Geographic Range
Parasitic jaegers breed in the Arctic, completely encircling the North Pole. They
arrive on the breeding grounds in May or June and leave in August or September. They
typically breed in the same place every year, so they prefer sites which have dependable
sources of food. They prefer more southern locations than Pomarine jaegers (
Stercorarius pomarinus
) and long-tailed jaegers (
Stercorarius longicaudus
), which have lower site fidelity. Only adults return to breeding territories, juveniles
are rarely seen. It is believed that parasitic jaegers spend the first two years of
their lives at sea before returning to breeding areas.
After leaving the Arctic, parasitic jaegers migrate to the Southern Hemisphere. They
arrive in October and November and leave again in February or March. Parasitic jaegers
spend their time in the southern oceans and are common in eastern Australia. During
migration, they can sometimes be blown off course and found in places they don't normally
occur, such as Borneo.
- Biogeographic Regions
- nearctic
- palearctic
- australian
- oceanic islands
- arctic ocean
- indian ocean
- atlantic ocean
- pacific ocean
Habitat
Parasitic jaegers nest in tundra habitats. They live near the coast and large rivers,
bordered by grassland and moorland. In Iceland, these rivers are often cut out by
glaciers and contain willows and rushes. They avoid agricultural areas in Iceland,
though in other places they may nest on pastures. When parasitic jaegers are not breeding
they are found only in the open ocean.
- Habitat Regions
- polar
- terrestrial
- saltwater or marine
- Terrestrial Biomes
- tundra
- savanna or grassland
Physical Description
Parasitic jaegers are relatively small, weighing about 450 grams, but they have talons
strong enough to catch and hold other birds. Adults have dark caps and white panels
on their primaries, which are visible dorsally and ventrally when the wings are outstretched.
Adults also have blunt tails with several pointed tail feathers that emerge from the
center. These longer tail feathers are not present in juveniles. Juveniles are dark
with bars in their plumage, they often pale as they age.
Parasitic jaeger plumage is described as two or three morphs. Pale morphs have light
venters, intermediate morphs have brown venters, and dark morphs are dark brown all
over. Some people refer to intermediate and dark morphs collectively as melanistic.
This dark coloration is caused by a single allele, melanocortin-1 receptor, which
is activated by melanocyte stimulating hormone. It shows incomplete dominance in the
melanistic morph and its frequency follows a latitudinal cline. Pale morphs occur
most in northernmost breeding areas and their frequency decreases toward the south,
with southernmost breeding birds being mainly melanistic morphs.
Female parasitic jaegers are generally larger than males. Females generally prefer
males smaller than themselves. Several studies have demonstrated that parasitic jaeger
males prefer pale females and females prefer melanistic males, even if pale males
have higher fitness.
- Other Physical Features
- endothermic
- homoiothermic
- bilateral symmetry
- polymorphic
- Sexual Dimorphism
- female larger
Reproduction
Parasitic jaegers usually breed every year, but about 3% of experienced breeders will
not breed in a given year. Non-breeding is often due to death of a mate or divorce,
though environmental features (such as lack of food) can cause the birds to defer
breeding until conditions improve. Parasitic jaegers are monogamous, and no polygamy
have been recorded. They reach maturity in their fourth or fifth year, at which time
they travel to breeding areas and establish territories. Juveniles often do not return
to breeding colonies but, if they do, they form clubs with other juveniles.
Parasitic jaegers breed in the Arctic tundra between May and September, after migrating
there from the Southern Hemisphere. They demonstrate site fidelity, with pairs returning
year after year to the same territory. Parasitic jaegers defend their territory with
vocalizations, chases, and distractions. Occasionally, three birds will defend a
territory, the third bird is usually a resident on the territory rather than a visitor.
When both dark and light color morphs are available, parasitic jaegers seem to choose
mates of the opposite color morph. Females of either color morph seem to prefer dark
males. Dark males form pairs sooner in the year and breed earlier than pale males,
possibly because pale birds are more aggressive. There is some evidence that pale
morphs will eventually be eliminated by selective breeding in parasitic jaegers.
- Mating System
- monogamous
Parasitic jaegers exhibit reversed sexual dimorphism, in which females are larger
than males. Larger females produce larger eggs and they have more body heat and surface
area for incubating eggs. Females lose weight after their chicks hatch, so larger
females are better adapted to stresses encountered post-hatching. Larger females can
also defend better defend nests from predators and compete better for mates than smaller
females. Smaller males, on the other hand, are more efficient at foraging. Kleptoparasitism
requires agile flight, which is easier for birds with shorter wings and less momentum.
