The
Northern gannet (
Morus bassanus) is a
seabird and the largest member of the
gannet family,
Sulidae. "Gannet" is derived from
Old English ganot "strong or masculine", ultimately from the same
Old Germanic root as "gander".
[2] Morus is derived from
Ancient Greek moros, "foolish" due to the lack of fear shown by breeding gannets and
boobies allowing them to be easily killed. The specific
bassanus is from the
Bass Rock in the
Firth of Forth, which holds the world's largest colony of northern gannets.
[3]
The northern gannet 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
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.
[4][5]
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.
[6] 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.
[7] 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.
[8]
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.
[9] 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.
[10] From the fifth week they are covered in dark brown feathers flecked with white.
[6]
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.
[7] The rear toe is strong and faces inwards allowing the birds to firmly grip onto vertical cliff faces.
[11]
Distinguishing anatomical features
Northern gannet on Bonaventure Island, in
Quebec
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.
[12]
The
lungs
are highly developed and probably also play a role in reducing the
effects of hitting water at high speeds and protecting the body from
these effects. 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 expelled by muscle contractions.
[12]
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.
[13]
By air, land and sea
The wings of the northern gannet are long and narrow and 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)
[14]
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%.
[15]
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.
[14] 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 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.
[16] Damaged or broken wings are a frequent cause of death in adults.
[17] The position of the legs towards the rear of the body means that they walk in a similar way to
ducks.
Call
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.
[18]
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.
[19] 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.
[20]
Distribution
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,
[21] 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).
[22] 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 Dr 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.
[23]
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.
[24] The southern limit of their distribution mainly depends on the presence of sufficient prey.
[25]
The species is a rare visitor in the Black Sea region.
[26]
Breeding colonies
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.
[27] 68% of the world population breeds around the coasts of
Great Britain and
Ireland. The biggest colonies include:
- Bass Rock off the east coast of Scotland, first recorded in 1448.[28] In 2004, it contained more than 48 000 nests.[29]
This is where part of the species’ Latin name comes from. The Bass Rock
is now the world's largest colony of Northern gannets. Visitors to the
Scottish Seabird Centre have the opportunity to see the Bass Rock via
interactive live cameras in the Centre and on seasonal boat trips.
- Saint Kilda and Sula Sgeir, in the Hebrides. Saint Kilda is the largest colony in Europe with more than 60 000 nests.[29]
- Sceilg Bheag one of the Skellig Islands located off the South West Coast is the largest colony in Ireland hosting around 30,000 pairs.
- Eldey off Iceland, where between 14 000 and 15 000 pairs breed.[30][31]
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.
[29]
Population
A 2004 survey counted 45 breeding colonies and some 361 000 nests.
[32] The population is apparently growing between 3% and 5% a year, although this growth is concentrated in just a few colonies.
[32] 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.
[24] 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.
[33] 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.
[34]
A count was undertaken on the Bass Rock population in 2014 - this was
an aerial count and confirmed the Bass Rock as the world's largest
colony with over 150,000 pairs.
Conservation status
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.
[35] Many of these gannets are taken from
Sula Sgeir, which is itself named after them.
Depredation
This species is not heavily predated. The only known habitual natural predators of adults are the
bald and
white-tailed eagles.
[36]
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.
[36]
Diet and foraging
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).
[37]
Diving
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).
[38]
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.
[39]
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.
[39]
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.
[40] 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.
[41]
Diet
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.
[42][43] 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 way as
gulls do. They fly around the boats to take fish from the
fishing nets or pick up the remains thrown into the sea.
[44]
Reproduction
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.
[45]
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.
[46]
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.
[47]
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.
[45]
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.
[48]
Nest construction
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.
[49] On average there are 2.3 nests per square metre.
[50]
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
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.
[51]
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.
[52]
Mating
"Billing", a mutual greeting gesture
[53]
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
Northern gannets only lay one egg that on average weighs 104.5
grams.
[54] This is lighter than for other seabirds.
[55]
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.
[56]
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.
[57]
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.
[58]
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.
[46]
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.
[57]
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.
[59]
Migration
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.
[37]
Gallery
-
-
-
Northern gannets usually nest close together on
cliff ledges
-
-
"Sky pointing", gesture made to signal that a bird is about to fly away from the nest
[53]
-
