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Skuas
Pomarine jaeger
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Charadriiformes
Suborder: Lari
Family: Stercorariidae
Gray, 1871
Genus: Stercorarius
Brisson, 1760
Type species
Larus parasiticus
Linnaeus, 1758
Species

See text.

The skuas (/ˈskjuːə/) are a group of predatory and kleptoparasitic seabirds with seven species forming the genus Stercorarius, the only genus in the family Stercorariidae. The three smaller skuas, the Arctic skua, the long-tailed skua, and the pomarine skua, are called jaegers in North American English.

The English word "skua" comes from the Faroese name for the great skua, skúgvur [ˈskɪkvʊɹ], with the island of Skúvoy renowned for its colony of that bird. The general Faroese term for skuas is kjógvi [ˈtʃɛkvɪ]. The word "jaeger" or Jäger is German for "hunter".[1][2] The genus name Stercorarius is Latin and means "of dung";[note 1] the food disgorged by other birds when pursued by skuas was once thought to be excrement.[3]

Skuas nest on the ground in temperate, Antarctic, and Arctic regions, and are long-distance migrants. They have even been sighted at the South Pole.[4]

Biology and habits

[edit]
Two brown skuas (S. antarcticus) and a southern giant petrel (Macronectes giganteus) fighting over a dead Antarctic fur seal

Outside the breeding season, skuas take fish, offal, and carrion. Many practice kleptoparasitism, which comprises up to 95% of the feeding methods of wintering skuas, by chasing gulls, terns and other seabirds to steal their catches, regardless of the size of the species attacked (up to three times heavier than the attacking skua). Larger species, such as the great skua, regularly kill and eat adult seabirds, such as puffins and gulls and have been observed killing birds as large as a grey heron.[5] On the breeding grounds, the three, more slender northern breeding species commonly eat lemmings. Those species that breed in the southern oceans largely feed on fish that can be caught near their colonies. The eggs and chicks of other seabirds, primarily penguins, are an important food source for most skua species during the nesting season.[6]

In the southern oceans and Antarctica region, some skua species (especially the south polar skua) will readily scavenge carcasses at breeding colonies of both penguins and pinnipeds. Skuas will also kill live penguin chicks and sick or injured adult penguins. In these areas, the skuas will often forfeit their catches to the considerably larger and very aggressive giant petrels. Skuas have also been observed to directly pilfer milk from the elephant seal's teats.[7][8]

Skuas are medium to large birds, typically with grey or brown plumage, often with white markings on the wings.[6] The skuas range in size from the long-tailed skua, Stercorarius longicaudus, at 310 grams (0.68 pounds), to the brown skua, Stercorarius antarcticus, at 1.63 kg (3.6 lb). On average, a skua is about 56 cm (22 in) long, and 121 cm (48 in) across the wings. They have longish bills with a hooked tip, and webbed feet with sharp claws. They look like large dark gulls, but have a fleshy cere above the upper mandible.

The skuas are strong, acrobatic fliers. They are generally aggressive in disposition. Potential predators approaching their nests will be quickly attacked by the parent birds, which usually target the heads of intruders – a practice known as 'divebombing'.[9]

Great skua leaving the nest
Skua nestling, with egg tooth still present on its beak

Taxonomy

[edit]

The genus Stercorarius was introduced by the French zoologist Mathurin Jacques Brisson in 1760 with the parasitic jaeger (Stercorarius parasiticus) as the type species.[10][11]

Skuas are related to gulls, waders, auks, and skimmers. In the three smaller species, all nesting exclusively in the Holarctic, breeding adults have the two central tail feathers obviously elongated, and at least some adults have white on the underparts and pale yellow on the neck. These characteristics are not shared by the larger species, all native to the Southern Hemisphere except for the great skua. Therefore, the skuas are often split into two genera, with only the smaller species retained in Stercorarius, and the large species placed in Catharacta. However, based on genetics, behaviour, and feather lice, the overall relationship among the species is best expressed by placing all in a single genus.[12] The pomarine and great skuas' mitochondrial DNA (inherited from the mother) is in fact more closely related to each other than it is to either Arctic or long-tailed skuas, or to the Southern Hemisphere species.[13] Thus, hybridisation must have played a considerable role in the evolution of the diversity of Northern Hemisphere skuas.