Long distance foraging is more efficient for smaller birds. Smaller, more agile males
are also better at territorial displays and courtship rituals.
When parasitic jaegers reach one year old, they begin to visit breeding colonies looking
for potential territories. Once a pair has established their territory, usually within
a colony, they will continue to use that spot for the remainder of their breeding
lives, unless a divorce or death occurs. Breeding pairs arrive at their territories
in late May or June and lay their eggs before the end of July. Pairs that are particularly
dimorphic (females are much larger than males) lay earlier in the year than pairs
that are more closely matched in size. Young birds (5 to 8 years old) lay their eggs
the latest. Eggs are 58 mm long and 40 mm wide and weigh about 48 grams, 6% of which
is the shell's weight. Two eggs are laid per nest, and only one brood is attempted
per pair per year. Both parents incubate the eggs, which hatch in 25 to 28 days. Hatching
success is from 70 to 80%. Hatchlings are altricial and covered with downy feathers.
About 85 to 95% survive to one week of age. Hatchlings fledge in 25 to 30 days. The
average number of chicks raised by a pair of birds in a particular colony ranges from
0.13 to 0.97. The average number of chicks per pair is dependent on several factors,
including food availability and how many pairs attempt to breed.
- Key Reproductive Features
- iteroparous
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- oviparous
Parasitic jaegers only attempt to raise a brood if environmental conditions are good
enough to support both parents and offspring. If conditions are poor, parasitic jaegers
delay breeding until the next year. Parasitic jaegers have long lifespans and high
adult survival rates, so their likelihood of surviving to attempt breeding the next
year is high.
Parasitic jaegers invest heavily in raising offspring. Males forage more than females
during courtship and incubation. Females do most of the incubation, though males do
help with incubation. After hatching, females begin to help with foraging to provision
hatchlings. Near the end of the breeding season both males and females forage for
their young equally. Parasitic jaegers defend territories and young vigorously; hatchlings
are almost constantly watched by at least one parent. Males defend nests more than
females, but females grow more aggressive after the eggs are laid. Sometimes an extra-pair
jaeger will assist with defense. This third jaeger can be a resident on the territory
or a visitor.
- Parental Investment
- altricial
-
pre-fertilization
- provisioning
-
protecting
- female
-
pre-hatching/birth
-
provisioning
- female
-
protecting
- male
- female
-
provisioning
-
pre-weaning/fledging
-
provisioning
- male
- female
-
protecting
- male
- female
-
provisioning
-
pre-independence
-
provisioning
- male
- female
-
protecting
- male
- female
-
provisioning
Lifespan/Longevity
Parasitic jaegers have long lifespans, high adult survival rates, and relatively late
maturity. Adults rarely suffer widespread mortality, although some die offs have been
attributed to environmental factors like the El Nino Southern Oscillation. The oldest
recorded parasitic jaeger was found in Finland. The information on its band indicated
it was 31 years and 1 month old.
Behavior
Parasitic jaegers defend nesting territories, either in nesting colonies or in more
solitary nesting areas. Neesting colony densities vary; lowest densities occur at
the highest latitudes. Pairs in low-density situations rarely share borders with more
than three other pairs. Pairs build nests on the ground on coastal gravel not containing
plant life or in moorland, grassland, and riverplain areas. They avoid agricultural
areas, except for the occasional pasture.
Parasitic jaegers with territories on coastal plains form mounds from which they observe
the local area. Mounds usually begin as a large object, like a rock or whale bones.
After years of resting on the spot, their guano builds up into a dome on which plants
grow. They grow up to 70 cm tall and 1 to 10 meters in diameter, and the interior
of the mound can have a water table higher than the surrounding tundra. Mounds can
be 4500 years old. The guano content of those mounts qualifies them as guanogenic
bogs, and they may contain peat layers up to 40 cm deep. Because the manure fertilizes
the spot so well, the plants are often bright green, making the mound easily visible
in the tundra. Peat is difficult to find in the tundra, so these mounds are valuable
to paleo-ecologists. They contain interesting plant material which decays slowly due
to the low temperatures and permafrost.
Home Range
At high latitudes, parasitic jaeger pairs defend large territories from which they
collect their food. They often live at densities of 0.02 to 0.2 pairs per square kilometer.
At lower latitudes, parasitic jaegers defend smaller territories within colonies.
Their nearest neighbors's nests are closer than 150 meters away.
