Northern gannet

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Northern gannet
Morus bassanus adu.jpg
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Suliformes
Family: Sulidae
Genus: Morus
Species: M. bassanus
Binomial name
Morus bassanus
(Linnaeus, 1758)
Basstölpel (Sula bassana) world.png
Northern gannet range
Synonyms

Sula bassana

The northern gannet (Morus bassanus) is a seabird and it is the largest member of the gannet family, Sulidae. It has the same colours as the Australasian gannet and is similar in appearance. Nesting in colonies as large as 60,000 pairs on both sides of the north Atlantic this bird undertakes seasonal migrations and is a spectacular high-speed diver.

Old names for the northern gannet include solan and solan goose.

Description[edit]

Northern gannet breeding pairs.
Northern gannet flying over the English Channel, in the 7 Islands Nature Reserve, northern France.

Adults are 81–110 cm (32–43 in) long, weigh 2.2–3.6 kg (4.9–7.9 lb) and have a 165–180 cm (65–71 in) wingspan.[2][3] Before fledging, the immature birds (at about 10 weeks of age) can weigh more than 4 kg (8.8 lb). Each wing measures between 47 and 53 cm (19 and 21 in) when outstretched and the beak measures between 9 and 11 cm (3.5 and 4.3 in) (measured from the head). The two sexes are a similar size.

The plumage of the adults is white with dark wing tips, with colours that range from brown to black. The colour of the head, cheeks and side of the neck depends on the season and the individual; during breeding, the head and neck are brushed in a delicate yellow, although this colouring may not be evident in some individuals.[4] The feathers are waterproof, which allows the birds to spend long periods in water. A water-impermeable secretion produced by a sebaceous gland covers the feathers and the birds spread it across their body using their beak or their head.[5] The eye is light blue, and it is surrounded by bare, black skin, which gives the birds their characteristic facial expression.

Young northern gannet. The front part of its body shows adult plumage.
Young birds are dark brown

Fledglings are brown with white wing tips. They have white spots on their head and on their back and a v-shaped white area underneath.[6] The plumage of one-year-olds can be almost completely brown. In the second year the birds’ appearance changes depending on the different phases of moulting: they can have adult plumage at the front and continue to be brown at the rear.[7] They gradually acquire more white in subsequent seasons until they reach maturity after five years.

Newborn chicks are featherless and are dark blue or black in colour. In the second week of life they are covered in white down.[8] From the fifth week they are covered in dark brown feathers flecked with white.[4]

Their beak is long, strong and conical with a slight downward curve at the end. The front part has a sharp edge. In adults, the beak is blue-grey with dark grey or black edges. It is brownish in immature birds.

The northern gannet’s eyes are large and point forwards, and they have a light blue to light grey iris surrounded by a thin black ring. The four toes of their feet are joined by a membrane that can vary from dark grey to dark brown. There are yellow lines running along the toes that continue along their legs; these lines probably have a role in mating.[5] The rear toe is strong and faces inwards allowing the birds to firmly grip onto vertical cliff faces.[9]

Distinguishing anatomical features[edit]

Northern gannet on Bonaventure Island, in Quebec.
Selecting a dive target
Plunge-diving with wings retracted

Northern gannets dive vertically into the sea at velocities of up to 100 km/h (62 mph) and the structure of their bodies is adapted for this practice. They do not have external nostrils and their secondary nostrils can be closed when they are in water. The opening of their auditory canal is very small and is covered with feathers; the openings can also be closed in water using a system that is similar to that used for the nostrils. The sternum is very strong and sufficiently long to provide protection for the internal organs from impacts with water.[10]

The lungs are highly developed and probably also play a role in reducing the effects of hitting water at high speeds and protect the body from these effects. There are subcutaneous air sacks in the lower body and along the sides. Other air sacks are located between the sternum and the pectoral muscles and between the ribs and the intercostal muscles. These sacks are connected to the lungs and are filled with air when the bird breathes in. The air can be expelled by muscle contractions.[10]

Individuals have a subcutaneous fat layer, dense down and tightly overlapping feathers that help them withstand low temperatures. A reduced blood flow in the webbing on their feet outside of the breeding season also helps to maintain body temperature when they swim.[11]

By air, land and sea[edit]

Silhouette in flight.

