Breeding

Pair Formation

Pairs form at start of breeding season in Mar and Apr (depending on latitude), either on male’s territory or in loafing areas. In areas covered by ice, pairs delay nest-building and egg-laying but not territory establishment. Male regurgitates food for female during egg-laying. No data on feeding frequency or amounts.

Nest-Building

Timing depends on latitude. In Maine, builds nest as early as 10 Apr (Butler and Janes-Butler 1982). In Maryland, begins in mid-Mar (Robbins and Blom 1996).

First/Only Brood Per Season

One brood/yr. Four follicles enlarge before egg-laying, but only 3 are ovulated; last is a reserve from which an egg can be laid in 1–2 d if an egg is lost (Murton and Westwood 1977). Most clutches are initiated late Apr–early May; can be initiated in mid-May. In Nova Scotia, peak laying date is 12 May (range 30 Apr–28 May; Lock 1973). On Appledore I., ME, peak laying date is 15 May (range 15 Apr–5 Jun; McGill-Harelstad 1985). On Little Duck I., ME, mean laying date is 4 May (earliest 24 Apr; Butler and Janes-Butler 1982). In New Jersey, initiates clutch 7–18 Apr (Burger 1978). In Great Lakes region, laying dates range from 28 Apr to 29 Jun (Peck and James 1994).

Typically initiates clutch 2 wk before sympatric Herring Gulls do (Erwin 1971). Takes 5–6 d to lay 3-egg clutch. Incubation begins after first egg, lasts 27–28 d; chicks fledge 7–8 wk after hatching (Cramp and Simmons 1983). Parents and chicks abandon territory in early fall. Chicks cared for off territory for up to 6 mo (TPG).

Selection Process

Male and female dig various scrapes, fill them with vegetation, feathers, etc. May start but not finish extra nests in territory (Witherby et al. 1941). Ultimate choice is where female lays eggs.

Microhabitat; Site Characteristics

Site often protected from prevailing wind(s); placed next to large object (log, bush, rock), which acts as visual barrier between nest and closest neighbors. Natural screens (barriers ≥30 cm high) adjacent to nests on Appledore I., ME, are not oriented to prevailing winds, but oriented equally in 4 major directions; prefers more open habitat than Herring Gull; extent of natural screens adjacent to nests (mean 65° ± 6.5 SE; range 0°–360°, n = 143) is half that of Herring Gull (mean 121° ± 4.3 SE (range 0°–360, n = 470; TPG). Nests on rocky outcrops, grassy knolls, dunes on barrier islands, or in depressions scraped into short vegetation. Located above high tide, splash zone on beaches, and marine terraces. In New Jersey, nests in salt marshes, generally under marsh elder (Iva frutescens) bushes (Burger 1978).

Construction Process

See Nest site, above. Pair builds nest during daylight several days before egg-laying. Adds vegetation through incubation.

Structure And Composition Matter

Scrapes bowl into substrate, lines it with vegetation, feathers, plastic, rope, etc. Collects nest material off territory or steals it from other gull species; often wets the material before adding it to nest (Verbeek 1979). Some nests in grass have little or no lining (Bent 1921, TPG).

Dimensions

Inner cavity 25–33 cm; outer dia-meter 20–56 cm; inner depth 6–10 cm, outer depth 5–12 cm (Bent 1921, Peck and James 1983).

Microclimate

In open or protected from prevailing wind. Shelter lowers wind speed above nest cup; reduces wind chill for incubating adult or uncovered eggs (Pierotti 1979, TPG).

Maintenance And Reuse Of Nests, Alternate Nests

Male and female add to nest throughout incubation; do not reuse nest, but often reuse specific site from year to year. Starts building alternate nests, but does not use them (Witherby et al. 1941, Pierotti 1979).

Nonbreeding Nests

Sometimes builds alternate nests on territory before choosing final nest site. Uses only final nest (where eggs are laid). Counts of nesting pairs may be overestimated because of inclusion of these (alternate) nests.

Shape

Variable; most ovoid.

Size

Appendix 1 . Data from various collections: length 77.9 mm (range 73–86.5); breadth 54.2 mm (range 51–57.5, n = 59; Bent 1921). In Nova Scotia and Labrador: length 76.51 mm (range 71.12–80.83); breadth 53.89 mm (range 51.25–56.53, n = 60 pre-DDT [pre-1947] eggs in 20 egg sets; Western Foundation of Vertebrate Zoology).