Species

[edit]

The genus contains seven species:[14]

Genus Stercorarius – Brisson, 1760 – eight species
Common name Scientific name and subspecies Range Size and ecology IUCN status and estimated population
Chilean skua

 Stercorarius chilensis
Bonaparte, 1857 		Breeds along the coasts of southern Chile and southern Argentina, winters along the Pacific coasts of Peru and Chile as well as the Atlantic coast of Argentina
Map of range 		Size:

Habitat:

Diet: 		 LC 


South polar skua

 Stercorarius maccormicki
Saunders, 1893 		Breeds along the coast on Antarctica, winters in the north Atlantic and north Pacific
Map of range 		Size:

Habitat:

Diet: 		 LC 


Brown skua

 Stercorarius antarcticus
(Lesson, 1831)

Three subspecies
  • S. a. antarcticus (Lesson, 1831)
  • S. a. hamiltoni (Hagen, 1952)
  • S. a. lonnbergi (Mathews, 1912)
Southern Ocean
Map of range 		Size:

Habitat:

Diet: 		 LC 


Great skua

 Stercorarius skua
(Brünnich, 1764) 		Breeds along the coastline of the northeast Atlantic, winters in the north Atlantic
Map of range 		Size:

Habitat:

Diet: 		 LC 


Pomarine jaeger or pomarine skua

 Stercorarius pomarinus
(Temminck, 1815) 		Breeds along the Arctic coastline, winters in tropical and subtropical oceans
Map of range 		Size:

Habitat:

Diet: 		 LC 


Parasitic jaeger or Arctic skua

 Stercorarius parasiticus
(Linnaeus, 1758) 		Breeds along the Arctic coastline, winters in the southern hemisphere
Map of range 		Size:

Habitat:

Diet: 		 LC 


Long-tailed jaeger or long-tailed skua

 Stercorarius longicaudus
Vieillot, 1819

Two subspecies
  • S. l. longicaudus Vieillot, 1819.
  • S. l. pallescens Løppenthin, 1932
Breeds in the Arctic, winters in the Southern Ocean
Map of range 		Size:

Habitat:

Diet: 		 LC 


References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Skua

View on Grokipedia
from Grokipedia
Skuas are a group of seven predatory seabirds comprising the Stercorariidae, closely related to and terns but distinguished by their aggressive foraging strategies and adaptations for life in harsh polar and oceanic environments. These robust, gull-like birds range in size from medium (jaegers, around 0.4–0.5 m in length) to large (great skuas and southern skuas, up to 0.65 m), with barrel-shaped bodies, broad wings for soaring, hooked bills for tearing prey, and strong, clawed webbed feet suited for both swimming and terrestrial movement. The family Stercorariidae is divided into two main groups: the larger skuas, including the (Stercorarius skua) that breeds in northern regions and the three southern skuas (brown skua S. antarcticus, Chilean skua S. chilensis, and S. maccormicki) that breed in and sub- regions, and the three smaller northern jaegers ( S. parasiticus, pomarine jaeger S. pomarinus, and long-tailed jaeger S. longicaudus) that nest in . All species exhibit circumpolar distributions, migrating extensively across oceans during non-breeding seasons, where they remain far from land and rely on . Their is typically dark brown or gray with distinctive white primary wing patches visible in flight, and some species show in size, with females often larger than males. Skuas and jaegers are renowned for their kleptoparasitic behavior, in which they relentlessly pursue and harass other seabirds—such as , terns, and —to force them to drop or regurgitate , which the skua then seizes; this accounts for up to 95% of their diet at sea. On breeding grounds, they supplement this by actively hunting small mammals like lemmings, preying on eggs and chicks of ground-nesting birds, and scavenging carrion, including penguin colonies in the south. Highly territorial and aggressive, they defend nesting sites—simple scrapes on the ground laid with two eggs—fiercely against intruders, with both parents sharing incubation (lasting 24–30 days) and chick-rearing duties until fledging after 25–40 days. These birds play key ecological roles as top predators in polar food webs, influencing prey populations and cycling, though some species face threats from and in fisheries.

