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Main Foods Taken
Surface-schooling fish, 2.5–30.5 cm in length; main fish species taken include mackerel and herring, although in some localities depends heavily on other species, such as capelin (Mallotus villosus) off Norway, coalfish (Pollachius virens) off Iceland, and cod (Gadus morhua) in North Sea (see Diet, below).
Microhabitat For Foraging
Relatively shallow continental-shelf waters; during breeding season, breeders forage most intensively within 60 km (Kirkham et al. 1985), but range up to 180 km from their colonies to obtain food (Montevecchi and Porter 1980, Montevecchi et al. 2002). Subadults and nonbreeders well outside this range, having been sighted as far north as 54° off the Labrador coast (Kirkham et al. 1985), and along southwest coast of Greenland (Salomonsen 1967). Range of foraging trips from breeding colonies varies with colony location, stage of breeding cycle, and distribution patterns of primary prey species (Nelson 1978a, Kirkham et al. 1985). Fishing trips usually 7–13 h, but may extend to 2–3 d (Cramp and Simmons 1977, Nelson 1978a).
Feeds on surface-schooling pelagic prey that move into inshore waters at different times through-out breeding season, in a range of water depths from surface to 15 m. In Newfoundland waters, sequence of prey availability through breeding season: herring, Apr–Jun and Aug–Oct; capelin, Jun to mid-Jul; mackerel, Jun–Oct; and squid, Jun–Oct (Montevecchi et al. 1988).
Food Capture And Consumption
Opportunistic generalist predator on pelagic prey; feeds diurnally; may hunt singly, but generally forages communally in large “frantic-flocks” of up to 1,000 over shoals of prey fish. Although a generalist predator, Northern Gannet is a feeding specialist whose unique combination of body mass, size, and shape allow it to utilize, almost solely, a feeding niche of exceptionally abundant and nutritious, large, oil-rich, shoaling fish under rigorous environmental conditions (short days, strong icy winds, sleet, snow, and rain; Nelson 1978a). Primary foraging strategy plunge-diving. In presence of shoaling fish, flies up-wind with bill pointing slightly downward and, from a height of 10–40 m, tips steeply, or gradually, into a vertical or slightly angled gravity-plunge; occasionally accelerates with power strokes. Before entry, wings extended backward, close to body, with wing-tips beyond tail, and penetrates water like an arrowhead at speeds >100 km/h. Depth of penetration from dive 3–5 m; occasionally descends to 12–15 m by swimming, using wings and feet; most submergences last 5–7 s, sometimes 10 s, and occasionally as long as 30 s (Garthe et al. 2000); generally emerges from water facing into wind. If unsuccessful, takes flight immediately and prepares for another dive; if successful, will often shuffle-bathe excitedly, dipping its bill and shaking its head, having swallowed its prey underwater. Sometimes fish is brought to surface, shaken vigorously, tossed, juggled, and swallowed, usually head-first; never flies with fish in bill (Nelson 1978a). Little known about actual way in which Northern Gannet captures prey; obviously it sees prey before plunging, tracks it on the way down, and makes adjustments as necessary; most likely captures deep prey on the way down by pursuit, or on the way up if prey near the surface; fish generally grasped in bill, but occasionally large fish speared (Nelson 1978a).
Using underwater data-logging systems attached to adult gannets at Funk I., Newfoundland, Garthe et al. (2000) distinguished 2 different dive types: relatively short, shallow, V-shaped dives and relatively long, deep, U-shaped dives. Most dives made were relatively shallow; only 10% of all dives (n = 336) were >10 m deep; mean depth of all dives 5.2 m (range <1.0–22.0 m); maximum depth of 136 V-shaped dives, 8.7 m, and of 78 U-shaped dives 22 m; U-shaped dives occur at all depths, but most numerous at depths >8.0 m. Mean dive duration of 214 dives was 10.9 s, with 21% lasting >15 s, and the longest 38 s; all dives <8 s were V-shaped and all >15 s were U-shaped; time between descending and ascending in U-shaped dives ranged from 4 to 28 s, and depth varied little during maximum depth phase. Dives throughout daylight period, with deepest dives during late morning. On basis of food collections of telemetered birds, U-shaped dives were directed at deep schools of capelin, a small pelagic fish; V-shaped dives aimed at larger, pelagic fish and squids. V-shaped dive appears advantageous in that it allows individuals to surprise their pelagic prey; this may be critical because maximum swimming speeds of prey likely exceed maximum dive speeds of gannets (Garthe et al. 2000).
In addition to plunging, gannets observed swimming with head immersed and catching fish sighted after a dive; swimming among fry and scooping them up (Nelson 1978a); fishing on foot in shallow water for sand eels (Ammodytidae; Cramp and Simmons 1977); diving from surface and pursuing fish by swimming (Nelson 1978a); robbing other birds (Tasker and Taylor 1984); consuming fish and offal while swimming (Furness et al. 1992); scavenging from fishing boats (King 1983, Martin 1989); and taking fish from fishing nets near surface (Cramp and Simmons 1977). Northern Gannets often feed in association with cetaceans (Camphuysen et al. 1995), and large predatory fish species such as bluefish (Pomatomus saltatrix; TBM), which herd fish shoals into “frenzied” concentrations near surface where they can easily be taken with rapid, shallow dives. Young fed by incomplete regurgitation; 2.7 feedings/chick/d (Cramp and Simmons 1977).
