Morus bassanus

Geographic Range

Northern gannets are found in the cold, temperate waters of the northern Atlantic over the continental shelves. They are found as far north as the arctic and as far south as subtropical east and west Atlantic coasts. They are typically concentrated within 500 km of breeding colonies during the summer and are more widely dispersed in the winter, occurring as far south as the Gulf of Mexico or rarely into the Caribbean in their western range and as far south as northwestern Africa and the Cape Verde Islands in their eastern range. Although some trans-Atlantic movements have been recorded, there seems to be no substantial exchange of individuals between the eastern and western Atlantic.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• atlantic ocean atlantic ocean
  • native native

Habitat

Northern gannets are found in coastal, marine waters on the west and eastern coasts of the north Atlantic. They are found on the ocean at all times of the year, except when they come to land to breed in the summer. Breeding colonies are densely populated and found on sea stacks, steep cliffs, or uninhabited islands. Nests on cliffs and ledges are from just above the high water splash zone to over 200 meters. The suitability of breeding areas is determined by their proximity to good foraging, absence of terrestrial (mammalian) predators, and the presence of good updrafts for taking off and landing.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial
• saltwater or marine saltwater or marine

• Aquatic Biomes
• coastal coastal

Physical Description

Northern gannets are the largest seabirds in the northern Atlantic. Males are slightly larger than females, although they are similar in plumage. Males are from 93 to 110 cm in length, females from 92.5 to 104 cm. Mass is from 2470 to 3610 grams, with females averaging heavier than males (female average: 3067, male average: 2932). Wing length is from 484 to 535 mm. Adults are white with black-tipped wings and a yellowish crown and nape. The bill is pale blue with black nasal grooves and a black, serrated mandible. The feet and legs are gray-black with a greenish line running down the front of the leg and onto the toes. The line is yellow-green in males and bluish-green in females. The feet have well-developed webbing. The skin of the face is blue-gray and the eye has a bright blue orbital ring with a pale blue-gray iris. The bill is stout and straight and the maxilla overhangs the mandible slightly. The tail is long and tapers to a point.

Juveniles are gray, mixed with white feathers. There is a V-shaped white patch on the rump. The bill, legs, and feet are black and they have bluish grey eyes. With each successive molt more white plumage develops, starting with the lower body and belly and moving progressively upwards to include the head, neck, and breast. Molts are erratic and there is no discernible pattern. Young at fledging weigh substantially more than adults, sometimes more than 4 kg. This weight is lost within 7 to 10 days of fledging, at which point they acheive an adult weight. Development of adult plumage takes 3 to 4 years.

In the northern part of their range northern gannets are unlikely to be confused with other seabirds, although they may be mistaken for shearwaters at a distance. In the southern part of their range they may be confused with masked boobies ( Sula dactylatra ) or other gannets ( Morus capensis ). There are no described subspecies of Morus bassanus .

Basal metabolic rate of northern gannets at a breeding colony was estimated at 0.231 kJ/g/d ± 0.035 SE. Basal metabolic rates are considered high, relative to those of other seabirds, because of the high cost of flapping flight at sea and the high cost of thermoregulation in their cold water environment.

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Reproduction

Northern gannets form monogamous bonds for life. At maturity, males attempt to secure a breeding territory and then attract a mate. Males initially compete for breeding spots, but after mating both males and females aggressively defend and fight for breeding sites. Fighting for good breeding spots - ones that are near or in a breeding colony - can be fierce and sometime result in death. Physical interactions are accompanied by calling and displays, fights generally involve locking bills and pushing. Gannets can even push themselves off of cliffs, where fights continue in the air. Jabbing with the bill is used to keep neighbors away from a nest site once it is established. Males attract females with a "headshake-and-reach," in which they shake their head and dip their bill towards the nest. The first few weeks of a new pair bond are tenuous and females may desert the male for another. After forming a longer term pair bond, however, mates are paired for life. The pair bond is reinforced with headshakes, nape biting, allopreening, and "mutual fencing," in which they stand facing each other and knock their bills together by shaking their hides side to side. Once mated, pairs return to the same breeding site every year; in one study 94% of males and 88% of females returned to the same nest site the next year.