Taxonomy and Systematics

Etymology

The term "skua" originates from the Faroese word skúgvur (or skúvur), which traces back to skúfr, meaning "tuft," "tassel," or "seagull," likely alluding to the bird's distinctive tufted ear feathers or overall shaggy appearance. This nomenclature entered scientific usage through New Latin in the late 17th century, during European explorations of northern seas, and was adopted into English around 1678, reflecting early encounters with these seabirds in Faroese and Scandinavian waters. Historically, the naming of skuas has been intertwined with that of jaegers, causing some taxonomic confusion in ornithological literature. "Jaeger" derives from the German Jäger, meaning "hunter," emphasizing the birds' aggressive kleptoparasitic behavior of chasing and robbing other seabirds of food. In European traditions, all species in the family Stercorariidae were broadly termed skuas, but 19th-century American ornithologists distinguished the smaller, more agile forms (such as the parasitic and long-tailed jaegers) as "jaegers," reserving "skua" for the larger, bulkier species like the , to highlight morphological differences. Regional vernacular names further illustrate the cultural embedding of these birds in local lore. In the Shetland Islands, the is known as the "bonxie," a term derived from bónki (or related Scandinavian words like Norwegian bunke), denoting something dumpy, heap-like, or thick-set, which captures the bird's robust build and its notorious "piratical" raids on other avifauna. These etymological roots underscore the skuas' recognition as formidable predatory seabirds across Nordic and Atlantic cultures.

Classification and Phylogeny

Skuas are classified within the family Stercorariidae, which comprises predatory seabirds including both skuas and jaegers, and is placed in the order . The family was elevated from subfamily status (Stercorariinae within ) to full family rank based on DNA sequence analyses demonstrating its distinct phylogenetic position as sister to Alcidae (auks) and (gulls, terns, and skimmers). Stercorariidae is monotypic at the subfamily level, containing a single , Stercorarius, with seven recognized . Traditionally, the species were divided between two genera: Stercorarius for the three smaller "jaegers" (S. longicaudus, S. parasiticus, and S. pomarinus) and Catharacta for the four larger "skuas" (S. antarcticus, S. chilensis, S. maccormicki, and S. skua), a separation supported by morphological and behavioral differences such as size, plumage, and foraging strategies. However, molecular studies using have revealed close genetic affinities across these groups, particularly showing that the pomarine jaeger (S. pomarinus) is more closely related to the (S. skua) than to other jaegers. This finding contributed to the merger of Catharacta into Stercorarius, as adopted in the American Ornithological Union Check-list in 2000 and supported by subsequent genetic analyses emphasizing hybridization events and shared evolutionary history as key factors in skua diversification. Phylogenetically, skuas form a monophyletic within the suborder Lari, with basal divergence from their closest relatives—gulls () and auks (Alcidae)—estimated at approximately 35 million years ago during the late Eocene, based on analyses calibrated with evidence. Early studies, such as (1987), provided a morphological framework highlighting skuas' evolution as aggressive hyperpredators adapted to colonies, while Cohen et al. (1997) used cytochrome b and 12S rRNA mitochondrial sequences to resolve intergeneric relationships, confirming and challenging traditional divisions. Species-level distinctions within Stercorarius rely on a combination of morphological traits (e.g., bill shape, leg color) and genetic markers, with taxa showing rapid post-glacial radiations around 0.5–1 million years ago.

Physical Description

Morphology and Size

Skuas (genus Stercorarius) display a robust body structure adapted for a predatory lifestyle, with overall sizes varying by from smaller jaegers to larger forms. Typical measurements include body lengths of 41–64 cm, wingspans of 110–160 cm, and weights ranging from 0.25–2.2 kg. Skuas exhibit reversed sexual size dimorphism, with females larger than males across . Key morphological features include a sturdy, barrel-shaped build that supports agile terrestrial and aerial pursuits. The bill is strong and distinctly hooked at the tip, facilitating the tearing of flesh from prey or scavenged items. Legs are short yet powerfully muscled, with sharp claws and partial webbing between the toes, enabling effective ground chases and occasional swimming. In jaeger species like the long-tailed jaeger, the central tail feathers are notably elongated, forming streamers that enhance aerodynamic maneuverability during flight. Anatomical adaptations emphasize endurance and speed, particularly in flight. The plumage is dense and waterproof, providing insulation and buoyancy essential for marine environments. Strong pectoral muscles anchor to an enlarged keeled sternum, delivering the power needed for sustained soaring over oceans and rapid aerial chases comparable in intensity to those of falcons.

