports of voles (Microtus pennsylvanicus; C. Adams unpubl.) and a snake (Bent 1922). Foraging is opportunistic and flexible. More than 250 species of fish from >60 families reported as prey.
Microhabitat For Foraging
Generally feeds in shallow, open water (<8 m deep) and close to shore (<5 km away). May feed over sandy bottoms or among rocks and in beds of sea grass or kelp. Often feeds at the bottom, but also in midwater. On Pacific Coast, more likely than other cormorants to forage in open water and in estuaries (Ainley et al. 1981). Responds rapidly to concentrations of prey (see Conservation and management: conflicts, below). Although captive Great Cormorants feed at night in Japan, Double-crested Cormorants are almost exclusively diurnal.
Food Capture And Consumption
Dives from surface and pursues prey underwater. Grasping of fish is aided by hooklike nail at tip of maxilla (upper half of bill) and by muscles attached to occipital style (xiphoid). This pointed bone, articulating at posterior part of skull, is present only in cormorants and anhingas (Siegel-Causey 1988). Wide jaw-opening is facilitated by nasal-frontal hinge at junction with cranium. Small prey may be swallowed underwater; those noticed at the surface are likely to be large or otherwise difficult to handle, such as eels, flounders, or spiny fish. Handling of larger prey includes shaking and hammering on the water. Legs may be shaken from crustaceans. Then prey may be thrown in the air, to be caught and swallowed headfirst. To catch skulking prey such as mudpuppy (Necturus) and burbot (Lota lota) in freshwater, or gunnels (Pholis) in the ocean, cormorants probably root around at the bottom.
When feeding on schooling prey, may form loosely coordinated foraging flocks, sometimes in lines or shallow crescents. The line rolls forward as individuals at rear fly short distances to “leapfrog” the diving birds (Bartholomew 1942). In Georgian Bay, Ontario, in late summer and early fall, coordinated flocks of 50–175 birds drive fish to heads of narrow bays (Glanville 1992). In Massachusetts, foraging in flocks is most noticeable by transient fall migrants (JJH). In Green Bay, WI, flock foraging during breeding season was associated with clear water, shifting to solitary foraging where water was muddied by rainfall (Custer and Bunck 1992); associated with turbid water according to van Eerden and Voslamber (1995). Feeding territoriality not reported.
Flying cormorants sometimes detect the presence of fish below and may rapidly descend to feed; aborted responses of that kind may explain mid-flight “hesitations” shown by flocks. Swimming cormorants occasionally hold their heads underwater before diving, apparently looking for prey. Several features of the eye indicate effective vision underwater as well as in the air; cornea is notably thick and flattened, and intraocular muscles are well developed, apparently to facilitate underwater accommodation (Sivak et al. 1977). However, Double-crested Cormorants commonly catch bottom-dwelling fish in turbid water where visibility is very low, so tactile sensitivity may be important for both locating and capturing prey.
Information on foods eaten has been widely gathered, but generally only in context of perceived conflicts with fishermen, and not in all seasons. Foods of cormorants wintering in marine areas not described. Representative studies and comparative papers are summarized below, and only major prey species are listed (see Appendix 1 for listing of scientific names of species by family); for additional lists of prey, see Palmer 1962 . Seasonal variation is evident in all appropriately designed studies, but ecological factors contributing to cormorant predation and its impacts are not revealed by mere lists of prey or simple percentages. Occurrence of individual specializations has not been examined. See Conservation and management: conflicts, below.
Most data are from regurgitations collected in nesting colonies, or from pellets (especially fish otoliths); contents of gizzards and esophagi of collected birds are less frequently reported. It is not known if chick regurgitations reflect the adult diet. Collecting adults is not only expensive but also likely to undersample known spatial and temporal variation in prey captured each day. Pellets may integrate meals over an entire day, but with unknown completeness. Difficulties with otoliths and other data from pellets include differential loss and digestion by size and/or species (Duffy and Laurenson 1983, Craven and Lev 1987, Johnstone et al. 1990), as well as secondary consumption (evidence for prey’s prey; Blackwell and Sinclair 1995, Johnson et al. 1997). Thus, otoliths are difficult to use for estimates of size or number of fish ingested. Reliance on the abundant dietary remains in pellets may lead to overestimates of invertebrate prey and underestimates of small or soft-bodied fish. Comparing food items from pellets to those from collected birds in Wyoming, Derby and Lovvorn (1997) concluded that some of the differences revealed by the methods had resulted from the cormorants’ daily schedules of hunting for different prey. For development of standardized methods, see Carss et al. 1997 .