• Mating System
• monogamous monogamous

Northern gannets breed in 32 colonies in the eastern Atlantic and only 6 colonies in North America. Breeding colonies are large and densely populated, found on rocky cliffs, islands, and stacks. Nests are re-occupied by pairs each year. They add nest materials to the nest after arriving at the breeding colony. Females lay a single egg from the end of April through mid June. Females may lay up to 3 replacement eggs if they are lost, even after up to 26 days of incubation. Eggs are about 105 grams and are pale blue-green that becomes thick with a chalky outer layer as incubation progresses. Hatching occurs from early June to early July, with a peak in mid-June. Young are then brooded for about 13 weeks until fledging, in September. Hatchling growth is rapid, going from about 79.3 grams at hatching to over 4 kg at 10 weeks old, at which point they weigh more than adults.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

Young are altricial at hatching and eggs and young are continuously incubated or brooded on the vascularized webbing of their parent's feet. Adults do not develop a brood patch. Young are naked with a thin layer of creamy down. All downy plumage is lost by 11 to 12 weeks old. Both males and females incubate, brood, feed, and protect the young. Females spend more time (74%) incubating than males. Eggs and young are continually attended by a parent. When a mate comes to take over incubation, an elaborate display ensues. Similar to breeding, males and females engage in mutual fencing and nape biting as they prepare to exchange places. The parent that has been relieved from the nest then does a "skypointing" display in which they stand with the bill held vertically, spreads the wings upwards, and alternates lifting the feet, accompanied by an "ooh-ah" vocalization. Hatchlings are fed by regurgitation by their parents. Once young gannets have fledged, they disperse from the breeding colony. Fledglings glide off a cliff ledge into the sea and begin to swim south towards their wintering range. Because of their inexperience and large body mass, they are unable to fly for their first week after fledging. Once they can fly, they continue their migration to wintering grounds.

• Parental Investment
• altricial altricial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • male
    • female

Lifespan/Longevity

The oldest wild northern gannet was estimated to be 21 years old. Mortality is highest in the first year after hatching, with very high mortality rates during the period just after fledging when immature individuals cannot fly. About 65% of immature northern gannets do not survive to adulthood. Little mortality is associated with the pre-fledging period because breeding colonies are found in areas with few predators and because gannet parents incubate and brood their young continuously in their cold-weather habitats. Yearly mortality rates of adults are estimated to be less than 6%.

Behavior

Northern gannets are gregarious birds both on breeding colonies and at sea. They nest in dense colonies, often with several nests occupying 2-3 square meters. They may forage singly, in small groups, or in flocks of over 1000. Northern gannets are active during the day, foraging mainly in the morning and late afternoon.

Northern gannets can fly over 60 km per hour. They have short legs and walk in an ungainly manner, waddling slightly. They have to run and hop to get up enough speed to attain flight. They use flapping flight which is described as strong and direct. They will soar on updrafts when possible. They seem to usually fly 10 to 40 meters above the water, although they have been observed flying directly above the water and at higher altitudes during migrations. Northern gannets also swim underwater well, using both their feet and wings to propel themselves. To protect themselves from exposure to the cold air and water of their North Atlantic habitat, they preen themselves frequently. One behavior is both a display and a preening technique. Called the "wingflap rotary headshake," in this display northern gannets vigorously flap their wings, head, neck, and tail - ruffling their body feathers and shaking off any loose feathers or soil. They also bathe in the ocean water.

Migration patterns vary with age class in northern gannets. In summer, northern gannets are found at high concentrations near breeding colonies at higher latitudes. Flocks observed away from breeding colony areas during these times are likely to be immature and non-breeding birds. Adults begin spring migration north towards breeding colonies in February, sub-adults in March, and immature birds in April. Adults arrive at breeding colonies in April to mid-May, younger birds arrive later. In Europe immature gannets may remain on the wintering grounds throughout the breeding season. In fall northern gannets begin their southward migration, as far south as Texas, Florida, and the Gulf of Mexico. Immature northern gannets in North America winter mainly in the Gulf of Mexico, whereas adults generally winter only as far south as the east coast of Florida. Concentrations of northern gannets are often found in the near-coast intermixing waters off the Outer Banks, NOrth Carolina. Individuals typically return to the nesting colony they hatched in after 2 to 3 years as a juvenile.

• Key Behaviors
• flies
• diurnal diurnal
• motile motile
• migratory migratory
• social social
• colonial colonial

Home Range

Northern gannets range widely in search of food, flying as much as 540 km, but more typically from 60 to 232 km. They nest in dense colonies, with nests spaced only as far apart as they can reach from the nest, about 2 to 3 square meters or about 80 cm apart.

Communication and Perception

Northern gannets communicate with a wide variety of calls and visual displays. Many displays seem to be associated with maintaining territorial control in their densely packed breeding colonies. Displays include several threat displays that involve stereotyped jabbing and gaping. Threats are also communicated with a bowing display that involves thrusting the head and body forward several times and then tucking the bill against the breast. Appeasement is communicated with tucking the bill against the breast or otherwise hiding the bill in both adults and nestlings.

