Northern gannet

Adaptations for diving

Northern gannets have streamlined bodies adapted for plunge-diving at high speed, including powerful neck muscles, and a spongy bone plate at the base of the bill.^[17] The nostrils are inside the bill and can be closed to prevent water entry; the eyes are protected by strong nictitating membranes.^[33] There are subcutaneous air sacs in the lower body and along the sides. Other air sacs are located between the sternum and the pectoral muscles and between the ribs and the intercostal muscles. These sacs are connected to the lungs and are filled with air when the bird breathes in. The air can be returned to the lungs by muscle contractions.^[34]

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 head.^[29] Individuals have a subcutaneous fat layer, dense down feathers 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 the birds swim.^[35]

Call

The northern gannet is a loud and vocal bird, particularly in the colony. Its typical call is a harsh arrah-arrah or urrah-urrah, which is emitted upon arriving or when challenging other gannets at the colony. The call is shortened to a rah rah when fishing or collecting nesting material, and lengthened to a ooo-ah when taking off.^[36] The calls of the sexes are similar.^[37] According to Nelson northern gannets can recognize the call of their breeding partner, their chicks and birds in neighbouring nests. Individuals from outside this sphere are treated with more aggression.^[38]

Breeding colonies

Some northern gannet breeding colonies have been recorded as being located in the same place for hundreds of years. The cliffs containing the colonies appear white when seen from a distance, due to the number of nesting birds present on them. There is a written record of a colony on the island of Lundy from 1274. There were only 70 nests by 1871, and the colony finally disappeared by 1909 at the latest.^[44] More than two-thirds of the world population breeds around the coasts of the British Isles.^[17] Colonies include:

• Scotland

• Bass Rock, in the Firth of Forth, first recorded in 1493.^[45] In 2004, it contained more than 48,000 nests.^[46] By 2014 this had increased to over 75,000, making it the largest colony in the world.^[47]
• Northern gannets began a colony at Troup Head in Aberdeenshire in 1988,^[48] and by 2014 it held an estimated 6,456 pairs.^[49]
• Saint Kilda and Sula Sgeir, in the Hebrides, Scotland.^[50] The former colony was estimated at 60,290 pairs and the latter 11,230 pairs in 2013. A colony on the Flannan Isles almost doubled in size to 5,280 pairs in 2013. Further south, the island of Ailsa Craig in the Firth of Clyde hosted an estimated 33,226 pairs in 2014.^[49] Ailsa Craig has been known as a colony since 1583.^[51]
• In Shetland, there were an estimated 25,580 breeding pairs at Hermaness, 11,786 on Noss, and 3,591 on Fair Isle in 2013, while Orkney had an estimated 4,550 pairs at Sule Stack. Sule Skerry's breeding population rose from 57 to 1,870 pairs between 2003 and 2013.^[49]

• Wales

• Grassholm off the Pembrokeshire coast in Wales had an estimated 36,011 pairs in 2015.^[49] Gannets began nesting here between 1820 and 1860, though never in great numbers, only exceeding 300 pairs by 1913.^[52]

• England

• Bempton Cliffs hosted an estimated 12,494 pairs in 2015, more than tripling in size in 12 years.^[49]

• Ireland

• Little Skellig, a small island located about twelve kilometres (6+1⁄2 nautical miles) off the Iveragh Peninsula of County Kerry is the largest colony in Ireland, hosting around 30,000 breeding pairs.^[50] Known as a gannetry before 1700, human impact had reduced the population to 30 pairs by 1880 before rapidly increasing to around 10,000 pairs by 1906, the fastest recovery ever recorded at a gannet colony.^[53] There are small colonies on Ireland's southern coast,^[54] on Bull Rock, County Cork, Clare Island, County Mayo and Great Saltee Island, County Wexford.^[55]