Plumage and Sexual Dimorphism

Skuas display notable plumage polymorphism, with dark and pale morphs varying in prevalence across . The dark morph, predominant in great skuas (Stercorarius skua) and south polar skuas (S. maccormicki), consists of uniformly dark brown to blackish overall , accented by prominent white flashes on the primary wing feathers that are visible in flight. In arctic skuas (S. parasiticus) and long-tailed skuas (S. longicaudus), the pale morph is more common, featuring lighter underparts that enhance against and snowy substrates during breeding. Intermediate morphs occur in some populations, blending elements of both, but are less frequent. Adults undergo a complete annual prebasic molt after breeding, typically at sea during migration or wintering, which replaces worn feathers and shifts from the brighter breeding condition—characterized by vivid yellow facial patches—to a duller, more uniform winter appearance. Juveniles emerge with distinct barred or scaled patterns on the body and wings, providing additional ; these molt progressively through a partial preformative molt in the first winter, attaining subadult , and reach full adult coloration by the second or third year. Sexual dimorphism in plumage is minimal, with males and females generally sharing identical morphs and patterns within . Although plumage shows little differentiation, skuas exhibit reverse sexual dimorphism, with females larger than males overall. The adaptive value of skua plumage lies in its dual role: cryptic tones and patterns provide concealment for nests on exposed breeding grounds, reducing predation risk, while bold contrasting elements like white wing flashes aid in recognition and elaborate aerial displays during and territorial interactions.

Distribution and Habitat

Global Range

Skuas exhibit a predominantly circumpolar distribution, with distinct species groups occupying high-latitude regions in both hemispheres. In the , four species—Great Skua (Stercorarius skua), Jaeger (S. parasiticus), Long-tailed Jaeger (S. longicaudus), and Pomarine Jaeger (S. pomarinus)—breed primarily in and and coastal areas. The nests in the North Atlantic, including , (including ), the , and . The Jaeger breeds along the northernmost coasts of and , while the Pomarine and Long-tailed Jaegers occupy from through to . These species are highly migratory, undertaking transequatorial journeys to winter in the Southern Hemisphere's oceans, often between the tropics and Antarctic waters, including coastal and the southern tips of . In the , three main species— (Stercorarius maccormicki), (S. antarcticus), and Chilean Skua (S. chilensis)—are centered on sub- and regions. The breeds circumantarctically along ice-free coastal areas, particularly in the region. The occupies the and sub- islands across the Atlantic, Indian, and Pacific Oceans, while the Chilean Skua breeds in Patagonia, southern Chile (from southward), and the . These southern species generally remain in southern latitudes year-round, with some northward into subtropical waters during non-breeding periods, though the occasionally migrates transequatorially to higher northern latitudes. Overlap between northern and southern skua groups is rare but occurs in the , where wintering northern migrants encounter breeding southern populations, potentially leading to inter-hemispheric interactions.

Habitat Preferences

Skuas exhibit distinct habitat preferences that align with their opportunistic predatory , favoring open, low-vegetation environments conducive to ground nesting and access to prey resources. Northern species, such as the (Stercorarius parasiticus) and pomarine jaeger (S. pomarinus), primarily breed in coastal , characterized by sparse vegetation and proximity to colonies for kleptoparasitic opportunities. The (S. skua) selects treeless northern islands with low vegetation, such as those in and around , often nesting close to mixed colonies to exploit food sources. In southern regions, brown skuas (Stercorarius antarcticus) and south polar skuas (S. maccormicki) prefer sub- grasslands, moorlands, and barren ground, including coastal on remote islands and the mainland, where they establish territories near penguin rookeries. These breeding sites generally avoid dense forests, as skuas thrive in open terrains that facilitate visibility and mobility essential for their predation strategies. During non-breeding periods, skuas shift to marine environments, utilizing open waters and pack edges across to equatorial latitudes, where they remain far offshore and out of sight of land. This pelagic preference supports their wide-ranging movements, with southern species often associating with dynamic oceanographic features like bathymetric fronts. Altitudinally, skuas tolerate low elevations, typically breeding between 0 and 500 meters above in cool, windy climates that match their physiological adaptations. Terrain elevation is a minor factor in nest site selection compared to proximity to prey colonies. In breeding grounds, summer temperatures of 5–15°C prevail, with individuals showing thermoregulatory stress above 9°C, indicating an upper thermal limit. Southern populations endure harsher conditions, including winter lows below −10°C during the breeding season onset and reliance on proximity for resource access. Human activities have altered skua habitats, particularly in less remote areas, though these birds generally favor isolated islands minimizing interference. In the , nesting sites face encroachment from agricultural expansion and waste-related ecosystem changes, potentially disrupting territory defense and prey availability. Near research stations, south polar skuas experience localized disturbances, including exclusion from construction and increased competition from human-associated food sources, leading to variable population responses such as localized growth from garbage scavenging.