The wings of the northern gannet are long and narrow and they are positioned towards the front of the body, allowing efficient use of air currents when flying. Even in calm weather they can attain velocities of between 55 and 65 km/h (34 and 40 mph)[12] even though their flying muscles are not highly developed: in other birds flying muscles make up around 20% of total weight, while in northern gannets the flying muscles are less than 13%.[13] The consequence of this is that northern gannets need to warm up before they begin flying. They also walk with difficulty and this means that they have difficulty taking off from a flat area. They take off from water by facing into the wind and strongly beating their wings. In light winds and high waves they are sometimes unable to take off and they can become beached.[12] They take advantage of the wind produced by the front of a wave in the same way as the albatross does. They are only seen inland when they have been blown off-course by storms.

They alight on water with their feet retracted. They rarely land on water with their feet stretched forward like pelicans or cormorants. When they are on the water their body is rather low in the water with their tail pointing diagonally upwards. They alight with difficulty on land and often alight with a bump as their narrow wings do not allow them to turn easily and they have to use their feet and tail to aid in these manoeuvres. Individuals often suffer damage to their legs or feet when they land on the ground if there is not sufficient wind.[14] Damaged or broken wings are a frequent cause of death in adults.[15] The position of the legs towards the rear of the body means that they walk in a similar way to ducks.

Call[edit]

The northern gannet does not have a very characteristic acoustic repertory. Its typical call is rab-rab-rab, which is emitted when fishing and also when on the nest.[16] They have a special call when they approach the colony: this call is often heard because there is usually a lot of toing and froing in a colony.[17] Males and females make similar calls.

According to the ornithologist Bryan Nelson northern gannets can recognize the call of their breeding partner, chicks and birds in neighbouring nests. Individuals from outside this sphere are treated with more aggression.[18]

Distribution[edit]

Breeding colonies in the north Atlantic.
Panoramic view of the Seven Island Nature Reserve that supports a northern gannet colony, in Brittany (France).

Their breeding range is the North Atlantic on coasts influenced by the Gulf Stream,[19] the exception being the colonies of the Gulf of Saint Lawrence and the islands off the east coast of Canada. They normally nest in large colonies, on cliffs overlooking the ocean or on small rocky islands. The waters near to these cliffs have a summer temperature at the surface of between 10 and 15 °C (50 and 59 °F).[20] The water temperature determines the distribution of Atlantic mackerel and herring, which are the main food source for the northern gannet. For this reason there is a close relationship between the location of northern gannet breeding colonies and the distribution of these fish. Northern gannet colonies can be found in the far north in regions that are very cold and stormy. The ornithologist Bryan Nelson has suggested that they can survive in these regions due to a number of factors including: the combination of body weight and a strong beak that allows them to capture strong muscly fish and the ability to dive to great depths and capture prey far from the cliffs. In addition they are able to stand long periods without eating owing to their large fat reserves.[21]

The northern limit of their breeding area depends on the presence of waters that are free of sea ice during the breeding season. Therefore, while Greenland and Spitsbergen offer suitable breeding sites, the arctic regions have summers that are too short to allow the northern gannets to lay their eggs and raise a brood, which requires between 26 and 30 weeks.[22] The southern limit of their distribution mainly depends on the presence of sufficient prey.[23]

The species is a rare visitor in the Black Sea region.[24]

Breeding colonies[edit]

Northern gannet colony on Bonaventure Island near to Percé, Quebec, Canada.

Some breeding colonies have been recorded as being located in the same place for hundreds of years. The cliffs containing the colonies appear to be covered in snow when seen from a distance, due to the number of nests present on them. There is a written record of a colony on the island of Lundy from 1274. It noted that the population was declining due to hunting and the theft of eggs. The colony finally disappeared in 1909.[25] 68% of the world population breeds around the coasts of Great Britain and Ireland. The biggest colonies include:

Other European colonies are found in the south west of Ireland, and off the west (Runde Island) and north of Norway (Syltefjord, Hovflesa and Storstappen). The most southerly European colony is on the island of Rouzic off the French Atlantic coast. There are breeding colonies along the coast of Newfoundland and on the islands in the Gulf of Saint Lawrence. The largest colony has 32 000 nests and is on Bonaventure Island off the south coast of Quebec.[27]

Population[edit]

A 2004 survey counted 45 breeding colonies and some 361 000 nests.[30] The population is apparently growing between 3 and 5% a year, although this growth is concentrated in just a few colonies.[30] Although northern gannet populations are now stable, their numbers were once greatly reduced due to loss of habitat, removal of eggs and killing of adults for their meat and feathers. In 1939, there were 22 colonies and some 83 000 nests, which means that the populations have increased fourfold since that time.[22] This increase in numbers could also be due to northern gannets benefiting from the growing activities of deep sea fishing.