• Alderney. Around 7,500 pairs nest on two islets—Ortac and Les Etacs—off the coast of Alderney in the English Channel, the first birds only nesting there in the 1940s. These birds migrate further south to the Mediterranean and west coast of Africa than more northerly breeding colonies.^[56]
• France. The French island of Rouzic in the Jentilez archipelago off the coast of Brittany hosts the southernmost breeding colony of northern gannets. Established in the late 1930s, it had grown to over 11,500 breeding pairs by 1995.^[57] Pairs have nested sporadically with varying success along the Mediterranean coast.^[58]
• Germany. Northern gannets have reappeared on Heligoland in 1991 and around 680 breeding pairs have been counted in 2016.^[59]
• Italy. A pair raised a chick successfully for three successive seasons from 2013 to 2015 on a moored boat at Porto Venere in northern Italy, after birds had been seen there since 1993.^[58]
• The Faroe Islands. Mykines in the Faroe Islands held about 2,000 breeding pairs in 2000,^[60] although that had risen to 2,500 pairs by 2014.^[61]
• Iceland. Eldey, a small island located about 16 kilometres (8+1⁄2 nautical miles) off the coast of the Reykjanesskagi Peninsula, Iceland, hosting around 16,300 breeding pairs in 1962,^[62] and a similar number in 2008.^[63] Iceland has several small colonies along its coast,^[64] and on Grimsey, around 40 km (20 nmi) north.^[65]
• Norway. As of 2016, there are an estimated 6,900 breeding pairs in Norway.^[66] Northern gannets first bred in the south of the country in the mid 1940s at Runde in Møre og Romsdal. In 1967 a colony was established at Syltefjord,^[67] within the Arctic Circle at 70°N.^[65] Colonies were also established on low islets in Lofoten and Vesterålen, but declined due to increasing numbers of white-tailed eagles. Once rare, these were protected in 1968 and their numbers have risen. It is unclear why the eagles have not impacted on gannet numbers further north.^[67] In 2011, gannets began breeding on Bear Island, southernmost island of Svalbard, likely due to the warming of the Barents Sea and influx of fish. The colony numbered 52 pairs in 2016.^[66]
• Russia. There are around 250 pairs in a colony at Kharlov on the Kola Peninsula in Russia. Established in 1995, this is also thought to be due to the warming of the Barents Sea.^[66]
• North America. There are six breeding colonies along the coast of Canada. The Gulf of Saint Lawrence hosts three colonies—Bonaventure Island, Bird Rocks and Anticosti Island—and there are three off the eastern and southeastern coast of Newfoundland (Cape St. Mary's Ecological Reserve, Funk Island, Baccalieu Island Ecological Reserve. Bonaventure Island off the south coast of the Gaspé Peninsula of Quebec is the largest colony with 60,000 nests as of 2009.^[68]

Migration

After the breeding season, adult northern gannets disperse over a wide area although they travel no more than 800 to 1,600 km (450 to 850 nmi) 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, much further south than the adults.^[69] The immature gannets migrate southwards for great distances and have been recorded as far south as Ecuador. In their second year some birds return to the colony they were born in, where they arrive later than the mature birds. They then migrate south again at the end of the breeding season, but travel shorter distances in this second migration.^[23] Gannets from Alderney have been tracked since 2015 to gain better knowledge of their movements.^[70] One individual was found to have travelled from its colony in Alderney to Scandinavian waters, a round trip of around 2,700 km (1,500 nmi).^[71]

The species has been recorded as a vagrant in many central and eastern European countries as far south and west as the Black Sea, and also in Bermuda, Cuba, Cyprus, Egypt, Kazakhstan, Jan Mayen and Syria.^[1][72] In February 2016, one was recorded from Ceará in northeastern Brazil—the first sighting in the Southern Hemisphere.^[73]

Feeding

Northern gannets forage for food during the day, generally by diving at high speed 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 (170 nmi) 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 (150 and 170 nmi) away. It is likely that they fly further than this while foraging, possibly up to double the distance; normally they fly less than 150 km (80 nmi).^[69] Some studies have found that the duration and direction of flights made while foraging for food are similar for both sexes, although 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.^[78]

Gannets will follow fishing boats or cetaceans to find discarded or injured fish.^[23] They forage from heights of up to 70 m (230 ft) with no clear preference, and typically dive between 11 and 60 m (35 and 195 ft).^[79] They dive with their bodies straight and rigid, wings tucked close to the body but angled back, extending beyond the tail, before piercing the water like an arrow. They control the direction of the dive using their wings and tail, and fold their wings against the body just before impact.^[80] Birds can hit the water at speeds of up to 100 km/h (30 m/s).^[34] This allows them to penetrate up to 11 m (36 ft) below the surface,^[81] and they will swim down to an average 19.7 m (60 ft), sometimes deeper than 25 m (80 ft).^[82] The bird's subcutaneous air sacs may have a role in controlling their buoyancy.^[34]

Gannets usually push their prey deeper into the water and capture it as they return to the surface. When a dive is successful, they 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 fish is stored in a branched bag in the throat and does not cause drag when in flight.^[83] 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. The colour also makes the gannet less visible to the fish underneath.^[26] Northern gannets also forage for fish while swimming with their head under water.^[84]

They eat mainly fish 2.5–30.5 cm (1–12 in) in length that 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 also eaten.^[23][85] M. bassanus takes huge quantities from the waters around Newfoundland for the energy requirements of their growing chicks.^[86] Given the economic importance of the prey taken, whether they are in competition with human fishing industry in the area is an important question which remains unassessed.^[86] (But given the small amount taken, NL's cod take is probably unaffected.)^[86] On the other hand, they profit off of discarded bycatch and processed parts^{[87][88][89][90]} and the reduction in competition if humans are taking predatory fish.^[91]

