Breeding

Phenology varies yearly, seasonally between colonies, and considerably throughout range of this species; considerable variation also between experienced breeders, inexperienced breeders, and birds pairing for first time during current breeding season (unpaired birds that successfully establish a territory early enough in season may breed late in season, or simply hold territory for current season without breeding); paired birds returning to their previously occupied nest site begin breeding activities shortly after arrival.

Pair Formation

Pair formation occurs at colony after male establishes a territory (see Behavior: sexual behavior, above); usually bond monogamously for life; pair bond continuously re-established. In w. North Atlantic, breeders arrive at colony in Mar (Palmer 1976, Montevecchi et al. 1980, J. Chardine pers. comm.).

Nest-Building

Pair reoccupies same nest site from year to year; begin to add new materials to nest shortly after arrival; nest-building closely linked with copulation, that also begins with arrival at site; continue to add material to nest through incubation (Nelson 1978a).

First/Only Brood Per Season

Figure 4 . Single clutch of 1 egg; replacement laid if egg lost (may lay up to 3 replacement eggs, as late as day 26 of incubation); mean interval between loss and re-laying 19 d (range 6–32, n = 28); experienced breeders more likely to re-lay (76.4%) than inexperienced breeders (12.5% [n = 50]; Cramp and Simmons 1977). On Bonaventure I., Québec, first eggs laid by last week of Apr, peak early to mid-May, last eggs laid by mid-Jun (Palmer 1976, Chapdelaine 1996). First eggs hatch early Jun, with peak mid-Jun to early Jul; young brooded and cared for by both parents for 13 wk, depart nest site by second week in Sep, with some lingering until Oct (Palmer 1976, Montevecchi et al. 1980, Chapdelaine 1996).

Selection Process

Nests in dense colonies, on cliff ledges, steep slopes, and occasionally flatter ground; male selects nest site, preferring locations as close to other nesters as possible near area where it was reared; after achieving ownership of site, attempts to attract a female (see Behavior: spacing, above).

Microhabitat

Socially acceptable sites as close to other nesters as possible; nests evenly spaced and nearly touching; of 408 nests on Bonaventure I., 72% had centers 60–80 cm apart, with maximum density of 2.3 nests/m²; nests reused in successive years (Poulin 1968, Cramp and Simmons 1977).

Site Characteristics

Prefers nest site on long, fairly broad, flat ledges of cliffs; will use narrow ledges (15 cm wide), sloping ledges and faces, and even knobs that can support only 1 pair; prefer sites facing into prevailing wind, immediately above splash-zone (10 to >200 m); normally on bedrock or shallow soil (Cramp and Simmons 1977, Nelson 1978a).

Construction Process

Typically collects nest material communally from nesting island, adjacent islands, or sea; male mainly responsible for gathering nest material. Before egg laid, female rarely collects any material; afterward both collect, but mainly the male. Male flies in, calling, with nest materials; either surrenders it to grasping female or deposits it and helps build it into nest structure. During peak of building, or rebuilding, male may bring material every few minutes for 2–3 h; seasonal peak of construction coincides with copulation (main laying period); minor peaks associated with late laying and replacement laying, and during periods of extremely wet weather.

Nest-building done by simple side-to-side movements of head, pushing materials into place with bill. Mud, grass, and seaweed carefully drawn up sides of nest with bill; materials cemented together, and to the substrate, with excreta; cup formed by turning and pressing with chest and belly. Building continues through incubation, with removal of small stones and rough items from floor of nest cup, and addition of fine materials to nest rim (Cramp and Simmons 1977, Nelson 1978a).

Structure And Composition Matter

Large, compacted pile of mud, seaweed, grass, flotsam, and feathers cemented together with excreta. Common building materials include seaweeds (Fucus spp.), grasses (Poaceae), molted feathers, straw, and synthetic fibers. Unusual items found in nests, suggesting gannets are beachcombers, include: netting, rope, fishing line, plastic wrapping, lobster-pot tags, shotgun-shell casings, a plastic frog, false teeth, a catheter, a gold watch, a fountain pen, and golf balls (Nelson 1978a, Montevecchi 1991).

