Food Habits

Main Foods Taken

Generalist predator on fishes; both pelagic and intertidal marine invertebrates; mammals; insects; and adults, eggs, and chicks of congeners, other breeding seabirds, and waterfowl. Opportunistic scavenger on fish, carrion, and human refuse. Individual specialization known (Lock 1973, Pierotti 1979, TPG), often age-specific (Ryder 1980, TPG).

Microhabitat For Foraging

Varies with food taken. Along rocky shores, forages primarily in low intertidal and shallow subtidal zones; dives into shallow water to take mussels (Mytilus edulis), crabs (green [Carcinus maenus], Jonah [Cancer borealis], rock [C. irroratus]), and sea urchins (Strongylocentrotus droebachiensis) (Good 1992b). Use of mussel beds is half that of Herring Gull (Hunt and Hunt 1973). On mudflats, follows retreating tide to capture worms, small bivalves. Use of mud habitat half that of Herring Gull (Hunt and Hunt 1973). At sea, congregates around submarine features (mounts, sandbanks, local upwellings) where prey concentrate. Cannot dive below 1–2 m; feeds on prey at or very near surface (Pierotti 1988). Captures crabs in low intertidal and shallow subtidal zones along rocky and sandy shores (Good 1992a). On colonies, feeds on eggs and chicks of Herring Gulls where the 2 species co-occur (Lock 1973, Pierotti 1979, TPG)

Food Capture And Consumption

In intertidal zones or mudflats, forages alone or in family groups primarily during daylight hours. In coastal areas, captures prey by walking or swimming along shore at low tide, by dipping from surface, or by shallow plunge-diving. Swallows small prey items whole; breaks up large prey items (gastropods, bivalves, sea urchins, crabs) and eats them on the site where found or drops them on rock or sand substrates to break open (TPG). Feeds on overturned horseshoe crabs (Limulus polyphemus) stranded on mudflats during and after spawning (Botton 1993, TPG). Foraging efficiency in gulls is a function of age, method, habitat, and food type; insufficient data for Great Black-backed Gulls (Burger 1987, 1988). Captures schooling fish and bycatch (non-target or undersized fish) discarded from stern of fishing vessels by surface-dipping or by landing and grabbing. Captures eels (Anguilla rostrata) in shallow water by plunge-diving from height of 2–3 m above surface of water, almost completely submerging (Seymour 1974). Obtains human refuse by following garbage scows, by roosting at landfills, or by waiting downstream of sewage outfalls (Bent 1921, TPG).

Observed at garbage dumps more in winter (Wells 1994) than during breeding season (Verbeek 1979). There is confusion over the role of “refuse”—fishery-generated waste and garbage from dumps—in diet. Fisheries waste (bycatch and fish offal) is higher in food quality than human food scraps from garbage dumps (Pierotti and Annett 1987). Landfills appear less important to this species than to Herring Gulls (but see Burger 1988), particularly during breeding (Mudge and Ferns 1982). Thought to be less affected by closing of landfills than is Herring Gull (Buckley and Buckley 1984, Cavanagh 1992). In Newfoundland, feeds no garbage to chicks (Pierotti 1979); on Appledore I., ME, pairs that fed chicks garbage had reduced breeding success (TPG).

Preys opportunistically on migrating passerines and colonial waterbirds, including Atlantic Puffins (Fratercula arctica), Common Murres (Uria aalge), Herring Gulls, Common Terns (Sterna hirundo), Roseate Terns (Sterna dougallii), Manx Shearwaters (Puffinus puffinus), and Horned Grebes (Podiceps auritis); adult waterfowl often attacked and killed when newly banded and released (Ryan 1990). Steals food (kleptoparasitizes) from conspecifics and other species (e.g., diving ducks, terns, puffins, murres, shorebirds). Observed kleptoparasitizing conspecifics and congeners (TPG), eagles (Haliaeetus leucocephalus) (Lien 1975), and sharks (Lamna nasus) (French 1982). Steals food from Herring Gulls at landfills, at intertidal areas, and during courtship-feeding by males (Verbeek 1979). Predation on Common Tern eggs and chicks observed only at night on Stratton I., ME (Nocera and Kress in press). Some pairs are specialist predators on conspecific chicks (Lock 1973) or Herring Gull chicks (Pierotti 1979). Numbers specializing on newly fledged chicks are increasing at Appledore I., Isles of Shoals, ME (TPG).

