Food Habits

Main Foods Taken

As a species, generalist predator on pelagic and intertidal marine invertebrates, fishes, insects, other seabirds, and adults, eggs, and young of congeners. Opportunistic scavenger on fish, carrion, human refuse. Individual specialization common (Pierotti and Annett 1987).

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 spp.) crabs and urchins (Good 1992b), and crayfish (R. D. Morris pers. comm.). On mudflats, follows retreating tide to capture worms, small bivalves. 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).

Food Capture And Consumption

In intertidal 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, dipping from surface, or shallow plunge-diving. Small prey items swallowed whole; large prey items (gastropods, bivalves, sea urchins, crabs) broken up and eaten in situ or dropped on rock or sand substrates to break open (Tinbergen 1960). Captures small schooling fish and bycatch or discards from stern of fishing vessels by surface-dipping or landing and grabbing. Human refuse obtained by following garbage scows, roosting at refuse tips, waiting downstream of sewage outfalls (Bent 1921). Piracy from gulls and other species (e.g., diving ducks, terns, puffins, murres) used by a few birds, typically males holding territories near other breeders (Pierotti 1980). A few males (< 0.1%; typically only 1–2 per colony) specialize on conspecific chicks (cannibals; Parsons 1971, RJP).

At sea, forages in large, widely scattered groups that coalesce quickly through rapid recruitment when prey concentrations located (Hoffman et al. 1981, Pierotti 1988). Often follows foraging humpback whales (Megaptera novaeangliae) or groups of delphinids. Hovers over feeding groups grabbing fish, squid, zooplankton concentrated at surface by mammals, diving birds, or large predatory fishes swimming underneath concentration (Pierotti 1988). Employs similar techniques around fishing boats hauling nets—contemporary functional version of feeding whales.

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, other seabirds. Sea urchins (Strongylocentrotus droebachiensis), jonah and rock crabs (Cancer borealis and C. irroratus), and green crabs (Carcinus maenus) commonly taken along coasts in New England (Good 1992b, Dumas and Witman 1993).

In Newfoundland takes primarily mussels (Mytilus edulis), Leach’s Storm-Petrel (Oceanodroma leucorhoa), and refuse during prelaying and incubation periods. Switches diet to capelin (Mallotus villosus) and other small fishes when chicks hatch, followed by secondary switch to squid (Illex illecebrosus) in early to mid-Jul (Pierotti and Annett 1987).

In Great Lakes, feeds mostly on small fishes, primarily alewife and smelt (Pseudoharengus spp.; Osmerus spp.) (Fox et al. 1990, Belant et al. 1993, Chudzik et al. 1994). Large numbers of gulls reported at refuse dumps, but many roosting or loafing; relatively few birds actually feed on refuse (Belant et al. 1993).

On Dutch Frisian I., diet predominantly marine fish and invertebrates during 1960s (Spaans 1971). Increased competition from Lesser Black-backed Gull led to decline in marine fish, increase in garbage and invertebrates by mid 1980s. Decrease in Herring Gull breeding success related to change in diet (Noordhuis and Spaans 1992).

Quantitative Analysis

At coastal sites in New England, percent prey taken (occurrences observed year-round): echinoderms 64%, crustaceans 27%, fish 6%, molluscs 3%. Prey remains from “anvils” (areas where prey dropped to break them open) during same time period included echinoderms (37%), crustaceans (44%), and molluscs (19%) (Good 1992b). Pellets, boli, and mate feedings from 1977 to 1978 in Newfoundland showed 79% dietary specialists (47.4% mussels, 20.1% refuse, 11.5% petrels) and 21% dietary generalists (combination of above) (Pierotti and Annett 1991). Pellets and boli of gulls breeding on Great Lakes included 16 species of fish (80% alewife and smelt), 8 orders of insects, and 11 families of birds (Fox et al. 1990, Chudzik et al. 1994). On coastal Maine islands, percent weight of food in stomach contents of young gulls: refuse and fish offal 44–61%, crabs 1–10%, urchins 1–5%, natural fish 4–14%, squid 1–10%, earthworms 0–30%, tern chicks 0–2%, mussels 1–4%, clams 1–4% (Hunt 1972). On Dutch Frisian I. from 1966 to 1968, pellets: 70% invertebrates, 9% fish, 1.5% garbage; regurgitations: 72% fish, 19% invertebrates; 1985–1987 pellets: >90% invertebrates, 5% fish, 5% garbage; regurgitations: >50% invertebrates, 30–40% fish, 10–50% garbage (Noordhuis and Spaans 1992).