Dimensions

Nest begins as a simple ring of material surrounding a scrape in ground, and eventually built up into a large pedestal or drum; new cup added annually; average nest in established colonies about 30 cm in diameter across top and 20 cm high, but pedestals of 50–100 cm in height common (Cramp and Simmons 1977, Nelson 1978a).

Microclimate

Just before egg laid and during incubation, nest materials (especially feathers) carefully tucked around breast and flanks to seal gap between bird and nest rim and prevent heat loss from nest cup. Nests on cliff ledges exposed to almost continuous winds (Nelson 1978a).

Maintenance Or Reuse Of Nests

Nests reused in successive years by same birds; materials added to nest throughout nesting period (Cramp and Simmons 1977).

Shape

Elongated, subelliptical to oval (Nelson 1978a).

Size

Ten eggs from Baccalieu I., Newfoundland, had an average length of 80.41 mm ± 3.90 SD and width of 49.59 mm ± 1.27 SD (Ricklefs and Montevecchi 1979); average length and breadth of a set of eggs from Bonaventure I. 82.3 × 49.7 mm (n = 20; Poulin 1968); average of 100 eggs from various colonies in Scotland 78.1 × 49.1 mm (Nelson 1978a).

Mass

Relatively small; eggs from older females tend to be heavier; average mass of 10 eggs from Baccalieu I. 114.1 g ± 9.37 SD, 3.6% of adult female body mass (Ricklefs and Montevecchi 1979); average mass of 57 eggs from Bonaventure I. 103.2 g, 3.2% of female’s mass (Poulin 1968); and, from Bass Rock, e. Scotland, 104.6 g (range 81.0–130.0, n = 393 eggs), 3.4% of adult female mass (Nelson 1978a).

Color

Pale blue or greenish; translucent when newly laid, becoming white and heavily stained to brown or even black (Cramp and Simmons 1977, Nelson 1978a).

Surface Texture

Initially smooth; eventually covered with a thick, chalky outer layer that soon chips and roughens (Nelson 1978a).

Eggshell Thickness

Average shell thickness of 10 eggs from Baccalieu I. in 1977—0.55 mm ± 0.06 SD (Ricklefs and Montevecchi 1979). Eggshell thickness decreases significantly with increasing DDE levels in egg residues; eggs from coastal England, 1973–1974, had significantly thinner shells (0.46 mm ± 0.01 SE; n = 10; p < 0.05) when contaminated with levels of DDE >30 ppm, than eggs with DDE levels <30 ppm (0.52 mm ± 0.01 SE [n = 10]; Cooke 1979); shell thickness of eggs collected at Ailsa Craig and Bass Rock, Scotland, inversely correlated (r = ≤0.807; p < 0.001) with DDE concentration in egg contents (increasing DDE concentration tenfold causes a 20.2% decrease in eggshell thickness; Parslow and Jefferies 1977).

Clutch Size

One egg (see Cooperative breeding, below).

Egg-Laying

Eggs laid anytime of day, and possibly at night; readily replaces eggs if lost before 25 d from laying; occasionally replaces lost second eggs (Poulin 1968). On Bonaventure I., 27.3% of lost eggs replaced; in 52.8% of replacements, first egg had survived <4 d, in 75.5% <7 d, in 87.4% <14 d, and in 95.0% <28 d (Poulin 1968). On Bass Rock, interval between loss and replacement 6–32 d, with most being replaced within 18 d; experienced females more likely to re-lay (26 of 34) than birds breeding for first time (2 of 16; Nelson 1978a).