From nw. Atlantic Ocean, Cairns (1998) summarized 40 sets of collections (1914–1996) from Gulf of Maine (16), Atlantic Nova Scotia (7), Prince Edward I. (9), and Quebec (8). Total >5,000 samples, mostly stomach contents and regurgitations; about 60 fish taxa reported. Salmon were important prey during smolt run in stocked rivers, but no data available from rivers where runs are exclusively wild. Estimates include percentage composition from biomass and from number (within groups, percentages sum to 100).
During Smolt Runs. (671 samples). Atlantic salmon 26%; other salmonids 4.6%; nonsalmonid freshwater and estuarine fish 53% (including yellow perch, 3.1; white sucker, 3.6; gaspereau [alewife and blueback herring], 7.0; rainbow smelt, 24.5; American eel, 3.8; sticklebacks, 3.3). Marine prey totaled 15.4%, including sand lance, 2.2; sculpins, 3.4; and crustaceans, 3.7.
Outside Smolt Runs. (>4,554 samples). Freshwater fish, 4.9% (including negligible salmon); brook trout, 1.9 (chiefly reflecting numbers from freshwater ponds on Prince Edward I.); nonsalmonid fish, 2.9. Estuarine/diadromous fish, 23% (including rainbow smelt, 6.4; gaspereau, 5.3; sticklebacks, 3.4; American eel, 2.6; cyprinodonts, 2.1). Pelagic fish, 14% (including sand lance, 5.5; herring, 4.2; capelin, 4.0). Marine bottom fish, 54% (including sculpins 15.7; cunner, 15.6; blennies, 9.4; Atlantic cod 2.9; pollock, 2.2; winter flounder, 2.1; and hake, 1.4. No equivalent studies found for cormorant diets near salmon rivers in Pacific Northwest, where consumption of salmon smolts angers fishermen (Bayer 1989).
In Penobscot Bay and River, ME, adult diets showed seasonal changes. In spring, birds feeding on the river consumed 12 fish species and concentrated at dams where they caught many salmon smolts in May. Adults feeding below head of tide consumed 28 species, including freshwater and seasonally available marine and estuarine forms (Blackwell and Krohn 1997, Blackwell et al. 1997). Nestling diets (n = 742 regurgitated samples collected in Jul at colonies on islands in the bay): >40 taxa; 5 benthic taxa were consistently important: cunner, rock gunnel, sculpins, wrymouth, and sand shrimp (Crangon septemspinosa). Anadromous and catadromous species occurred inconsistently (Blackwell et al. 1995). In contrast to Mendall’s (1936) report from same area in 1935, economically important groundfishes (Atlantic cod, winter flounder) were negligible components of nestling diet in 1993. From the St. Lawrence, a shift from benthic to schooling species over a similar period was reported by Rail and Chapdelaine (1998).
From Pacific Ocean, samples of pellets and regurgitations from 7 sites from Mandarte I. (British Columbia) to Gulf of California in breeding season included 95 taxa, 2,722 individuals; predominantly schooling fish from well above flat bottoms (Ainley et al. 1981), or inshore benthic species (Robertson 1974). Contributions of >5% (by number) at 1 or more sites were derived from 21 species (in 15 families).
Information from breeders in se. U.S. is very limited, and no studies of coastal wintering birds are reported. In Biscayne Bay, FL, fish samples collected below nests included 507 individuals of 32 species: Gulf toadfish, 35.9% by number (40.8% by weight); bluestriped grunt, 9.1 (6.6); white grunt, 7.3 (5.2); bucktooth parrot fish, 5.7 (5.3); and pinfish, 5.1 (5.6). Parrot fish (Scaridae) together made up 20.5% by number (21.9% by weight); grunts (Hemulidae), 14.4 (12.9); filefish (Balistidae), 6.5 (1.9); barbfish, 3.2 (3.9); sandperch, 2.4 (4.9); and gray snapper, 1.4 (2.7). Mean weight of individual fish was 39 g (Cummings 1987). From freshwater sites in Florida, important fish in 67 cormorant stomachs were shad, brown bullhead, sunfish, and black crappie at both natural wetlands and phosphate-mine settling ponds, but in the latter habitat mosquito fish was the most frequent and insects were present in unusual numbers (but <0.4% aggregate volume). These included dragonfly nymphs (Aeschnidae, Libellulidae), midge larvae (Chironomus), and water boatmen (Corixidae; O’Meara et al. 1982). It is unclear if some of these insects resulted from secondary consumption.