Chooses prey easily handled and swallowed; prefers fish over squid; squid over shellfish; small clams, mussels, crabs over large ones (C. A. Annett pers. comm., RJP, TPG). Preferences change in relation to nutritional requirements, e.g., egg formation, feeding offspring (see Nutrition and Energetics) (Pierotti and Annett 1987, 1990, 1991). Prefers red or silver food over other colors (C. A. Annett pers. comm.). No food storage observed.

Mean daily metabolizable energy intake per day for captive Great Lakes chicks fed marine smelt and vitamin supplement increased almost linearly to 275 Kcal/d during first 30 d after hatching, leveled off until 60 d, declined to 120 Kcal/d by 70 d (Norstrom et al. 1986). Breeding adult birds require approximately 14 kJ/h (males) and 12 kJ/h (females) for normal maintenance, yielding daily requirement of 1,460 kJ/pair/d (Pierotti and Annett 1991). Fresh-caught fish most nutritious food taken (304 Kcal, 30 g protein, 29 g fat/meal), followed by squid (162 Kcal, 23 g protein, 2 g fat/meal), refuse (150 Kcal, 19 g protein, 13 g fat/meal), birds (61 Kcal, 8 g protein, 3 g fat/meal), and intertidal invertebrates (32 Kcal, 5.2 g protein, 1 g fat/meal) (Pierotti and Annett 1987). Despite rankings of last 3 items, most birds specialized on marine invertebrates; these birds had largest, heaviest eggs, highest hatching success (Pierotti and Annett 1987, 1990, 1991).

During mate-feeding, males deplete endogenous fat, females gain fat (Hario et al. 1991). During egg formation, females deplete protein and skeletal calcium reserves (Houston et al. 1983); this may lead to female dietary preference for marine invertebrates or fish, good sources of protein and calcium, during this period (Pierotti and Annett 1987, 1990). Reserves replenished during incubation (Hario et al. 1991). Chick-rearing most demanding period energetically, particularly latter stages when birds must feed each chick up to 200 g of food per day (Morris 1987, Pierotti 1987a). Food demands much lower during nonbreeding season.

More sensitive to direct solar radiation than to ambient temperature (Lustick et al. 1978). Orients toward sun on hot or sunny days; temperatures under white plumage lower than under dark plumage. Incubating gulls often pant on hot day or in direct sunlight. Most heat loss through bare areas, either through mouth lining during panting or through legs and feet. Gulls with feet exposed pant less in windy conditions (RJP). Gulls in water rarely pant. In cold conditions, use counter-current heat exchanger with thin-walled veins surrounding arteries in legs; unsaturated fats used as lubricants in joints (Scholander 1955 in Schmidt-Nielsen 1983).

Regularly drinks fresh water and visits freshwater sites when possible. When drinking seawater, uses salt glands located over eyes to remove salt from water (Schmidt-Nielsen 1983). Salt glands excrete far more salt than kidneys, generate fluid pure 5% sodium chloride (compared with 3% in seawater). Gland has parallel cylindrical lobes, each containing several thousand branching tubules, which extract salt from blood using counter-current flow and active transport; pump sodium and chloride ions against gradient. Fluid drips out nostrils and off end of bill (Schmidt-Nielsen 1983).

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 (Pierotti and Annett 1987, 1990, 1991). See Food Habits: diet. Defecates on breeding territory and on roosting areas. Defecations can also be used to identify dietary components (Spaans 1971).