On Bonaventure I., Poulin (1968) examined 1,112 nests, and recorded no 2-egg clutches; in Scotland, only 2-egg nests recorded by Nelson (1978a) were those suspected of involving 2 females.

Onset Of Broodiness And Incubation In Relation To Laying

Incubation by both sexes; experienced breeders begin incubating immediately, first-time breeders may be inattentive briefly after egg laid; male generally takes first incubation spell after laying (Cramp and Simmons 1977); incubate in turn until egg hatches; incubation stints shorten a few days before hatch (Nelson 1978a).

Incubation Patch

No brood patches; egg incubated beneath highly vascularized webs of feet. Egg oriented in nest parallel to long axis of bird, which places its overlapping webs over egg and settles into nest cup; may transfer egg to top of webs toward end of incubation, and may cover partly exposed egg in very warm weather (Cramp and Simmons 1977, Nelson 1978a).

Incubation Period

Average 43.9 d on Bonaventure I. (Poulin 1968); 43.6 d (range 42–46, n = 82) on Bass Rock (Nelson 1978a).

Parental Behavior

Incubation by both sexes; at Cape St. Mary’s, Newfoundland, females incubate significantly more (74%) than males (26%; Montevecchi and Porter 1980); on Bass Rock, both sexes share incubation duties equally, though male shifts slightly longer; during first half of incubation period, average male shift 37.2 h (range 7–84), female 30.8 h (range 4–70); during second half of incubation period, these shorten to 33.2 h (range 6–62) and 29.1 h (range 10–46) for male and female, respectively; length of incubation and site-attendant spells partly dictated by duration of foraging trips; incubating gannets spend more time sleeping than site-attending birds (Nelson 1978a).

Incubation changeover an elaborate event. Upon returning to nest site, reunited pair goes through ritual of Mutual Fencing and Nape-biting by male, and in case of returning males, can involve presentation of nest material; when changeover occurs and relieved gannet prepares to depart site, it goes through elaborate Skypointing display (stretches its neck, points its bill vertically skyward, lifts wings and raises their tips but doesn’t spread them out sideways, while lifting its feet in an exaggerated manner and uttering a peculiar ooh-ah on a descending pitch just as it departs cliff; see Nelson 1978a).

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

No specific information on hardiness against temperature stress, but often incubates eggs through periods of snow and extremely cold weather; eggs seldom unattended (Nelson 1978a).

Preliminary Events And Vocalizations

Incubation significantly shortened during last days of incubation, probably cued by calls from chick within egg; normally adult transfers egg to top of webs once it has pipped; first-time breeders often fail to switch egg, crushing it in nest (Nelson 1978a).

Shell-Breaking And Emergence

No information available on emergence of chick from egg. High proportion of nests with hatching chicks have both mates present (Nelson 1978a).

Parental Assistance And Disposal Of Eggshells

Eggshells left in nest up to 4 d after hatching, then placed on nest rim, dropped over edge, or trampled underfoot by adults (Cramp and Simmons 1977, Nelson 1978a).

Condition At Hatching

Altricial and nidicolous, with almost no down, and little motor coordination; skin loose and blackish, especially in gular region; length about 11 cm; little thermoregulatory capacity (Nelson 1978a, Montevecchi and Porter 1980). Average body mass of 12 chicks from Great Bird Rock, Magdalen Is., Québec, and Baccalieu I., Newfoundland, 79.3 g ± 11.2 SD at hatching (Kirkham and Montevecchi 1982), 70% of fresh egg mass (114.1 g ± 9.4 SD [n = 32]; Ricklefs and Montevecchi 1979), and 2.5% of adult mass (3,236.4 g ± 41.9 SD [n = 40]; Montevecchi et al. 1984); on Bonaventure I., 81.0 g ± 8.0 SD (n = 26; Poulin 1968).