Behavior and Ecology

Feeding Strategies

Skuas employ a range of opportunistic feeding strategies, with kleptoparasitism—commonly known as food piracy—serving as their primary tactic across many species. In this behavior, skuas aggressively pursue other seabirds, including gulls, petrels, auks, and penguins, harassing them in mid-air or on the ground until the victims drop or regurgitate their catch. This strategy is particularly well-developed in species like the Great Skua (Stercorarius skua) and parasitic jaeger (S. parasiticus), where individuals specialize in aerial chases to steal fish or other prey. Success rates in these pursuits vary by victim species, environmental conditions, and skua plumage phase, but can reach up to 79% for light-phase parasitic jaegers targeting puffins, though rates are often lower (around 30-63%) against more agile prey like guillemots. Complementing kleptoparasitism, skuas frequently resort to direct predation, targeting the eggs, chicks, and adults of smaller seabirds, as well as occasional small mammals near breeding colonies. For instance, South Polar Skuas (S. maccormicki) primarily prey on (Pygoscelis adeliae) eggs and chicks during the austral summer, using stealthy ground approaches to ambush nests. Scavenging forms another key method, with skuas feeding on carrion such as washed-up marine mammals or fishery discards, which can constitute a reliable source in nutrient-poor areas. Piscivory occurs sporadically via plunge-diving or surface-seizing techniques, particularly in pelagic species like the , which exploit fish schools or from trawlers. These alternative tactics allow skuas to adapt to fluctuating prey availability, with kleptoparasitism often yielding higher energetic returns than independent hunting in competitive environments. Feeding patterns exhibit seasonal shifts tied to breeding cycles and habitat transitions. During the breeding season, skuas prioritize terrestrial prey, such as penguin and eggs, to meet elevated energy demands near colonies, often defending feeding territories within rookeries. In contrast, non-breeding periods and winter months see a pivot to oceanic , emphasizing scavenging on carrion and discards alongside of migratory seabirds. This flexibility is evident in Great Skuas, where 76% of tracked individuals focus on marine including year-round, but breeding pairs increase colony-based predation to support chick-rearing. Energetic models suggest that stolen food can account for a majority of intake in kleptoparasite-specialized populations, underscoring the strategy's efficiency despite the costs of pursuit.

Breeding and Reproduction

Skuas typically form monogamous pairs that maintain strong, long-term bonds, often lasting for life, although may occur if breeding attempts fail repeatedly. These pairs exhibit high site fidelity, returning to the same nesting territories year after year. In the , breeding occurs during the summer months from May to July, while in the , it takes place from October to February, aligning with periods of extended daylight and food availability in polar and subpolar regions. Nests are constructed as simple scrapes on the ground, often lined with grass, moss, or pebbles, and situated in loose colonies with low densities of 1-6 pairs per km², depending on quality and proximity. Pairs aggressively defend territories around the nest, typically within a radius of 30-100 m, using displays, dives, and physical attacks to deter intruders, including other skuas and potential predators. Females lay clutches of two eggs, with an of 24-32 days shared by both parents; the female often performs more incubation early on, while males provide most during this phase. Hatching is asynchronous, with the second egg laid 1-3 days after the first, which can lead to competitive interactions among chicks. Chick-rearing is biparental, with both adults foraging to provision the young, often relying on or predation on eggs and chicks to meet nutritional demands. Chicks are altricial and brooded closely for the first few weeks, fledging after 25-50 days, during which time they remain dependent on parents for . Annual averages 0.5-1 fledgling per pair, heavily influenced by food availability; poor conditions can result in complete brood failure. In some species, such as the , asynchronous hatching promotes , where the dominant chick may kill or evict the subordinate, reducing brood size to one and enhancing offspring under resource limitation.