Northern gannets are noisy birds, especially when in large groups. They use a wide array of vocalizations. Young give cheeping calls when hatching, yap in response to trespassers in their nesting area, and beg for food from parents. Adult vocalizations have been grouped into 3 types: 1) landing calls are harsh calls used when landing and in bowing, mutual fencing, and threat displays - they are described as loud, metallic, repeated "urrah"s, "rah rah" calls are alarm versions of the "urrah," which are staccato and loud, 2) hollow groans are used when taking off or after short hops or runs, 3) soft "krok krok" sounds are given when gannets are swimming at sea or in low flight over the water.

• Communication Channels
• visual visual
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

A substantial body of literature documents foraging behavior in northern gannets. Northern gannets eat mainly schooling fish found at the surface of oceans or seas, up to 15 m deep. Prey fish are from 2.5 to 30.5 cm in length. They also eat surface schools of squid. They often feed in association with predatory fish and cetaceans, such as bluefish ( Pomatomus saltatrix ), white-beaked dolphins ( Lagenorhynchus albirostris ), and Atlantic white-sided dolphins ( Lagenorhynchus acutus ). Northern gannets forage over shallow, continental shelf waters. They typically range 60 to 232 km from colonies to forage, but can range up to 540 km. Foraging expeditions are typically 7 to 14 hours long, but can last several days. Northern gannets do forage on their own, but more commonly they forage in large flocks (up to 1000) over schools of fish. Research indicates that when resources are predictable, northern gannets learn and remember feeding locations, revisiting them over a period of time. This is not observed in areas where resources are less predictable. Northern gannets generally spend about half of foraging trip duration in flight to a foraging area. Northern gannets travel at an average speed of 15 km per hour during foraging trips, although their maximum flight speed is 55 km per hour. Foraging activity is concentrated in mornings and late afternoon, with a mid-day lull and no activity at night.

Northern gannets are generalists and opportunistic in foraging, although their body size and foraging style allows them to take advantage an oil-rich source of fish prey that is abundant in the size class they take. Their size also helps them to withstand the punishing environmental conditions in the areas these fish are found. Northern gannets use mainly "plunge-diving," in which they dive from 10 to 40 meters above the water, entering at over 100 km/hr to depths of 3 to 5 meters. They can then swim to depths up to 15 meters after a dive. They use both wings and feet when they swim and can be submerged up to 30 seconds, although 5 to 7 seconds is more typical. Most dives (90%) are less than 10 meters deep. Northern gannets have also been observed feeding from the water's surface by dipping their heads into the water, diving in from the water to pursue prey, foraging in shallow water on foot, or stealing prey from other seabirds.

The composition of the diet varies substantially with region. The dominant prey species throughout most of their range are mackerel and herring ( Clupea harengus ) species. In some areas dominant prey are capelin ( Mallotus villosus ), coalfish ( Pollachius virens ), cod ( Gadus morhua ), whiting ( Merlangius merlangus ), haddock ( Melanogrammus aeglefinus ), sprat ( Sprattus sprattus ), pilchard ( Sardina pilchardus ), and garfish ( Belone bellone ), and short-finned squid ( Illex illecebrosus ). Other recorded prey species include: sandlance ( Ammodytes hexapterus ), sandeels ( Hyperoplus ), smelt ( Osmerus mordax ), menhaden ( Brevoortia ), flounder ( Pleuronectes ), long-finned squid ( Loligo pealei ), and shrimp ( Crangon ). Northern gannets also follow commercial fishing ships and consume both fish discarded from catches and fish in nets, including species of fish not normally part of their diet because they are not found at the surface. Northern gannets are most successful at taking larger fish discarded from fishing vessels. They are one of the few species that has been recorded preying on marine-phase salmon , especially Salmo salar , which can make up a significant portion of the diet in some colonies (up to 6.37%).

• Primary Diet
• carnivore carnivore
  • piscivore piscivore

• Animal Foods
• fish
• mollusks

Predation

Northern gannets suffer relatively small amounts of predation. Eggs are occasionally taken by great black-backed gulls ( Larus marinus ), herring gulls ( Larus argentatus ), common ravens ( Corvus corax ), red foxes ( Vulpes vulpes ), or short-tailed weasels ( Mustela erminea ). Nestlings may be taken by the same predators as well as bald eagles ( Haliaeetus leucocephalus ). Adult northern gannets are generally safe from predation, although fledglings and occasional adults that are on the water may be taken by a large fish, shark, or seal. Northern gannets are large and will aggressively encounter a predator that approaches the breeding area.

Ecosystem Roles

Northern gannets have been found with protozoan ( Sarcocystis ) infections in the brain linked to their definitive hosts, Virginia opossums ( Didelphis virginianus ). This has been taken to suggest that wastewater discharge into marine environments can result in infection of marine species with terrestrial-based parasites.