Major Food Items
Wide variety of surface schooling fish and squid; varies geographically and seasonally; no differences between sexes; differences between adults and young, if any, quite small (Kirkham et al. 1985). Diet during breeding season well established, little known about diet of birds during nonbreeding season; information on diet based largely on observations during bouts of feeding, or analysis of freshly regurgitated crop contents collected at breeding and roost sites (Nelson 1978a, Kirkham et al. 1985).
During breeding season, most important prey species in North American waters are mackerel, short-finned squid (Illex illecebrosus), capelin, and herring; other prey species include Atlantic saury (Scomberesox saurus), post-smolt Atlantic salmon (Salmo salar), sandlance (Ammodytes hexapterus), smelt (Osmerus mordax), pollack or coalfish (Pollachius virens), menhaden (Brevoortia spp.), flounder (Pleuronectes spp.), long-finned squid (Loligo pealei), and shrimp (Crangon sp.; Palmer 1976, Montevecchi and Porter 1980, Kirkham et al. 1985, Montevecchi et al. 2002). In Western Palearctic, major prey species include cod, coalfish, whiting (Merlangius merlangus), haddock (Melanogrammus aeglefinus), sprat (Sprattus sprattus), pilchard (Sardina pilchardus), and garfish (Belone bellone). Gannets also take fish from fishing nets near surface that include species normally out of diving range, including blue whiting (Micromesistius poutassou), dolphin fish (Coryphaena spp.), blue ling (Molva dipterygia), lemon sole (Microstomus kitt), dab (Limanda limanda), and long rough dab (Hippoglossoides platessoides). (For a complete list of all foods taken by Northern Gannets, see Cramp and Simmons 1977; Nelson 1978a, 2002 .)
Analysis of stomach contents of 18 adult Northern Gannets, 84% fish, 14% miscellaneous animal material, and 2% mollusks (mollusks thought to be from fish eaten; Cramp and Simmons 1977). Composition of prey collected during summer of 1978 on Bird Rock, Magdalen Is., Québec, was 52% mackerel, 45% sandlance, 1% capelin, and 2% flounder (Burton and Pilon 1980). At Bonaventure I., Québec, total mass of prey consumed by Northern Gannets late Aug 1995, 68% mackerel, 10% herring, 4% sandlance, and 18% unidentified spp. (Rail et al. 1996).
Composition of prey collected from Northern Gannets, during breeding season 1977–1980, 1982, off e. Newfoundland and Labrador: 41% mackerel, 20% capelin, 17% short-finned squid, 11% herring, 9% Atlantic saury, and 2% other. Mean mass of prey regurgitations varied by species: mackerel 324.4 g ± 123.5 SD (n = 109), herring 310.3 g ± 148.5 SD (n = 45), Atlantic saury 211.1 g ± 106.1 SD (n = 48), capelin 194.3 g ± 59.1 SD (n = 40), and squid 161.5 g ± 96.6 SD (n = 28); mean mass of mackerel taken by breeders in 1978 significantly greater than nonbreeders, 438.6 g ± 130.0 SD (n = 18) versus 276.1 g ± 92.8 SD (n = 20, p < 0.001; Kirkham et al. 1985). Of total mass of prey consumed by Northern Gannets during Jul and Aug (1977–1982), 62% mackerel, 16% herring, 11% squid, 6% capelin, and 5% Atlantic saury; between 1982 and 1986, estimates of gannet population’s average annual harvest of squid and mackerel in Newfoundland often exceeded that of total commercial landings (Montevecchi et al. 1988).
At Skarvklakken and Hovvsflesa, Norway, during breeding season 1984–1986, 57% of diet by mass herring, 41% saithe (or Atlantic pollack) and 2% sand eel; in terms of energy, herring 68%, saithe 29%, and sand eel 2% (Montevecchi and Barrett 1987). In the Shetland Is., 1981–1988, sand eel, mackerel, and herring 3 most important prey species in diet of Northern Gannet chicks, making up 78–95% of diet by mass; during the period percentage mass of sand eels in chicks’ diet declined steadily from 90% in 1981 to 6% in 1988, while proportion of herring increased steadily from 0% in 1981 to 51% in 1988; during period no variation in reproduction, suggesting Northern Gannet able to switch prey without affecting breeding success (Martin 1989). At Ailsa Craig, w. Scotland, 1975, 1976, mackerel main component of chicks’ diet, 72% and 70% by mass, respectively; contribution of other species variable, indicating ability of Northern Gannet to utilize a variety of prey species as their availability within foraging range fluctuates (Wanless 1984).