For the Great Lakes, at least 6 diet studies of cormorants, 1986–1998, have been published. In these ecosystems, populations of prey fish have fluctuated widely in response to almost complete loss of native large piscivorous fish in 1940s and stocking of several salmonids (Christie et al. 1987). Introduced alewife, abundant in Lake Ontario by 1880s and in remaining Great Lakes (except Lake Superior) by mid-1950s, has been important in every study. In Lakes Huron, Michigan, and Superior during May–Aug 1986–1989, Ludwig et al. (1989) collected 8,512 regurgitated food items from adults and young birds handled. Alewife and nine-spine sticklebacks each accounted for 41% by number (1 species of crayfish, all the rest were fish, in 18 taxa). By biomass, the most important species were alewife (57%), yellow perch (13%), rainbow smelt (8%), and white sucker (7%); weights of sticklebacks averaged 1.9 g, of alewives 19 g. Diet varied both seasonally and geographically; by Aug, diet at all 4 study areas was 100% alewife in all colonies sampled. Overall, forage fish accounted for 88% of food items (66% of biomass). Fish species of local commercial importance (such as panfish) constituted 12% by number (34% by weight), including yellow perch (3%, 13%), smelt (7, 8), and sucker (<1, 7). No lake trout or common whitefish, the two most important commercial fish in Lake Superior, were found. Cormorant diet in Lake Superior in 1983 was examined by Craven and Lev (1987).
On Lakes Huron, Erie, and Ontario, 2,133 pellets and boli, collected Apr/May–Jul 1992 and 1994, revealed significant spatial and temporal variation among 8 colonies in frequency of occurrence of fish species (Neuman et al. 1997): smallmouth bass, 5.3–87.5%; alewife, 0–49.6; smelt, 2.0–65.3; slimy sculpin, 2.5–41.3; white perch, 0–21.4; yellow perch, 4.3–77.8. Nonfish items included crayfish (0–6.4%) and snails (0–1.0%). Variation was attributed to movements of fish into shallow spawning areas and to spatial heterogeneity of fish habitat. Average length of prey consumed varied by species: 7.6–30.5 cm (26 species identified).
On Little Galloo I., e. Lake Ontario, 38,301 fish (24 taxa) were identified from 3,827 pellets collected during Apr–Jul 1992–1994 (Ross and Johnson 1995). Forage fish accounted for approximately 50% of diet (by frequency), including alewife (25%), trout perch (11), minnows (3) and slimy sculpin (3). Panfish made up about 33% of diet, including yellow perch (18%), centrarchids (12), and white perch (3). Game fish made up 1.6% of diet, mainly smallmouth bass (1.3%) and salmonines (0.3%; see Ross and Johnson in press); 14% of total fish were unidentified. Proportion of ale-wife increased and that of yellow perch decreased in diet after chicks hatched; the proportions also varied between years. Predation losses from individual releases of stocked salmonids were as high as 9%.
From Les Cheneaux Is. area of n. Lake Huron in 1995, contents of 373 cormorant stomachs were analyzed to evaluate cormorant impact on yellow perch fishery (Belyea et al. in press). Yellow perch (75–150 mm) varied from 47% of diet (by weight) in early spring to 2% in late spring and early summer (overall 10%). Sticklebacks and alewives varied from 0 to 45% and from <30 to 85% of diet, respectively, in various months (together, 62% overall). Diet and feeding locations varied according to spawning of the various prey species. Diet of cormorants in Lake Erie was similarly examined by Bur and Belant (in press).
On Lake Winnipegosis, Manitoba, in 1987, 6,169 prey samples from regurgitates represented 16 fish species and 1 crayfish (Orconectes sp.; Hobson et al. 1989). Three species constituted >90% frequency (biomass): yellow perch, 63.8 (27.6); white sucker, 14.2 (46.4); and tullibee, 14.0 (20.4). Commercially valuable species, walleye and sauger, accounted for maximum 0.1 and 0.2% of prey biomass, respectively.