In 1992, the International Union for the Conservation of Nature (IUCN) estimated the bird’s population to be some 526 000.[31] However, taking into account an estimate produced for BirdLife International in 2004 of the European population, the IUCN revised its global population to between 950 000 and 1 200 000 individuals.[32]

Conservation status[edit]

Nests among the rocks. The population of this species appears to be increasing.

The IUCN lists northern gannets as a species of least concern, as they are widely distributed, do not fulfil many of the criteria set for vulnerable species, and as there is a large population that appears to be growing.[1]

In the United Kingdom, gannets are a protected species. However, a legal exception is made for the inhabitants of the district of Ness (also known as Nis) of the Isle of Lewis in Scotland's Outer Hebrides, who are allowed to cull up to 2,000 gannets (locally known as guga) annually to serve as a traditional local delicacy—the taste is described as fishy.[33] Many of these gannets are taken from Sula Sgeir, which is itself named after them.

Depredation[edit]

This species is not heavily predated. The only known habitual natural predators of adults are the bald and white-tailed eagles.[34]

Predators of eggs and nestlings include the great black-backed gull and American herring gull, common ravens, ermine, and red fox. Predation at sea is insignificant though large sharks and seals may rarely snatch a gannet out at sea.[34]

Diet and foraging[edit]

Northern gannets forage for food during the day, generally by diving into the sea. They search for food both near to their nesting sites but also further out to sea. Birds that are feeding young have been recorded searching for food up to 320 km (200 mi) from their nest. It has been found that 2% of birds nesting in the colony on Bass Rock search for fish at Dogger Bank, between 280 and 320 km (170 and 200 mi) away. It is likely that they fly greater distances than this while searching for food, possibly up to double this distance; however, they normally fly less than 150 km (93 mi).[35]

Diving[edit]

When feeding, these birds are spectacular high-speed divers. They can locate their prey from heights of up to 45 m (148 ft), but they normally search from a height of between 10 and 20 m (33 and 66 ft).[36] When they see a fish they will dive into the water. They dive with their bodies straight and rigid, wings tucked close to the body but reaching back, extending beyond the tail, before piercing the water like an arrow. They control the direction of the dive using their wings.[37] Just as it is going to hit the water a bird will fold its wings against its body. A bird’s head and neck are stretched out in front of the body and the beak is shut.[37] Birds can hit the water at speeds of up to 100 km/h (62 mph). This allows them to penetrate 3–5 m (10–16 ft) below the surface, and occasionally they will swim down to 12–15 m (40–50 ft).

Northern gannet searching for fish.

They usually push their prey deeper into the water and capture it as they return to the surface. When a dive is successful, gannets swallow the fish underwater before surfacing, and never fly with the fish in their bill. Larger fish are swallowed headfirst, smaller fish are swallowed sideways or tail first. The bird’s subcutaneous air bags aid their rapid return to the surface.

Their white colour helps other gannets to identify one of their kind and they can deduce the presence of a shoal of fish by this diving behaviour; this in turn facilitates group foraging, which makes capturing their prey easier.[38] Northern gannets also forage for fish while swimming with their head under water.

Some studies have found that the duration and direction of flights made while foraging for food are similar for both sexes. However, there are significant differences in the search behaviour of males and females. Female northern gannets are not only more selective than males in choosing a search area: they also make longer and deeper dives and spend more time floating on the surface than males.[39]

Diet[edit]

They eat mainly fish 2.5–30.5 cm (0.98–12.01 in) in length which shoal near the surface. Virtually any small fish (roughly 80–90% of their diet) or other small pelagic species (largely squid) will be taken opportunistically. Sardines, anchovies, haddock, smelt, Atlantic cod and other shoal-forming species are eaten.[40][41] In the case of the larger fish species northern gannets will only eat the young fish.