At Funk I., Newfoundland, during a period of mega-scale physical and biological change on the Newfoundland and Labrador Shelf (1980s–1990s) in which the abundance of the gannet’s warm-water prey such as mackerel, squid, and saury was greatly reduced; the amount of post-smolt Atlantic salmon increased from 0.29% to >2.5% of their total food intake, demonstrating an ability to shift their dietary intake in response to changing relative abundance of prey (Montevecchi et al. 2002).
Food Selection And Storage
Food selection varies seasonally, geographically, and by availability in habitat; positively correlated with occurrence; similar for both sexes (Nelson 1978a, Montevecchi et al. 1988, Martin 1989). Actual prey selection appears to be a compromise between preferred larger, energy-rich prey and species availability within foraging distance of breeding colonies (Montevecchi et al. 1988). Owing to seasonal fluctuations in exploitation of different prey, average size of a parental regurgitation during Jul (216.5 g ± 30.0 SD), compared to Aug (359.6 g ± 24.7 SD; n = 50, p < 0.005), when a greater proportion of mackerel are taken; size of regurgitations of breeding adults not significantly larger than immature-plumaged birds, and percentage of different foods regurgitated same for the 2 age classes (Montevecchi and Porter 1980, Kirkham et al. 1985).
Nutrition And Energetics
Basal metabolic rate of Northern Gannets at Cape St. Mary’s and Funk I., Newfoundland, with a mean mass of 3.03 kg ± 0.14 SE, average 701 kJ/d ± 131 SE (range 530–979, n = 10) or 0.231 kJ/g/d ± 0.035 SE (Birt-Friesen et al. 1989). During chick-rearing at Funk I., field metabolism of 20 free-rang-ing Northern Gannets with a mean mass of 3.21 kg ± 0.21 SE, averaged 4,865 kJ/d ± 450 SE, or 6.6 × basal metabolism; metabolic rate at rest or on water 144 kJ/h ± 25 SE, during flight 349 kJ/h ± 107 SE, and 250 kJ/h ± 52 SE during diurnal (foraging) time at sea; field metabolic rate correlated strongly with both basal metabolic rate and metabolism at the nest; metabolic rates high, relative to other seabirds, because of costs of thermoregulation and flapping flight (Birt-Friesen et al. 1989). In Norway, herring and saithe dominant parental foods during chick-rearing, 57% and 41% of total biomass, respectively. However, because of its higher fresh-energy density (5.1–9.2 kJ/g vs. 3.0–5.6 kJ/g), herring makes up 68% of total energy harvested, and thus parental food loads of herring much more energetically valuable (Montevecchi and Barrett 1987).
Over 13-wk nestling period, Montevecchi et al. (1984) determined captive chicks consume an average of 24,174 g (n = 4) of food containing 190,550 kJ; assuming 76.1% efficiency (Cooper 1978), this leaves 145,000 kJ of metabolizable energy for growth, maintenance, temperature regulation, and activity. Estimated metabolizable energy during nestling period rises rapidly from 952 kJ during week 1 to 19,318 kJ during week 6, after which it fluctuates between about 9,000 and 16,400 kJ/wk (Montevecchi and Porter 1980, Montevecchi et al. 1984). At Ailsa Craig, nestlings receive an estimated average maximum daily intake of 650 g, with an energy value between 2.2 and 5.7 kJ depending on prey: pollack or mackerel (Wanless 1984).
Metabolism And Temperature Regulation
Hatchlings altricial; continuously brooded on parent’s highly vascularized webbed feet for first 2 wk and sporadically in week 3; during this period, nestlings maintained at temperatures >30°C (Kirkham and Montevecchi 1982); mechanisms of heat conservation and thermogenesis develop gradually during this period, with a marked transition during week 3; thermal independence acquired by day 19–24. Thermal independence improves slightly during early development (weeks 1 and 2) as a result of a 25% decrease in relative surface area, and slight increases in down feathering and shivering capacity; marked improvement in thermal competence by 13- to 18-d-old nestlings associated with significant increase in shivering (metabolic) thermogenesis; thermal independence of 19- to 24-d-old nestlings result of further increases in shivering thermogenesis, especially in leg muscles which constitute a significantly larger proportion (50%) of relative adult size in nestling than do pectoral muscles (12%; Montevecchi and Vaughan 1989). By delaying homeothermy and freeing caloric allotments that might otherwise be channeled into temperature regulation, Northern Gannet makes more energy available for rapid growth (Montevecchi and Porter 1980).
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).
Drinking, Pellet-Casting, And Defecation
Obtains water from body fluids of prey, from drinking seawater, and from water derived from metabolism of fats and stomach oils. Like other marine birds, has salt glands above eyes that are connected by a duct to nasal cavity. Secretion of these glands is rich in salt and is primary method of eliminating excess salt. Kidneys also used to rid body of excess salt (Proctor and Lynch 1993, Gill 1994). No pellet-casting. Defecation often over nest edge and cliff edge and in flight just after nest departure (W. A. Montevecchi pers. comm.).