Hatchlings have exceptionally rapid growth; young chicks increase in mass >10-fold in 3 wk; by 7 wk, chick as heavy as adult, and by 10 wk, weighs >4 kg, 25–50% more than adult; chick experiences a slight mass recession before fledging (Kirkham and Montevecchi 1982, Montevecchi et al. 1984). On Ailsa Craig, mean mass of young increased from 193.0 g ± 33.3 SE (n = 3) 4–6 d after hatching to 4,150.0 g ± 50.0 SE (n = 2) 61–63 d after; mass increase linear from day 4 to 56 (r = 0.91, n = 93, p < 0.001); young reached mean adult mass (3,090.0 g ± 25.0 SE, n = 118) by day 40; rate of mass increase decreases after day 56, when young averaged >3,800 g (Wanless 1984).

Growth Of Body Parts

Growth of tarsi, culmen, midtoes, and wings of hatchlings representative of altricial chicks; tarsi and midtoes relatively well developed at hatching; by 3 wk, tarsi fully grown, and by 5 wk, feet near adult proportions; culmen of hatchling short relative to adult’s, but still reaches adult length about same time as midtoe. Wing takes longest to develop to adult dimensions, not reaching full length until slightly after fledging age (Montevecchi and Porter 1980, Kirkham and Montevecchi 1982).

Northern Gannet chicks exhibit well-developed thermolytic behavior (panting, gular-fluttering, posturing, etc.) by day 2; early onset of heat tolerance suggests that heat stress is a greater threat to nestling survival than cold, and that parental behavior can cope more effectively with cold than heat (Montevecchi and Vaughan 1989). Nestlings brooded for 2–3 wk after hatching, during which they are maintained at temperatures >30°C; mechanisms of heat conservation and thermogenesis develop gradually during this period, with a marked transition during week 3 and thermal independence by day 19–24 (see Food habits: metabolism and temperature regulation, above; Kirkham and Montevecchi 1982).

See Appearance: molts and plumages (hatchlings), below, for description of hatchling feather coat and Prejuvenal molt.

Brooding

Hatchling brooded, preened, and fed by both parents; throughout nestling period, chicks continuously attended by at least 1 parent; brooded continuously for first 10–14 d, on top of or between parent’s webbed feet. Both parents present at nest site an average of 8% of the time during first 2 mo, and up to 20% toward end of nestling period; pairs present at 12.5% (58 of 465) of nests checked at dusk (Nelson 1978a, Montevecchi and Porter 1980). At colonies in Québec and Newfoundland, diurnal chick attendance shifts by parents average 217.1 min ± 9.7 SE (n = 451) during week 1–8 of the nestling cycle; shifts decrease gradually from 432.9 min ± 53.1 SE (n = 13) in first week to 139.6 min ± 21.1 SE (n = 73) in week 7; parental shifts by females during most periods significantly longer than those of males. Male makes more visits to nest site than female does, with most visits not resulting in changeovers, but presentation of nest material to female; changeover rates relatively constant through day (n = 10 all-day watches); gannets do not enter or leave nesting area after dark and before first light (Montevecchi and Porter 1980).

Feeding

Nestlings fed by partial regurgitation, putting head into parent’s bill; begging behavior changes markedly as motor coordination develops; by 1 wk of age, chicks can orient begging response to parent’s bill; by 1 mo, begging relatively evenly distributed in time with reference to parent arrivals, and by 2 mo, chicks initiate begging soon after parent arrives (Cramp and Simmons 1977, Montevecchi and Porter 1980). Begging does not always result in feeding, and entries into parent’s bill do not always result in food transfer; at Great Bird Rock, Magdalen I., Québec, food transfer occurred in 84% (660 of 784) of instances in which chicks (1–7 wk of age) inserted their heads into parent’s bill. Parental feeding frequency decreases over nestling period and drops markedly just before fledging, while ratio of begging bouts to parental feeds increases. Average size of parental regurgitation smaller in Jul during neonatal period (216.5 g ± 30.0 SE), compared to Aug (359.6 g ± 24.7 SE). Males feed neonatal chicks more than females do, whereas females feed older chicks more than males do (Montevecchi and Porter 1980).