They will also follow fishing boats with the hope of finding food in the same was as gulls do. They fly around the boats to take fish from the fishing nets or pick up the remains thrown into the sea.[42]

Reproduction[edit]

The oldest birds are the first to return to the breeding colonies. The exact duration of the breeding season depends on the colony’s geographic location: the breeding season on Bass Rock starts in the middle of January, that of Iceland at the end of March or in April.[43] The birds that are not of breeding age arrive a few weeks later. In general, birds first return to a colony (not necessarily the one they were born in) when they are two or three years old.[44] It is not unusual for birds to change colony before they reach breeding age, but once an individual has successfully bred in a colony it will not change to another.[45]

Immature birds stay on the edges of the colony. They may even make a nest but they will not breed until they are four or five years old.[43] Some birds of this age will occupy empty nests that they will aggressively defend if they have sat on them for two or three days. If an apparently empty nest has an owner the immature bird will abandon it without putting up a struggle when the owner arrives to claim the nest.[46]

Nest construction[edit]

Northern gannet transporting material for its nest.

The preferred nesting sites are on coastal hillsides or cliffs. If these sites are not available northern gannets will nest in groups on islands or flat surfaces. As they find it more difficult to take off from these locations they will often cross the area occupied by an adjacent nest causing an aggressive reaction from the pair occupying that nest; this means that the stress levels are higher in this type of colony than in those on more vertical surfaces. Notwithstanding this, nests are always built close together and ideal nesting sites will not be used if they are some distance from a colony.[47] On average there are 2.3 nests per square metre.[48]

Nests are made from seaweed, plants, earth and all types of object that float on the sea. The males usually collect the materials. Nests measure between 50 and 70 cm (20 and 28 in) in diameter and are some 30 cm (12 in) in height; during the course of a breeding season they will sustain damage from the wind and other causes and they require frequent maintenance. The area which a nest occupies grows throughout the breeding season as the breeding pairs throw their excrement outside the nest.

Aggressive behaviour on the nest[edit]

A female will not react if a male approaches her nest, but she will defend it fiercely if another female approaches.

Northern gannets exhibit many types of aggressive behaviour while they are nesting. Confrontations normally only take place between birds of the same sex. Females will lower their heads before an aggressive male that is defending its nest: this will expose the back of the female’s neck and the male will take it in its beak and expel the female from the nest. A female will not react if a male approaches a nest but it will react fiercely if another female approaches.[49] The fights between males that occupy nests for the first time are particularly intense. Such fights can lead to serious injuries. The fights are preceded by threatening gestures, which are also seen outside the breeding season. Males will demonstrate ownership of a nest by gesturing towards their neighbours with their head with the beak pointing down and the wings slightly outstretched.[50]

Mating[edit]

«Billing», a mutual greeting gesture.[51]
Mating.

Once males have found a place to breed they try to attract an available female. The females will fly over the colony a number of times before landing. Their posture, with the neck stretched out, tells the male that they are available for courtship. The male will then shake their heads in a similar way to when they are guarding their nest but with their wings closed.

Gannet pairs are monogamous and may remain together over several seasons, if not for all their lives. The pairs separate when their chicks leave the nest but they pair up again the following year. Should one of the pair die the other bird will leave the breeding ground and pair up with another single bird.

They fiercely defend the area around their nest. Where space allows, the distance between nests is double the reach of an individual.

Eggs and chicks[edit]

Northern gannets only lay one egg that on average weighs 104,5 grams.[52] This is lighter than for other seabirds.[53] Where two eggs are found in a nest this is the result of two females laying an egg in the same nest or one of the eggs has been stolen from another nest. Northern gannets will lay another egg if the first one is lost. Incubation takes 42 to 46 days. During incubation the egg is surrounded by the brooding bird’s webbed feet that are flooded with warming blood. The process of breaking the eggshell can take up to 36 hours. When this is about to take place the brooding bird will release the egg from its webbed feet to prevent the egg from breaking under the adult's weight as the chick breaks it open. This is a frequent cause of death for chicks of birds that are breeding for the first time.[54] The webbed feet are also used to cover the chicks, which are only rarely left alone by their parents. Chicks that are left unattended are often attacked and killed by other northern gannets.[55]

Young chicks are fed regurgitated semi-digested fish by their parents. Older chicks receive whole fish. Unlike the chicks of other species, northern gannet chicks do not move about the nest or flap their wings to ask for food: this reduces the likelihood that they will fall from the nest.[56]

The adults feed their offspring for 11 or 12 weeks, until they are strong enough to leave the nest for good. A chick will glide from the nest down to the sea after 75 days, which will mark the point at which it separates permanently from its parents.[44] Young birds that weigh 4 kg (8.8 lb) at this point will not be able to fly so they are unable to return to the cliff. Their fat reserves allow them to pass two or three weeks without eating. If the young birds leave the nest in bad weather they can be mortally wounded as they can be blown against the rocks.[55]

The young birds are attacked by adults if they enter the breeding ground, so they stay at sea learning to fish and fly. A high proportion of the young birds can die if storms occur at this time.[57]

Migration[edit]

The young birds migrate southwards for great distances and have even been recorded as far south as Ecuador. In their second year a number of birds return to the colony they were born in, where they arrive after the mature birds, they will then migrate south again at the end of the breeding season. They travel shorter distances in this second migration.