Amphibians are rarely reported as prey, but salamander Ambystoma occurred in up to 25% of stomach samples in Wyoming (Derby and Lovvorn 1997) and mudpuppy Necturus in New York (C. Adams unpubl.). Crustaceans are frequently recorded as prey, but usually in small numbers (e.g., 0.2% frequency; Hobson et al. 1989), and some may be derived from gut contents of fish prey. They are less frequent in stomach samples than in pellets (Johnson et al. 1997). In Penobscot Bay, ME, however, sand shrimp was among the highest-ranking prey taxa in nestling regurgitations (average percentage by volume 0.9–3.3; Blackwell et al. 1995) and also for spring adults feeding in the estuary (to 45.8%; Blackwell et al. 1997). Eight species of crustaceans were identified in nestling regurgitates (Blackwell et al. 1995). At aquaculture sites in Louisiana, some wintering Double-crested Cormorants feed on crawfish (Procambarus spp.) and shrimp (Macrobrachium spp.; J. Huner pers. comm.). Other invertebrates reported in small numbers include gastropods and pelecypods, which may often be secondarily consumed (Johnson et al. 1997).
Winter diets infrequently reported, except near catfish farms. From 420 wintering cormorants shot on 8 reservoirs in Texas, however, stomach contents included only fish, of 29 species. Largest was a catfish 415 mm long, but 90% were ≤125 mm, ≤200 g. Most were shad (79.2% by number, 26.1% by weight) or sunfish (7.8, 15.0). Sport fish (3 catfishes, 2 bass, crappies) together accounted for 3% by number (32% by weight). Other prey included common carp (0.2, 2.2) and blue tilapia (5.2, 18.2). From Nov to Mar, consumption of shad decreased while that of sunfish increased (Campo et al. 1993).
Near catfish farms in Mississippi, wintering cormorants that were collected at night roosts in 1989–1991 had eaten >10 species of fish, but chan-nel catfish and gizzard shad constituted >90% of diet by weight. Proportions of these 2 species were approximately equal overall, but differed by month and location, and between sexes of cormorants sampled. Proportion of catfish ranged from 0–27% in Oct–Nov to 90–96% in Feb–Apr. Catfish was most often consumed by males during spring (Feb–Apr) and in areas with more farmed catfish (Glahn et al. 1995).
Some of the recent changes in diet (and increases in cormorant numbers) reported in both marine and freshwater areas are associated with declines in major predatory fish that may be attributable to overfishing—e.g., cod and other groundfish in Penobscot Bay, ME (Blackwell et al. 1995), and walleye in Lake Winnipegosis, Manitoba (Hobson et al. 1989). Overfishing disrupts whole ecosystems and may contribute to increases in cormorant numbers.
Food Selection And Storage
Opportunistic feeder, taking wide range of prey species depending on availability. The studies summarized under Diet, above, do not show prefer-ences of cormorants, because availability of fish is unknown. In many habitats, potential prey present cannot be measured accurately; e.g., in a 1982 Massachusetts study, the most numerous fish in 498 regurgitations was rock gunnel, a bottom-living fish that was absent from extensive trawl samples taken in nearby sandy areas (JJH). In an aquaculture pond containing both gizzard shad and catfish, cormorants preferred gizzard shad, probably because it is easier to handle (Stickley et al. 1992). Captive nestlings fed by Lewis (1929) accepted any white-fleshed fish and favored sculpins, but were reluctant to eat trout. Likely to concentrate on species when they come into shallow water to spawn or are easily caught for other reasons (e.g., naive hatchery-reared individuals, or captives without shelter). No evidence of food storage, and abundance of rotting fish in colonies suggests that regurgitated prey are unlikely to be reswallowed.
Nutrition And Energetics
Captive young fed unsupplemented, previously frozen rainbow smelt showed symptoms of rickets (Nichols et al. 1983). For free-living nestlings in New Hampshire that were fed fish (average caloric content 1.14 kcal/g), food intake peaked at 600 kcal/d at 4 wk and then declined; digestive efficiency 85% (Dunn 1975a). Existence energy expenditure (200–300 kcal/d) for these birds was estimated to include 38% for thermoregulation (Dunn 1976); for energy budget, see Dunn 1980 . For 5 Florida nestlings, food consumption peaked at 40 d of age and exceeded that of adults from 21 to 168 d; assimilation efficiencies averaged 75.8% ± 6.4 SD (Cummings 1987). Metabolizable energy coefficients (nitrogen-corrected) were approximately 75% for bluegill (Lepomis macrochirus), 78% for gizzard shad, and 79% for channel catfish (Brugger 1993).