At sea, forages in widely scattered groups; often joins other small groups when concentrations of prey are located. In Georges Bank area, adults and juveniles often forage in pairs; often follows foraging large predatory fishes, hovers over feeding groups grabbing fish or squid concentrated at surface by bluefin tuna (Thunnus thynnus; Pierotti 1988). Employs similar techniques around fishing boats hauling nets (Furness et al. 1992) and lobster boats dumping bait and undersized lobsters (TPG). Follows boats trawling and discarding at night north of Skagen, Denmark; takes experimentally discarded fish at night at same levels as do Lesser Black-backed and Herring gulls (Garthe and Hüppop 1993).

Major Food Items

Direct observation possible year-round; sampling pellets, boli, and prey possible on breeding territories. Observed diets include marine invertebrates, fish, insects, refuse, and seabirds. Jonah and rock crabs, sea urchins, green crabs, and sea stars (Asterias forbesii and A. vulgaris) commonly taken along coasts in Maine, New Hampshire, and Massachusetts (Good 1992b, Dumas 1996). Large numbers of gulls reported at refuse dumps throughout Maine (Wells 1994), but many roosting or loafing; in Massachusetts, relatively few birds (19%) actually observed foraging (Cavanagh 1992).

On islands in Witless Bay, Newfoundland, diet in breeding season consisted primarily of fish and birds (Threlfall 1968a). On Great I., Newfoundland, major food items are fishes (capelin [Mallotus villosus], Atlantic cod [Gadus morrhua], and Atlantic tomcod [Microgadus tomcod]), birds (Leach’s Storm-Petrel [Oceanodroma leucorhoa], Atlantic Puffin, and Herring Gull chicks), and squid (Illex illecebrosus; Pierotti 1979).

Quantitative Analysis

Stomach contents of adults during breeding season from Great I., Newfoundland (n = 16): 32% prey fish, 19% birds, and 19% squid; in collected regurgitants (n = 53), 68% fish, 10% birds, and 15% squid. In pellets, 5% fish and 91% birds; in chick and mate feedings (n = 57), 53% fish, 20% birds, and 25% squid (Pierotti 1979).

On Sable I., Nova Scotia, of pellets and food remains collected during breeding season (n = 627): 25% fish, 17% tern eggs, 15% invertebrates, 10% pelagic birds, 11% terns, 11% seal (Phoca vitulina) remains, 3% gull eggs, 2% passerine birds, 1% gull chicks, and <1% garbage; of chick regurgitants (n = 82): 96% fish (46% mackerel (Scomber scombrus), 18% herring (Clupea harengus), 1% sand lance (Ammodytes hexapterus), 4% eel, 1% pleuronectids, and 26% unidentified) and 4% birds (Lock 1973).

Year-round observations at coastal sites in n. New England (n = 75): 23% echinoderms, 63% crustaceans, 14% fish (TPG). Prey remains from “anvils” (areas where birds drop prey to break them open) during same time (n = 454): 37% echinoderms, 44% crustaceans, and 19% mollusks (Good 1992b).

On Monomoy I., MA, in breeding season of 1988, stomach contents of adults: 11.6% fish, 3.6% birds, 10.3% mollusks, 2.9% crustaceans, 13.1% insects, 42.0% refuse, and 10.2% vegetation; of chick regurgitants, 74.6% fish, 0.7% birds, 5.6% mollusks, 4.1% crustaceans, 1.4% insects, and 14.7% refuse (Cavanagh 1992). Pellets, boli, and mate-feedings on Appledore I., ME, during breeding in 1991 showed no dietary specialists; prehatching pellets and boli included 54% refuse, 27% intertidal invertebrates, 15% fish, and 4% mammals and birds; posthatching pellets and boli included 36% refuse, 30% fish, 23% intertidal invertebrates, and 11% vertebrates (primarily muskrat [Ondatra zibethicus] and gull chicks; TPG).

Males forage at greater distances and exploit different prey than females (Belopol’skii 1961). In general, species chooses prey that are easily found, handled, and swallowed. Preferences change in relation to nutritional requirements—e.g., egg formation, feeding of offspring (see Nutrition and energetics, below; Lock 1973, TPG). No food storage reported.

