Stercorarius parasiticus
Arctic jaeger
(Also: parasitic jaeger; parasitic jaeger;arctic jaeger)

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. (Cohen, et al., 1997; Kjellen, 1997)

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. (Harrisson and Smythies, 1961; Robinson, 2005)

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. (Gunnarsson, et al., 2006; Kjellen, 1997; Phillips, et al., 1998; Robinson, 2005)

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. ("BirdGuides", 2008; Cohen, et al., 1997; Robinson, 2005)

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. ("BirdGuides", 2008; Hoekstra and Price, 2004; Janssen, et al., 2005; Phillips and Furness, 1998a)

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. (Catry, et al., 1999; Janssen, et al., 2005; Mundy, et al., 2004)

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. (Catry, et al., 1998; Catry, et al., 1999; Kjellen, 1997)

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. (Catry, et al., 1999; Elmberg, 1991; Kjellen, 1997)

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. ("BirdGuides", 2008; Janssen, et al., 2005; Mundy, et al., 2004; Phillips and Furness, 1998a)

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. (Catry, et al., 1999)

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. (Caldow and Furness, 2001; Catry, et al., 1999; Kjellen, 1997; Phillips and Furness, 1998b; Phillips, et al., 1998; Robinson, 2005)

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. (Phillips, et al., 1996)

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. (Catry, et al., 1999; Elmberg, 1991)

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. (Phillips, et al., 1996; Robinson, 2005)

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. (Elmberg, 1991; Gunnarsson, et al., 2006; Phillips, et al., 1998)

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. (Van der Knaap, 1988)

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. (Phillips, et al., 1998)

Parasitic jaegers rely mainly on visual and auditory signals. (Taylor, 1978; Williamson, 1948)

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. (Birt and Cairns, 1986; Caldow and Furness, 2001; Phillips, et al., 1996; Phillips, et al., 1998)

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. (Taylor, 1978)

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. (Belisle, 1998; Furness, 1983)

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. (Bety and Gauthier, 2001; Birt and Cairns, 1986; Phillips, et al., 1998; Wilson and Bromley, 2001)

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. (Elmberg, 1991; Nordstrom, et al., 2003; Phillips, et al., 1998)

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. (Catry, et al., 1999; Elmberg, 1991; Phillips, et al., 1996)

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. (Williamson, 1948)

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. (Williamson, 1948)

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. (Williamson, 1948)

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. (Birt and Cairns, 1986; Caldow and Furness, 2001; Cohen, et al., 1997; Furness, 1983; Phillips, et al., 1996; Taylor, 1978)

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. (Bety, et al., 2002)

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. (Berg, 2000; Bety, et al., 2002; Logue, 2002; Rountrey and Myers, 2006; Van der Knaap, 1988)

Due to their Arctic habitat and avoidance of agricultural areas, it is unlikely these birds negatively impact humans in a significant way. (Gunnarsson, et al., 2006)

Parasitic jaegers have a global range and populations are relatively stable, they are listed as "Least Concern" by the IUCN Red List. (BirdLife International, 2008)

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. ("Fleet Air Arm Archive", 2000; Robinson, 2005)

Parasitic jaegers are also known as parasitic skuas, Arctic jaegers, and Arctic skuas.