Mass

Mass 90–120 g. Second egg sometimes heaviest; third egg lightest because of length reduction (see Appendix 1), although difference may be reduced in eggs from well-nourished females. Runt eggs (strange miniature eggs) reported in 3- and 4-egg clutches at nests in Norway; thought to result from repeated egg-robbing by humans and subsequent relaying by exhausted females (Crick 1995). Clutch represents 22% (range 19–27%) of female mass (calculated from Appendices 1 and 3).

Color

Usually pale olive, buff, or greenish; may vary from pale whitish buff to deep brownish buff. Speckled, spotted, blotched blackish brown, olive, or olive brown. Pinkish variety known. Terminal egg (usually third) typically scrawled or streaked, especially on wide end (Harrison 1978).

Surface Texture

Smooth, nonglossy or slightly glossy, with finely granular surface.

Eggshell Thickness

In Barents Sea (n. Europe) colonies, mean shell thickness 0.32 mm (range 0.26–0.33; Belopol’skii 1961). In Britain, eggs contaminated with high level of DDE (18 ppm ± 1 SE in lipid) have thinner (mean 0.29 mm ± 0.009 SE) and weaker shells than those contaminated with low level of DDE (10 ppm ± 1 SE; mean 0.32 mm ± 0.0004 SE; Cooke 1979).

Clutch Size

Modal clutch size 3; some birds in poor condition lay 1–2 eggs (see Demography and populations: measures of breeding activity, below, for details).

Egg-Laying

Nest is mostly complete before first egg is laid. Eggs are usually laid in early morning at roughly 2-d intervals (Harrison 1978). Male regurgitates food for female throughout laying; female rarely leaves territory. Male is often absent obtaining food for female, reducing mate-guarding opportunities. If first egg lost, female re-lays for up to 24 d, depending on physiological condition (Pierotti 1979). If clutch lost, female requires refractory period to produce new clutch. Replacement clutches often have either smaller or fewer eggs than first clutch.

Onset Of Broodiness And Incubation In Relation To Laying

Incubation begins before clutch is completed. Effective incubation begins after laying of first or second egg, but not continuous until clutch is complete (Witherby et al. 1941).

Incubation Patches

Three incubation patches: one on either side of keel, and third posterior to these. Present in both sexes (Lewis 1982).

Incubation Period

First egg laid on day 0, hatches on day 26–28; second egg laid on day 2, hatches on day 30; third egg laid on day 4, hatches on day 31–32 (Witherby et al. 1941).

Parental Behavior

Incubation shared by male and female; each parent, 3 bouts/d (Burger 1980). Most data from Little Duck I., ME (Butler and Janes-Butler 1982, 1983); patterns likely to vary in other locations. Mean ± SE nest relief interval is 5.3 h ± 0.6. Before chicks hatch, female occupies territory (67% of observation time) more than male (59%; t = 51.1, n = 19, p < 0.05), and female incubates (57%) more than male (43%; t = 32.0, n = 19, p < 0.005); no difference between sexes in frequency of egg-shifting or bouts of nest-substrate collecting. Changeover occurs when mate returns, pair exchange Long Calls, and returning mate approaches nest; may give Mew Call or Head-Tossing display. Incubating bird either gets off nest, or gives Choking display, indicating tendency to remain. If incubating bird gets off, arriving bird walks onto nest cup, lowers breast into cup, paddles feet to adjust eggs and feathers. Once settled, preens, adjusts eggs with beak, and settles to rest or sleep. If bird on nest gives Choking display and remains, mate may leave, pick up nesting material, and return giving Mew Call. If this induces partner to leave nest, arriving bird settles on nest as described above, placing nesting material around nest. Time away from territory increases after chicks hatch, presumably as foraging demands grow (Butler and Janes-Butler 1983); time period that chicks are left unattended decreases with nest density and intraspecific intrusions (Butler and Janes-Butler 1982).

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

Eggs can be left unattended for several hours without affecting hatchability; eggs that are left unattended may show hatching delay (TPG). Nests can cool by 2°–2.5°C during 10-min absence and by 3.3°C in 60 min with no embryo death (Belopol’skii 1961). Eggs lost to predation are generally lost during periods of inattention. No data on egg temperature and shift between adult and chick in contribution to egg temperature.