Mean daily metabolizable energy intake for captive Nova Scotia chicks fed cat food, beef liver, cod, and haddock averaged 310 kcal/d during first 45 d after hatching, peaking at 550 kcal/d at 28 d; leveled off at 350 kcal/d after 45 d (Lock 1973). Fresh-caught fish are the most nutritious food taken (304 kcal, 30 g protein, and 29 g fat/meal), followed by squid (162, 23, and 2), refuse (150, 19, and 13), birds (61, 8, and 3), and intertidal invertebrates (32, 5.2, and 1; Pierotti and Annett 1987). Despite rankings of last 3 items, many Great Black-backed Gulls specialize on birds, possibly because of ease of capture relative to energy expenditure.

As with Herring Gulls, endogenous fat depletion in male and fat gain in female are likely during mate-feeding (Hario et al. 1991); no data reported for Great Black-backed Gull. As with Lesser Black-backed Gull, female likely depletes protein and skeletal calcium reserves during egg formation (Houston et al. 1983); no data reported for Great Black-backed Gull. This depletion of protein and calcium may lead to female preference for marine invertebrates or fish, which are good sources of protein and calcium during this period (Pierotti and Annett 1987, 1990). As with Herring Gull, reserves likely replenished during incubation (Hario et al. 1991). Chick-rearing is the most demanding period energetically, particularly latter stages. Food demands are much lower in nonbreeding season.

No data on metabolism or temperature regulation. More sensitive to direct solar radiation than to ambient temperature; orients toward sun on hot or sunny days (TPG). Temperatures under white plumage are lower than under dark plumage; difference disappears as angle of solar incidence increases (Lustick et al. 1980). Incubating gulls often pant on hot day or in direct sunlight. As in Herring Gull, most heat loss occurs through bare areas, either through mouth-lining during panting or through legs and feet (Lustick et al. 1978). In general, gulls with feet exposed pant less in windy conditions; gulls in water rarely pant (TPG). In cold conditions, gulls use countercurrent heat exchanger (thin-walled veins surrounding arteries in legs) to reduce heat loss in arterial blood flowing to feet; unsaturated fats used as lubricants in joints. No specific data.

Regularly drinks fresh water and visits freshwater sites when possible. When drinking seawater, uses salt glands located over eyes to remove salt from water. Gland has parallel cylindrical lobes, each containing several thousand branching tubules, which extract salt from blood using countercurrent flow and active transport; these mechanisms pump sodium and chloride ions against gradient. Fluid drips out nostrils, off end of bill. Fractional excretion of sodium (91%) through salt gland is double the osmolality of ureteral urine; fractional excretion of water (43%) through salt gland is less than half that of ureteral urine (Skadhauge 1982). Excreted fluid is more concentrated in Great Black-backed Gull than in Herring Gull (Schmidt-Nielsen 1960).

Regurgitates pellets of indigestible material around nests during breeding and on roosting areas. Pellets contain bones, shells, glass, paper; useful for quantification of dietary components (Lock 1973, TPG). See Food habits: diet, above. Defecates on breeding territory and on roosting areas. Full-grown hand-reared individuals that were fed as much frozen herring as they wanted excreted the following quantities of soluble compounds (in µg-at., except as noted): per gram dry weight of excreta—1,288 NH³, 2.8 NO², 50.8 NO³, 12.7 SiO², 479 PO⁴; per bird per day—24.5 NH³, 0.1 NO², 1.0 NO³, 0.2 SiO², 9.1 g-at. PO⁴nitrogen release (µg-at./l/g excreta) by female increases 10% in 2-d period following laying of egg (Bedard et al. 1980).

Defecations can also be used to identify dietary components (Lock 1973). Mean defecation rate 4.38/h (n = 30) and total phosphorus excreted is 48.4 mg/defecation (n = 21); estimate of phosphorus loading at a kettle pond was 35 kg/yr from 1.7 x 10⁶h/yr of pond use (Portnoy and Soukup 1990). Jaws of worms (Nereis virens) are the only infaunal invertebrate samples deposited in feces (Ambrose 1986).