After the breeding season the adults spread out over a wide area although they travel no more than 800 to 1,600 km (500 to 990 mi) from the breeding colony. It is not known if all birds from one colony migrate to the same over-wintering area. Many adults migrate to the west of the Mediterranean, passing over the Strait of Gibraltar and flying over land as little as possible. Other birds follow Africa's Atlantic coastline to arrive in the Gulf of Guinea. Immature northern gannets from colonies in Canada fly to the Gulf of Mexico while the adults do not fly that far.[35]

Gallery[edit]

References[edit]

  1. ^ a b BirdLife International (2012). "Morus bassanus". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013. 
  2. ^ Reinsch 1969, S. 15
  3. ^ Nelson 2005, S. 587
  4. ^ a b Reinsch 1969, S. 13
  5. ^ a b Nelson 2005, S. 133
  6. ^ Nelson, 2002, S. 9
  7. ^ Reinsch 1969, S. 14f
  8. ^ Reinsch 1969, S. 61
  9. ^ Reinsch 1969, S. 9
  10. ^ a b Reinsch 1969, S. 16
  11. ^ Nelson 2005, S. 134
  12. ^ a b Nelson 2002, S. 27
  13. ^ Nelson 2005, S. 130
  14. ^ Reinsch 1969, S. 63
  15. ^ Nelson 2005, S. 158 Although Nelson states that this data was mainly collected from one colony.
  16. ^ Nelson 2005, S. 129
  17. ^ Nelson 2002, S. 27f
  18. ^ Nelson 2005, S. 129, 310
  19. ^ Cramp & Simmons 1977
  20. ^ Reinsch 1969, S. 35–49
  21. ^ Nelson 2002, S. 24
  22. ^ a b Nelson 2005, S. 315
  23. ^ Nelson 2005, S. 138
  24. ^ Boev 2009
  25. ^ Reinsch 1969, S. 33, 99f
  26. ^ Reinsch 1969, S. 33
  27. ^ a b c Nelson 2005, S. 311
  28. ^ Reinsch 1969, S. 31
  29. ^ "Eldey island". Eldey island webpage. Retrieved 26 November 2010. 
  30. ^ a b Nelson 2005, S. 312
  31. ^ del Hoyo, Elliot & Sargatal 1992
  32. ^ del Hoyo 2004
  33. ^ "Birdwatching in the Outer Hebrides". Visit Scotland. Retrieved 16 April 2013. 
  34. ^ a b Mowbray, Thomas B. "Northern Gannet — Behavior — Birds of North America Online". Bna.birds.cornell.edu. Retrieved 17 October 2011. 
  35. ^ a b Nelson 2005, S. 320
  36. ^ Reinsch 1969, S. 54
  37. ^ a b Reinsch 1969, S. 53
  38. ^ Nelson 2005, S. 141
  39. ^ Lewis et al. 2002
  40. ^ Reinsch 1969, S. 50
  41. ^ Mowbray, Thomas B. "Northern Gannet — Food Habits — Birds of North America Online". Bna.birds.cornell.edu. Retrieved 17 October 2011. 
  42. ^ Reinsch 1969, S. 51
  43. ^ a b Reinsch 1969, S. 56
  44. ^ a b Nelson 2002, S. XIII
  45. ^ Nelson 2002, S. 37
  46. ^ Reinsch 1969, S. 75
  47. ^ Reinsch 1969, S. 74
  48. ^ Nelson 2005, S. 328
  49. ^ Nelson 2005, S. 326
  50. ^ Reinsch 1969, S. 77
  51. ^ a b "Nature’s Calendar: Series 1: Rocky islands: Body language". BBC. Retrieved 3 October 2013. 
  52. ^ Reinsch 1969, S. 59
  53. ^ Nelson 2005, S. 150
  54. ^ Nelson 2005, S. 153, 332
  55. ^ a b Nelson 2005, S. 334
  56. ^ Reinsch 1969, S. 73
  57. ^ Reinsch 1969, S. 88f

Bibliography[edit]

External links[edit]