Chilean skua

Stercorarius chilensis is a skua that breeds on the coast and islands of southern Chile and in southern Argentina, as far as Cape Horn ^[1],[2]. Outside the breeding season it disperses northward and reaches Peru, where it is regarded as a rare visitor to the coast ^[3],[2]. It steals food from other seabirds ^[3], and also eats eggs, chicks, carcasses and waste from human activities ^[2].

The word skua comes from the Faroese skügver, meaning a tuft of feathers, and from the island of Skúvoy in the Faroe Islands ^[4]. In Chile these birds are called salteadores, meaning robbers, for their habit of harassing other birds to take their food ^[4].

It is 55 to 61 cm long. The upperparts are greyish brown with small whitish and ochre spots, the neck has yellowish or whitish streaks and the wing carries a white patch ^[4].

Among skuas it stands out for a cinnamon body that contrasts with a uniformly blackish-brown cap; the sides of the neck and the mantle have streaks paler than the rest of the plumage, and the pale flashes under the primaries are narrower than in other skuas ^[1]. Adults differ from immatures in overall colour, a grey bill with a black tip and streaked rather than barred wings ^[1]. An adult examined in Brazil measured 544 mm in total length, 57 mm in bill and 382 mm in wing ^[1].

The upperparts are greyish brown and contrast with the underparts, including the wing coverts, which are cinnamon chestnut with yellowish or whitish streaks ^[2]. Where it breeds alongside the brown skua there are hybrids, some with a reddish or cinnamon chest and belly and dark wing coverts ^[2].

Similar species

• Stercorarius antarcticus: It is larger (52 to 64 cm), blackish brown above and paler below ^[4]; the head is uniformly dark, whereas the Chilean skua has cinnamon-chestnut underparts ^[2].
• Stercorarius parasiticus: It is smaller (41 to 50 cm) ^[4] and off Valparaiso it was seen closer to the coast, within the first 8 km, than the large skuas ^[3].

It breeds on the coast and islands of centre-southern Chile, including Cape Horn, and in southern Argentina, from Golfo San Jorge to the Beagle Channel in Tierra del Fuego ^[2]. It is marine but not pelagic: it stays tied to the coast and uses southern fjords and channels ^[4].

Outside breeding it occupies the South American coast between central Peru and northern Argentina, with occasional records in Ecuador, Brazil, Uruguay and Antarctica ^[1]. Off Valparaiso, at 33° S, it was recorded all year round, within the first 10 to 12 km from the coast ^[3]. In July 2011 a debilitated adult was rescued on a beach in Espírito Santo, in south-eastern Brazil, the first documented record for that state ^[1].

It breeds from October or November to March or April on the coasts and islands of centre-southern Chile and southern Argentina ^[2]. In Laguna de los Cisnes National Park, in Tierra del Fuego, it was recorded as a summer resident that nests, preferring open and exposed ground, above all the high, bare plateaus of the lagoon's islands ^[4].

The nest is made of plant debris from rushes and the egg is oval, olive green at the base with irregular brown blotches; adults can be seen in groups of 5 to 8 individuals ^[4]. The number of breeding pairs is unknown and there is no information on its breeding success ^[1].

The extent and timing of its migration are poorly known ^[3]. Off Valparaiso densities were lower in summer, when the birds are at their colonies, and rose between August and October, which suggests a staging area before breeding ^[3].

It is an opportunistic kleptoparasite: off Valparaiso it stole food from birds of similar size, such as shearwaters and fulmars, and from larger ones, such as boobies, including the Peruvian booby, and it also associated with 9 seabird species to steal, feed or rest ^[3]. It was almost always seen alone, and in groups of up to 3 individuals ^[3].

According to the studies summarised in an Argentine paper, in the breeding season it preys on gentoo penguin colonies on Isla Martillo and rockhopper penguin colonies on Isla de los Estados, taking eggs, chicks and carcasses; it also eats fish, human waste and seal carcasses in Tierra del Fuego ^[2]. In the southern Pacific it feeds on coastal invertebrates and on refuse at rubbish dumps ^[2].

Between 2008 and 2014, aboard high-seas trawlers in the Argentine Sea, 45 individuals were recorded in 27 sightings, mostly between 53 and 55° S ^[2]. Its relative occurrence was 3.8 % of counts on freezer trawlers (n = 545) and 0.4 % on fresh-fish trawlers (n = 1126); its abundance correlated with the fishing effort of the freezer fleet ^[2].

It is estimated at about 6000 mature individuals, although the number of pairs is not known ^[1]. The population is believed to be stable, but there are no systematic assessments of its trend ^[2].

No bycatch or incidental mortality associated with the trawlers was recorded between 2008 and 2014 ^[2]. However, because it regularly associates with those vessels in summer, fishing could affect it during the critical chick-rearing period ^[2].

In Peru it is a rare visitor to the coast, according to a 2014 study citing earlier works ^[3]. In its non-breeding season it disperses north as far as northern Peru, according to a study from 2017 ^[2].