Communication and Perception
Parasitic jaegers rely mainly on visual and auditory signals.
Food Habits
Parasitic jaegers are mainly piscivorous, although they may eat insects and berries.
Parasitic jaegers can catch their own food, they also are kleptoparasites. They steal
fish, such as sand eels (
Ammodytes marinus
), from other fish-eating birds. Kleptoparasitism saves energy spent looking for food,
although they have to chase other birds to steal their fish prey. Frequently victimized
birds have developed methods to counteract theft attempts. Colonial birds may respond
by foraging in large groups to make it more difficult for parasitic jaegers to steal
fish. Also, colonial birds may mob parasitic jaegers to deter theft.
One study (Taylor, 1979) examined attack behavior of parasitic jaegers on terns (
Sterna
). When attacking terns in Aberdeenshire, parasitic jaegers fly low above the water
at speeds up to 50 mph and try to sneak up behind and below the target. Their dark
plumage helps camouflage them against the water. When they are in position, they fly
up at the bird and chase until the tern drops the fish. They rarely physically assault
the bird. Chases are initiated by one bird, but other jaegers may join in and catch
the stolen food for themselves. Attacks are not guaranteed to end in theft. The encounter
may end with the tern eating the fish or otherwise escaping. Jaegers may also abandon
the chase. Success is higher when the victimized bird surprised. If only one jaeger
is involved in a chase, terns are more likely to drop the fish early during the chase
or not at all. If jaegers attack as a group, tern are less likely to drop fish until
later in the chase.
Belisle (1998) found that chasing behavior could induce a tern to regurgitate a fish
it had already swallowed, although fish are more likely to be abandoned before being
eaten. Belisle (1998) also found group attacks are generally more successful than
single ones. Single birds are slightly more successful than groups of four (at rates
of 33.7% and 30.0%). Groups of two or three are much more successful (59.5% and 58.7%).
A five-member group was only observed once, and they were unsuccessful. Another study
(Furness, 1983) on the kleptoparasitism of auks found that when many potential victims
are available, jaegers spend less time chasing each bird before giving up, but if
prospects are limited, they will chase longer.
Parasitic jaegers also search for and capture their own prey. They prey on the nests
of waterfowl, including snow geese (
Anser caerulescens
), and eat the eggs and young they find. They eat small mammals, including lemmings
(
Dicrostonyx groenlandicus
and
Lemmus trimucronatus
). They also eat small birds and occasional insects and berries.
- Animal Foods
- birds
- mammals
- fish
- eggs
- insects
- Plant Foods
- fruit
Predation
Parasitic jaegers are preyed on by other birds, including ravens (
Corvus
) and great skuas (
Stercorarius skua
). Because parasitic jaegers are rare in the United Kingdom, some people are debating
whether or not to control great skua populations by culling individuals that specialize
on seabirds, presumably including parasitic jaegers. Parasitic jaegers are also eaten
by mammals, including American mink (
Neovison vison
). Efforts to reduce feral mink populations in the United Kingdom have improved the
breeding success of parasitic skuas.
Parasitic jaeger pairs guard their nesting territories almost constantly, with at
least one adult on site at most times. Nesting territories are usually only guarded
by the nesting pair, though incidences of a third, extra-pair bird helping with defense
have been recorded.
Parasitic jaeger defensive behaviors are usually attacks or distractions and have
great variation. Solitary pairs practice displays together, learning gradually over
the season. Colony mates often perform together, learning the moves more quickly.
When attacking, parasitic jaegers swoop at the intruder and tries to hit it. Sometimes
pairs work together, with one bird flying at the intruder from the side or behind
and calling “tik-a-tik” while the other swoops silently at the intruder’s head. They
strike intruders with their feet or, rarely, with a wing.
Distraction displays seem to be juvenile begging behavior mixed with the bowing and
asymmetrical wing movements used in courtship displays. These displays are used to
distract predators from nests. They commence when a bird jumps five feet upward and
then drops to the ground and spreads its wings as if injured. It then flaps its wings
quietly and crawls a foot or so before jumping up again and repeating the sequence.
It crawls or walks upright during the display, and may drag its wings or flap them,
often at different speeds or in different directions.
Distraction behavior has many variations and can be mixed with attack behavior. Parasitic
jaegers may remain in one spot or move slowly toward the intruder. As in the attacks,
one bird may work alone, or the pair may display together. They might be silent or
make noises like “mew-mee-oo” or “choop, choop,” or some variation on those sounds.