Migration and Movements

Skuas display varied migratory behaviors influenced by their breeding origins and species-specific adaptations. Species breeding in the , such as the long-tailed jaeger (Stercorarius longicaudus), are long-distance migrants that travel southward 10,000–15,000 km annually to reach and sub-Antarctic waters during the non-breeding season. In contrast, many species, including the (Stercorarius antarcticus), exhibit more sedentary tendencies, often remaining within sub-Antarctic zones but undertaking post-breeding dispersal to nearby productive areas. The (Stercorarius maccormicki), however, performs extensive trans-equatorial migrations northward to temperate and subtropical waters in the North Pacific and North Atlantic Oceans. Satellite and geolocation studies have elucidated these patterns, particularly for south polar skuas breeding in , . These birds follow looping routes that leverage westerly winds in the and northward, minimizing energy expenditure while largely skirting the low-productivity tropical doldrums. One-way migrations can span over 10,000 km, with annual round trips reaching up to 20,000 km or more, marking some of the longest avian migrations recorded. Juveniles show greater dispersiveness on initial migrations, often exploring wider areas and facing elevated mortality due to inexperience and environmental challenges. Philopatry is pronounced among skuas, especially in south polar populations, where adults exhibit high site fidelity to breeding colonies, with over 80% returning to the same nest sites in subsequent seasons. Despite this, vagrant south polar skuas have been documented in equatorial regions, potentially as part of irregular exploratory forays during non-breeding periods.

Species

Recognized Species

The genus Stercorarius comprises seven extant species of skuas, predatory seabirds divided into three smaller northern species commonly known as jaegers and four larger southern species known as skuas. These species exhibit distinct geographic ranges, with the jaegers breeding in the and regions and migrating to southern oceans, while the southern skuas are largely confined to and waters. The recognized species are as follows:
Scientific NameCommon NameAverage WeightIUCN StatusDistribution Notes
Stercorarius pomarinus500–800 gLeast ConcernBreeds in high of and ; winters in southern oceans.
Stercorarius parasiticus300–650 gLeast ConcernBreeds across northern Holarctic; highly migratory to tropical and southern seas.
Stercorarius longicaudusLong-tailed Jaeger250–350 gLeast ConcernBreeds in ; longest migrant among jaegers, wintering off southern continents.
Stercorarius maccormicki900–1,600 gLeast ConcernBreeds on coastal margins; non-migratory but disperses northward in winter.
Stercorarius antarcticus1,200–2,180 gLeast ConcernBreeds on and nearby islands; some populations sedentary.
Stercorarius chilensisChilean Skua1,100–1,700 gLeast ConcernBreeds in southern ( and ); disperses northward in non-breeding season.
Stercorarius skua1,200–2,000 gLeast ConcernBreeds in northern Atlantic and islands; pelagic in non-breeding season.
Skuas display a clear size gradient, ranging from the smallest long-tailed jaeger at approximately 250 g to the largest at up to 2.2 kg, with corresponding variations in body length and that influence flight and capabilities. The northern jaegers and southern skuas show strong geographic exclusivity, with minimal overlap outside migration periods, reflecting their phylogenetic divergence into two main clades within the genus. All species are currently assessed as Least Concern by the IUCN (as of 2025), though some southern populations face localized pressures from human activity. Recent taxonomic refinements, including the elevation of the from a of the complex, have been supported by genetic studies demonstrating distinct lineages, such as analyses from the 2010s.

Interspecific Relationships

Skuas engage in complex interspecific relationships characterized by aggression, hybridization, predation, , and scavenging, often influencing community dynamics in breeding and foraging areas. In mixed colonies along the , brown skuas (Stercorarius antarcticus) and s (S. maccormicki) exhibit conspecific aggression through territorial fights, particularly during breeding, as they compete for nesting sites in overlapping ranges. This aggression facilitates hybridization in a documented 500 km-wide hybrid zone, where up to 12% of breeding pairs consist of a male and a female, producing offspring with intermediate and morphology that blur boundaries. Symbiotic interactions include scavenging opportunities arising from seal predation, where brown skuas opportunistically feed on carrion from (Mirounga leonina) kills and placentas at subantarctic colonies such as , supplementing their diet without direct conflict. Antagonistic relationships are prominent with penguins, as south polar skuas prey on (Pygoscelis adeliae) eggs and chicks while employing to force regurgitation of or fish, reducing penguin reproductive success in shared breeding grounds. Skuas also compete with for food resources, with great skuas (S. skua) frequently chasing species like herring gulls (Larus argentatus) to steal catches, establishing dominance in coastal foraging areas. These interactions extend to broader ecological impacts, such as the displacement of smaller seabirds; aggressive by jaegers (northern skua relatives like the , S. parasiticus) can evict terns and from productive fishing spots, altering local prey availability and community structure in and zones.