Preliminary Events And Vocalizations

Five to 6 d before hatching, fine web of cracks appears at a point on shell adjacent to widest egg circumference. After 1–2 d, small pipping hole appears in spot, through which tip of chick’s beak with egg tooth is visible. Once hole exists, chick produces peeping sounds (TPG). Adults respond with Mew Calls. Adults known to increase proportion of fish in own diet upon hatching of chicks (Belopol’skii 1961, TPG).

Shell-Breaking And Emergence

Each egg takes about 2 d from appearance of pipping hole to chick emergence. Shell breaks just above greatest diameter; chicks push out using feet.

Parental Assistance And Disposal Of Eggshells

Parents give no physical assistance; watch over hatching chicks, giving Mew Calls at intervals. Some adults remove shells from nest; others allow shell to be crushed into nest-lining (TPG).

Condition At Hatching

Semiprecocial with open eyes; covered with thick gray down marked with black spots over entire body; stay in nest until able to walk (nidifugous). Weigh 80–90 g; hatching weight is 5% of adult weight (Belopol’skii 1961). On Appledore I., ME, first- and second-hatched chicks averaged 85 g on day of hatching (Lewis 1982). Beak black, except for pinkish egg tooth (retained for 2–3 d after hatching); legs brownish black to dark gray (Bent 1921, Dwight 1925). Remain in nest for several hours while drying; may remain in nest for 1–2 d (Bent 1921). At 2–3 d of age, chicks move about nest vicinity; at 1 wk, can run about freely, but always return to nest. Live mostly off yolk reserves for first few days, although fed by parents within hours of hatching.

Growth And Development

Chicks show linear growth phase between 6 and 30 d of age; longer developmental period than that of Herring Gull (Lock 1973). On Skomer I., Wales, mean weight (range) of 11 chicks on day 5, 150 g (85–175); on day 10, 275 g (100–300); on day 15, 400 g (150–600); on day 20, 600 g (200–1,000); on day 25, 800 g (300–1,300); on day 30, 850 g (400–1,400); on day 35, 1,050 g (550–1,500); on day 40, 1,200 g (850–1,500; Harris 1964a). Weight of chicks (2 d old) at 35 nests on Appledore I., ME, averaged 383 g ± 13 SE (Lewis 1982). Wild chicks averaging weight gain of <40 g/d are unlikely to survive; chicks with no siblings at 20 d averaged weight gain of 50 g/d, while those with siblings at 20 d averaged 45 g/d (Lock 1973).

Flight-feathers emerge between days 15 and 20; primaries and accompanying coverts emerge first, followed by coverts on back and belly; rectrices emerge on days 25–30. All feathers fully emerged by fledging, day 45–55.

Young chicks are brooded on cold or rainy days through first week. Chicks capable of thermoregulation within 24–48 h of hatching (Belopol’skii 1961); tolerate lower ambient temperatures during first 5 d than do Herring Gull chicks (McGill-Harelstad and Taigen 1981). Chicks ambulatory and leave nest within 24 h; start stretching wings at 15 d, beating wings at about 30 d, jumping up and down and beating wings between 30 and 40 d; first become airborne at about 40–45 d; capable of flight by 45–55 d (Pierotti 1979). Chick mortality higher before 10 d old than after 10 d old (Lock 1973). Chicks 40 d and older actively defend territory against intruding chicks and adults (Butler and Janes-Butler 1982). When held on back for 15 s, chicks lie still and hold legs out stiffly (tonic immobility); older and heavier chicks show higher mean time of immobility and higher percent of individuals with the maximum response (600 s) (Montevecchi 1978).

Brooding

Both parents brood young; female spends more total time brooding than male does (Butler and Janes-Butler 1983). Brooding behavior begins with hatching of first egg and continues until chicks are 7–10 d old. Chicks >5 d old are brooded only during inclement weather. In successful pairs, 1 adult is present and attending chicks until they are >30 d of age.

Feeding

Both parents feed chicks. On Duck I., ME, no sexual differences in types of food items fed to chicks; mean ±SE feeding frequency of chicks by male (0.50/h ± 0.06) similar to that of female (0.45/h ± 0.06), and mean ± SE frequency of reconsumption of food offered to chicks is similar in male (mean 0.12/h ± 0.04) and female (0.13/h ± 0.03; Butler and Janes-Butler 1983). Adults leave territory to forage, return with food in proventriculus. When parent returns with food, either chicks rush up and beg, or adult gives Mew Calls, attracting chicks (Cramp and Simmons 1983). Young chicks (<10 d old) peck at red spot near gonys on adult; may stimulate regurgitation by adult, but many adults regurgitate before chicks peck. Older chicks (>10 d old) do not peck at red spot, but give Begging Call while oriented at base of adult’s mouth (TPG). If parent regurgitates, chicks grab food, often pulling it from parent’s mouth. Parent may hold food in bill for young chicks (<10 d old); regurgitate bolus onto ground for larger chicks. Competition appears to be scramble; no obvious aggression among siblings. Larger (earlier-hatched) chicks usually win scrambles. Chicks pick up pieces or entire prey items and swallow them whole.