They can perform on land or on water. The performance can be toned down to an open-winged
waddle. Sometimes, the solitary brooder will simply sneak a few feet away from the
nest and sit back down, pretending to brood a nest that isn’t there.
Ecosystem Roles
Parasitic jaegers are named for their kleptoparasitism. They steal food from a wide
variety of other sea birds, including
auks, guillemots, puffins, and murres
and
terns and gulls
. Parasitic jaegers are vulnerable to feather lice.
In areas where
geese
and
lemmings
are both available, their populations levels go through opposite cycles with each
other because they share predators. When goose populations are high, lemmings are
less heavily preyed on, which allows their populations to grow. When lemming populations
are high, geese are preyed on less, allowing their populations to grow again. This
is an example of an indirect trophic interaction, in which two species indirectly
affect each other. Parasitic jaegers are a predator of both
geese
and
lemmings
.
- Ecosystem Impact
- parasite
- auks ( Alcidae )
- common terns ( Sterna hirunda )
- Sandwich terns ( Sterna sandvicensis )
- Arctic terns ( Sterna paradisaea )
- kittiwakes ( Rissa tridactyla )
- black guillemots ( Cepphus grille )
- guillemots ( Uria aalge )
- Atlantic puffins ( Fratercula arctica )
- razorbills ( Alca torda )
- Hartlaub’s gulls ( Larus novaehollandiae hartlaubii )
- Sabine’s gulls ( Xema sabini )
- feather lice ( Haffneria grandis )
- feather lice ( Austromenopon )
- feather lice ( Quadriceps normifer )
Economic Importance for Humans: Positive
Through predation, parasitic jaegers control pest species.
Lemmings
can carry disease,
geese
and
lemmings
sometimes eat crops and pastures, and
snow goose
overpopulation in some areas has led to degraded habitats. Paleoecologists can learn
interesting things from parasitic jaeger mounds, some of which are 4500 years old.
These mounds contain pollen, spores, and other plant material, which can act as evidence
of climatic changes.
- Positive Impacts
- research and education
- controls pest population
Economic Importance for Humans: Negative
Due to their Arctic habitat and avoidance of agricultural areas, it is unlikely these
birds negatively impact humans in a significant way.
Conservation Status
Parasitic jaegers have a global range and populations are relatively stable, they
are listed as "Least Concern" by the IUCN Red List.
Other Comments
Parasitic jaegers are such agile flyers that the Blackburn Skua was named after them.
The Blackburn Skua was the first Royal Navy dive bomber. It was first built in 1937
and was built entirely of metal, whereas previous planes were covered in fabric. It
could carry one 500 pound bomb, had 4 machine guns on its wings, and had a flexibly-mounted
machine gun in the rear cockpit. It could travel at 225 mph, much faster than its
namesake.
Parasitic jaegers are also known as parasitic skuas, Arctic jaegers, and Arctic skuas.
Additional Links
Contributors
Tanya Dewey (editor), Animal Diversity Web.
Aqua Nara Dakota (author), Special Projects.
- Nearctic
-
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Palearctic
-
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Australian
-
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- oceanic islands
-
islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Arctic Ocean
-
the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Atlantic Ocean
-
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Pacific Ocean
-
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- polar
-
the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.
- terrestrial
-
Living on the ground.
- saltwater or marine
-
mainly lives in oceans, seas, or other bodies of salt water.
- tundra
-
A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.
- tropical savanna and grassland
-
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
- savanna
-
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
- temperate grassland
-
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
- pelagic
-
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
- coastal
-
the nearshore aquatic habitats near a coast, or shoreline.
- riparian
-
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
- estuarine
-
an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.
- endothermic
-
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
- bilateral symmetry
-
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
- polymorphic
-
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
- monogamous
-
Having one mate at a time.
- iteroparous
-
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
- seasonal breeding
-
breeding is confined to a particular season
- sexual
-
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- oviparous
-
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
- altricial
-
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
- diurnal
-
- active during the day, 2. lasting for one day.
- motile
-
having the capacity to move from one place to another.
- nomadic
-
generally wanders from place to place, usually within a well-defined range.
- migratory
-
makes seasonal movements between breeding and wintering grounds
- solitary
-
lives alone
- territorial
-
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
- colonial
-
used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- acoustic
-
uses sound to communicate
- parasite
-
an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death
- carnivore
-
an animal that mainly eats meat
- piscivore
-
an animal that mainly eats fish
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- acoustic
-
uses sound to communicate
- chemical
-
uses smells or other chemicals to communicate
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