Conservation

Population Status

Populations of skuas (family Stercorariidae) vary widely across species, with the three northern jaegers numbering in the millions of breeding individuals globally while the four southern skuas have much smaller populations in the tens of thousands. For the (Stercorarius skua), the breeding population is confined to and estimated at 16,300-17,200 pairs, equivalent to 30,000-35,000 mature individuals, with the majority (around 60%) breeding in , particularly the Islands. The (Stercorarius maccormicki) maintains a stable population of 10,000-20,000 individuals worldwide (as of 2018 estimates), with approximately 9,500 breeding pairs globally based on 2017-2018 surveys. (Stercorarius antarcticus) populations total 26,000-28,000 mature individuals, primarily in sub-Antarctic islands. Population trends vary regionally. In northern breeding areas, great skua numbers have historically been stable or increasing, supported by access to fisheries discards as a food source, contributing to a marked rise over the last century. However, recent sharp declines have occurred, with populations dropping by up to 90% in some colonies between 2021 and 2023 due to highly pathogenic outbreaks; as of 2024, numbers in colonies have shown slight increases but remain significantly below pre-2021 levels. In southern regions, trends are generally declining; for instance, populations have decreased in key sub- sites, with the largest known colony of the nominate experiencing a 47% drop linked to low breeding success. populations show stability or slight increases in monitored colonies, with annual growth rates of 1.3-1.9% observed in areas like Ryder Bay through the . Monitoring efforts rely on standardized colony censuses conducted during breeding seasons to count nests and pairs, often integrated into national programs like the UK's Seabird Monitoring Programme. At-sea surveys using vessel observations and tracking technologies complement these to assess non-breeding distributions and abundance. Key indicators include breeding success rates, measured by chick fledging per pair, which provide early warnings of population changes; for example, great skua productivity has been tracked annually in Shetland to detect fluctuations tied to food availability. These methods enable global assessments through collaborative efforts by organizations such as BirdLife International.

Threats and Management

Skuas face several major threats that impact their populations across their global range. Bycatch in longline and trawl fisheries is a primary , particularly for southern such as and south polar skuas, where interactions with baited hooks can lead to significant incidental mortality during foraging at . Invasive alien , including , cats, and foxes on breeding islands, pose the greatest overall threat to colonies, preying on eggs and chicks of skuas and exacerbating population declines in vulnerable sub- and sites. further compounds these pressures by altering prey availability; for instance, declines in due to loss and ocean warming indirectly affect south polar skuas, which rely on penguin prey that depend on . Human activities contribute additional risks to skua populations. In breeding grounds, localized egg collection and persecution historically impacted species like the Arctic jaeger, though these threats have declined with legal protections. , particularly ingestion, affects skuas as and predators; studies using regurgitated pellets from south Atlantic populations show persistent plastic loads in prey, with little reduction since the despite global efforts. Climate-driven range shifts are projected to cause substantial loss, with models predicting potential cessation of breeding for Arctic jaegers in Britain and by 2050 due to physiological intolerance to higher temperatures, while southern species like the Chilean skua may lose 9-27% of suitable areas under various emission scenarios. Conservation management focuses on mitigating these threats through targeted actions. Protected areas under the safeguard breeding sites for Antarctic and sub-Antarctic skuas, prohibiting harmful activities and supporting monitoring. Bycatch mitigation strategies, including bird-scaring lines (tori lines) and weighted sinkers on longlines, have been adopted in fisheries overlapping skua ranges, reducing mortality by up to 90% in some operations. International frameworks like the Convention on Migratory Species (CMS) promote cooperation for migratory skuas, while regional efforts in have stabilized populations through prey stock management and persecution controls, with colonies showing resilience post-avian influenza outbreaks via ongoing monitoring.

References

  1. https://pubmed.ncbi.nlm.nih.gov/36546866/
  2. https://nhpbs.org/wild/stercorariidae.asp
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  4. https://www.birds.cornell.edu/clementschecklist/introduction/updateindex/october-2024/
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