Young chicks (<10 d old) are fed small prey items (small fishes, euphausiid, shrimp, copepods, insects, earthworms), or well-digested prey that breaks into pieces small enough to be handled (e.g., fish). Small chicks (<10 d old) cannot handle entire large fish or invertebrates (mussels, crabs, squid, urchins) or human refuse. In Newfoundland, chicks are fed capelin until 2–3 wk old, squid when >2 wk old; fed refuse less than are Herring Gull chicks (Pierotti 1979). Chicks are fed 1.8 times/observation day (50% less than Herring Gulls) in poor food year and 1.4 times/day (33% less than Herring Gulls) in good food year; mean foraging bouts were 85 min for capelin and 103 min for squid (Pierotti 1979). On Little Duck I., ME, 47% of food items in chick feedings were fish (alewife, herring, lumpfish [Cyclopterus lumpus], mackerel, sculpin Myoxocephalus sp.), and pelagic invertebrates (krill [Euphausiacea] and squid [Loligo borealis]; Butler and Janes-Butler 1983). Feeding of refuse to small chicks was correlated with low fledging success on Appledore I., ME (TPG). No data on amount of food provided per feeding.

Nest Sanitation

Chicks (and adults) defecate on territory but away from nest. Adults also defecate while flying. Young can become infested with fleas (Siphonaptera) and ticks (Ixodes uriae; Roy et al. 1986) and chewing lice (Mallophaga; Threlfall 1968b).

Carrying Of Young

Does not occur.

Extremely rare. Chicks observed gathering in communal groups (creching) on Great I., Newfoundland (Pierotti 1979).

Intraspecific nest parasitism and dumping not observed. Chick adoption uncommon; communal feeding and guarding observed on Great I., Newfoundland (Pierotti and Murphy 1987). May lay eggs in nests of Herring Gulls. Three mixed clutches owing to usurpation of Herring Gull nest and territory on Walney I., England; no chicks hatched (Verbeek 1979). Within the species, chicks may be cared for when they wander into territories of neighboring pairs (Pierotti and Murphy 1987). Experimental cross-fostering between Great Black-backed and Herring gulls found no treatment or species effect in survival to 5, 6, or 7 wk (Firth 1974).

Departure From Nest

Chicks leave nest within 24 h; remain on nesting territory and around nest about 40 d. Within 10 d, leave territory and take to channels and bays (5–20 m away) when disturbed (Burger 1983). First fly at 45 d; leave nesting territory at 50–55 d (Burger 1980). Chicks fully grown, fully feathered, and at or above adult mass at fledging (Pierotti 1979). Return to nesting territory to rest and be fed until 10–11 wk old (R. Pierotti pers. comm.).

Growth

No data. Young probably lose mass while learning to forage for themselves. Wings and tail may continue to grow after fledging.

Association With Parents Or Other Young

Some chicks up to 74 d old roost on territory at night with adults, and are fed by adults on territory during day (Butler and Janes-Butler 1982). Postfledging care lasts about 40 d (Burger 1980); some chicks associate with solitary pairs for months. At Appledore I., ME, newly fledged chicks feed on Herring Gull fledglings drowned by adults and dragged to shore (TPG). Newly fledged chicks gather in groups around colony, also concentrate on areas where food is predictably obtained—e.g., in rocky intertidal zones, near fishing activities, at refuse dumps. In winter, large numbers of juveniles associate with feeding humpback whales (Megaptera novaeangliae) in Gulf of Maine, off s. Nova Scotia, in fall and winter; feed on fish driven to surface by these foraging whales (Pierotti 1988).

Details unknown from period of independence to first breeding attempts. Disperse from breeding site; range farther south than adults (Southern 1980). In winter in Maine and New Hampshire, forage at landfills (Wells 1994), at fishing docks (TPG), and along rocky shores (Good 1992b).