Northern gannets do not seem to be highly susceptible to disease epidemics, although some mortality associated with Salmonella typhimurium , Newcastle disease virus, and aspergillosis ( Aspergillus fumigatus ) has been reported. Northern gannets are parasitized by mites ( Neottialges evansi ), trematode worms ( Cryptocotyle lingua and Diplostomum spathaceum ), and diplostomes ( Bursatintinnabulus bassanus and Bursacetabulus morus ).

Northern gannets feed in association with larger, predatory fish and cetaceans, including bluefish ( Pomatomus saltatrix ), white-beaked dolphins ( Lagenorhynchus albirostris ), and Atlantic white-sided dolphins ( Lagenorhynchus acutus ).

Economic Importance for Humans: Positive

Monitoring northern gannet diet has been used as a way of estimating prey species abundance and distribution, such as populations of Atlantic salmon in eastern Canada ( Salmo salar ).

Economic Importance for Humans: Negative

Because of their large population sizes and concentration near breeding colonies, northern gannet foraging can impact fish availability. Estimates in Newfoundland suggest that the annual intake of mackerel and squid by northern gannets is greater than the total commercial take.

Conservation Status

Northern gannet populations appear to be stable and the IUCN lists them as least concern. Population estimates are approximately 530,000 individuals globally and a range extent of from 50,000 to 100,000 square kilometers. In portions of their range, northern gannet populations seem to have grown substantially - with increases by a factor of 2.4 between 1977 and 1999. Overall, colonies seem to be increasing by 3 to 3.5% yearly. Increases in North American populations may be partially the result of bans on DDT use. The Migratory Birds Convention Act of 1917 in Canada and Migratory Bird Treaty Act of 1918 in the U.S. protected breeding colonies. Previously, gannet colonies were exploited by fishermen for bait and were persecuted as competition for fish. Their habit of feeding on fish in nets leads to a fair amount of mortality through entanglement with nets and gear or through direct killing by fishermen. Northern gannets are not substantially affected by oil spills, but toxic chemicals and heavy metals, such as PCB's, mercury, and cadmium, accumulate in their tissues because of their trophic status. Populations may be limited by the availability of suitable breeding colony sites.

Other Comments

Northern gannets were previously known by the name Sula bassana and were originally described as Pelecanus bassanus by Linnaeus in 1758, from a bird recovered on Bass Rock, Scotland.

The genus Morus is known from the middle Miocene in California.

Additional Links

Encyclopedia of Life

Contributors

Tanya Dewey (author), Animal Diversity Web.

References

BirdLife International 2008, 2008. " Morus bassanus " (On-line). IUCN Redlist of Endangered Species. Accessed February 02, 2009 at http://www.iucnredlist.org/details/144611 .

Camphuysen, C., H. Heessen, C. Winter. 1995. Distant feeding and associations with cetaceans of gannets Morus bassanus from the Bass Rock. Seabird , 17: 36-43.

Garthe, S., S. Benvenuti, W. Montevecchi. 2003. Temporal patterns of foraging activities of northern gannets, Morus bassanus, in the northwest Atlantic Ocean. Canadian Journal of Zoology , 81: 453-461.

Garthe, S., O. Huppop. 1994. Distribution of ship-following seabirds and their utilization of discards in the North Sea in summer. Marine Ecology , 106: 1-9.

Hamer, K., R. Phillips, J. Hill, S. Wanless, A. Wood. 2001. Contrasting foraging strategies of gannets Morus bassanus at two North Atlantic colonies: foraging trip duration and foraging area fidelity. Marine Ecology , 224: 283-290.

Hamer, K., R. Phillips, S. Wanless, M. Harris, A. Wood. 2000. Foraging ranges, diets and feeding locations of gannets Morus bassanus in the North Sea: evidence from satellite telemetry. Marine Ecology , 200: 257-264.

Huettmann, F., A. Diamond. 2000. Seabird migration in the Canadian northwest Atlantic Ocean: moulting locations and movement patterns of immature birds. Canadian Journal of Zoology , 78: 624-627.

Montevecchi, W., D. Cairns, R. Myers. 2002. Predation on marine-phase Atlantic salmon ( Salmo salar ) by gannets ( Morus bassanus ) in the Northwest Atlantic. Canadian Journal of Fisheries and Aquatic Science , 59: 602-612.

Mowbray, T. 2002. Northern Gannet ( Morus bassanus ). Pp. 1-10 in The Birds of North America Online , Vol. 693. Ithaca: Cornell Lab of Ornithology. Accessed January 28, 2009 at http://bna.birds.cornell.edu/bna/species/693 .

Spalding, M., C. Yowell, D. Lindsay, E. Greiner, J. Dame. 2002. Sarcocystis meningoencephalitis in a northern gannet ( Morus bassanus ). Journal of Wildlife Diseases , 38: 432-437.