M. bassanus inflicts significant wastage on the northwest Atlantic Salmo salar fishery, but does not actually eat much of them. This makes it difficult to resolve this conflict with the fishing industry in the area, as the only option would be to exterminate the birds, which is ecologically unacceptable.^{[S 1]}

Breeding

The oldest birds are the first to return to the northern gannet's breeding colonies.^[92] Birds not of breeding age arrive a few weeks later. In general, birds first return to a colony (not uncommonly the one in which they were hatched) when they are two or three years old. Once an individual has successfully bred in a colony it will not normally change to another.^[93] Nesting starts in March or April.^[23]

Immature birds stay on the edges of the colony. They may even make a nest but they do not breed until they are 4 or 5 years old. Some birds of this age 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 leave without a struggle when the owner arrives to take possession.^[92]

The preferred nesting sites are on coastal hillsides or cliffs. If these are not available northern gannets will nest in groups on islands or flat surfaces. As they find it more difficult to take off from such locations they will often cross the area occupied by an adjacent nest causing an aggressive reaction from the sitting pair; this means that the stress levels are higher in this type of colony than in those on steeper surfaces. Notwithstanding this, nests are always built close together and otherwise ideal nesting sites will not be used if they are some distance from a colony.^[92] On average there are 2.3 nests per square metre (1.9 per square yard).^[94] Both sexes fiercely defend the area around their nest.^[95] Where space allows, the distance between nests is double the reach of an individual.^[96]

Nests are made from seaweed, plants, earth and debris from the sea. The males usually collect the materials.^[97] Nests are compact cups typically 30–60 cm (12–24 in) in height.^[98] The area which a nest occupies grows throughout the breeding season as the breeding pairs throw their excrement outside the nest.^[92] Over years, nests can reach 2 m (6+1⁄2 ft) in height.^[98]

Northern gannets lay one egg that on average weighs 104.5 g (3+5⁄8 oz),^[99] which is light for such a large seabird.^[100] The egg is around 79 mm (3+1⁄8 in) long by 50 mm (2 in) wide and the shell is pale blue and translucent initially before fading to a chalky white surface that is easily stained.^[98] M. bassanus was among those birds most affected by the shell-thinning effect of DDT, which contributed in large degree to its ban.^{[S 2]} Where two eggs are found in a nest this is the result of two females laying an egg in the same nest or one egg being stolen from another nest. Northern gannets will lay a replacement egg if the first is lost. Incubation takes 42 to 46 days, during which time the egg is surrounded by the brooding bird's warm, webbed feet. Just before hatching begins, the brooding bird releases the egg from its 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. The process of breaking the eggshell can take up to 36 hours.^[101] 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.^[102]

Newly hatched chicks are featherless and are dark blue or black in colour.^[26] In the second week of life they are covered in white down, replaced over the next five weeks by dark brown feathers flecked with white.^[103] Young chicks are fed regurgitated semi-digested fish by their parents, who open their mouths wide for their young to fetch the food from the back of their throats.^[104] 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.^[105]

The adults feed their offspring for around 13 weeks, right up until the time they leave.^[106] The young birds fledge between 84 and 97 days old, departing by launching themselves off a cliff and flying—a procedure for which it is impossible to practice beforehand.^[107] If they leave the nest in bad weather they can be mortally wounded as they can be blown against the rocks.^[102] The young birds are attacked by adults if unattended. Once they leave the nest they stay at sea learning to fish and fly, their flight skills being too poor for them to return to the breeding ledges.^[105]

Northern gannets have only one brood a year. The survival rate for young birds for their first four years is 30% and the annual survival rate for adults is 91.9%. The typical lifespan after becoming adult is 17 years, and the maximum known age is 37 years 4 months 16 days.^[108] Gannet pairs are monogamous and may remain together over several seasons, if not for all of their lives. The pairs separate when their chicks leave the nest but they bond again the following year. Should one of the pair die, the other bird will find another mate.^[109]

Displays

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.^[110] The fights between males occupying nests for the first time are particularly intense. Such fights can last for up to two hours and lead to serious injuries. Birds lunge at each other and lock bills, wrestling for extended periods while neighbours peck at them. The fights are preceded by threatening gestures, which are also seen outside the breeding season. Males demonstrate ownership of a nest by gesturing towards their neighbours in a bowing display; their heads and beaks point down, and their wings are held up and away from the body, yet folded backwards. The male moves his head from side to side before bowing forwards.^[92]

Males try to attract an available female after establishing a territory. The females will fly over the colony several 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. Mated pairs engage in a fencing display when the male arrives back at the nest. The two birds stand breast to breast with wings spread and bills extended vertically. They fence and scissor with their bills rapidly, calling loudly at the same time. Fencing is interspersed with bill bowing.^[92]