Nest Sanitation

Adults do not defecate into nest; chicks tend to void clear of nest for first 2–3 wk after hatching, during which nest remains fairly clean; after that, it becomes increasingly fouled and more flattened (Nelson 1978a).

Carrying Of Young

Parents do not carry young, but sometimes move chick in nest by placing mandibles over it and nudging it along (Nelson 1978a).

Not known to occur. In rare instances, 2 eggs in a nest, with 1 of the 2 most likely laid by a second female. However, in a twinning experiment on Bass Rock, where an extra egg or chick was placed in a nest, Nelson (1964) found that gannets were capable of successfully raising 2 chicks. If twins were same age and together from hatching, they lived together amicably; if not, the larger tended to dominate the smaller. By the time twins reached maximum body mass, they weighed almost as much as single young of same age, but took 4 d longer to fledge (Nelson 1964). In a similar experiment on Ailsa Craig, Wanless (1984) recorded similar results: Single young grew at same rate at both colonies, twins survived less well, stayed longer in nest, and had a similar pattern of growth to single young, although their growth rate was slightly retarded; no significant differences recorded in bill and wing lengths of twins and single young. Survival from day 1–3 until going to sea for undisturbed young was lower for twins than singles (0.60 and 0.92 respectively); however, pairs with twins still reared more young (1.2 young/pair) than normal pairs (0.92 chicks/pair); no indication of survival once leaving nest (Nelson 1964, Wanless 1984).

Not known to be affected by brood parasites.

Departure From Nest

Fledging period about 90 d; no significant difference in time taken to fledge at different gannetries. On Bonaventure I., chicks fledge at 90.6 d of age (range 82–99, n = 214); at Bass Rock, 89.9 d (range 84–97, n = 111); chicks of first-time breeders 90.1 d (range 83–96) versus experienced breeders 89.9 d (range 83–96); chicks reared in cliff-edge nests 89.9 d (range 84–97) versus inland nests on slopes or plateaus 88.9 d (range 83–96; Poulin 1968, Nelson 1978a).

Fledging involves irrevocable departure from nest site; once fledged, juvenile completely independent. Fledging occurs when a young gannet moves from nest to cliff edge (young from inland nests may make 2 or 3 abortive attempts before making it through nesting ranks to reach cliff edge), where it assumes long-necked, forward leaning posture, flaps its wings a few times, and jumps off ledge. Once in air, chick flies somewhat erratically for 400–800 m before landing in sea (no published records of juveniles flying >3–5 km; Okill and Wanless 1986), feet or belly down, with a great splash, and immediately begins to bathe excitedly. At first, young gannet cannot raise itself from sea surface because it may have too much fat and its wings may not be fully grown. Or, it might be that juvenile is capable of flight on biometric criteria, but some other factor, such as muscle development, may be limiting. Evidence suggests that juveniles simply do not attempt to fly away from colony but disperse by swimming (Nelson 1978a, Okill and Wanless 1986).

Little known about development of juveniles once they are at sea. One young recaptured, still unable to fly, 3 d after it left its nest; had lost 500 g (12% of its leaving weight) and its wing length had increased 34 mm (9%). If these changes had continued at same rate, young would have reached mean adult body mass and wing length 7 d after leaving nest (Wanless 1984). Most have fat reserves sufficient to last for 1–2 wk; assumed that sometime during this period young begin to fly and learn to fish (Nelson 1978a).

First-year birds migrate southward to subtropical waters, where they spend their first winter; some number of juveniles may remain away from breeding colonies until they are 3 yr of age; upon returning to breeding areas, immatures spend several years in colonies as nonbreeders before gaining a nest site and breeding at 5 or 6 yr of age, or rarely 4 (Cramp and Simmons 1977, Nelson 1978a).