Daily consumption of fish by an adult Double-crested Cormorant is about 320 g (range of estimates 208–537 g), or 20–25% of body mass/d (Dunn 1975a, Schramm et al. 1984, Glahn and Brugger 1995). (This amount is far less than extravagant claims, widely circulated in the past, of >1 body mass/d.) Large differences in body mass, breeding status, and environmental temperature contribute to variability. Measurements of daily energy expenditure by free-living birds (field metabolic rates) are needed (Carss et al. 1997). In a nonrandom sample of 14 catfish ponds, median consumption was 2 fish/cormorant-hour (Stickley et al. 1992).
Metabolism And Temperature Regulation
Basal metabolic rates (kJ/kg.d, mean 1.36 kg) reported for adults and immatures (7 mo–1 yr) from Florida (Cummings 1987); by day, adult 254, immature 272; by night, adult 243, immature 256. Day and night values were not significantly different, but both were lower than predicted (e.g., by Aschoff-Pohl equations). No reports of field metabolic rates.
Outer feather coat over entire body is underlain by thick layer of pale brownish down. Adults respond to cold by tucking one foot into flank-feathers and bill under wing. Characteristic spread-wing posture is not primarily thermoregulatory (it serves to dry feathers; Henneman 1988; see Behavior: self-maintenance, below). Heat stress for adults on their nests at midday can be severe, especially when there is no wind. Responds to high temperature by fluttering gular area; this behavior is sometimes mistakenly called panting. Driven by hyoid, this behavior moves air across moist mucosa of buccal cavity and esophagus and is an effective method of evaporative cooling. Rate of gular flutter is relatively constant over wide range of ambient temperatures (645–730 flutters/min), but amplitude and continuity increase as heat load increases (Bartholomew et al. 1968). Gular flutter rates of 450–540 flutters/min reported by Van Scheik (1985). The behavior is also shown by both feathered and featherless nestlings who cannot thermoregulate before day 10. Under severe heat stress, nestlings droop wings and raise scapular feathers. Breathing increases in rate and amplitude with increasing body temperature and is not synchronized with gular flutter. Young chicks are usually shaded by parents who feed them water on very hot days; if exposed to midday sun they may die in <20 min (see Conservation and management: effects of human activity, below).
Drinking, Pellet-Casting, And Defecation
Drinks by dipping open bill in water, then raising head; bill-dipping observed after dives may be associated with successful captures.
Ionic homeostasis achieved by salt glands located in shallow depressions inside skull above orbits (Siegel-Causey 1990). This species was the first for which functional significance of these glands was demonstrated, by Schmidt-Nielsen in 1957. The secretion is a salt solution more concentrated than seawater (500–600 m M Na+, 73 µl/kg/h). On a diet of fish, this species does not require fresh water to drink.
Pellets of fishbones, crustacean exoskeletons, and/or small pebbles (function unknown), or rarely snails (especially Littorina), are common at nesting and roosting sites in both marine and freshwater locations; residual pebbles may be abundant. Pellets produced by adults and older chicks typically weigh 5–60 g (dry). Fresh pellets are enveloped in soft mucus, which hardens as pellets dry and eventually disintegrates. Captive birds commonly produce 1 pellet/d, but less if fed soft-bodied fish (Brugger 1993), and a single pellet cannot be taken to represent accurately food intake of 1 d (Duffy and Laurenson 1983, Russell et al. 1995, Neuman et al. 1997). Adults and young readily regurgitate food if disturbed.
Defecates when on water, when perched or incubating, or in flight. Gut transit time <2 h (time to first appearance of matter in feces), but complete digestion takes up to 3 d (Brugger 1993). By age of about 10 d, nestlings project their liquid feces beyond nest cup. Accumulated feces make sites of roosts and colonies conspicuous, kill trees and other vegetation, and cement nest materials. The role of cormorants in cycling nutrients within and between aquatic ecosystems has not been examined.