Devillers, P. 1978.
Distribution and relationships of South American skuas.
Le Gerfaut, 68: 374–417

Abstract

Two skuas of the Stercorarius skua superspecies breed in South America, S. antarcticus mainly in the Falklands, S. chilensis mainly in Fuegia and the Chilean Fjords. The interactions of the two forms and the limits of their respective ranges have been poorly known.

The Falkland Skua was studied in two large colonies of the Falkland Islands (New Island and Carcass Island). Adult plumages, individual variation, including frequency distribution of plumage types, juvenal plumage and behavior, particularly the Long-Call performance, are considered. The adult plumage is very variable, with extreme light and extreme dark types bridged by more numerous intermediates. All have blackish underwing coverts. The Long-Call performance is extremely similar to that of S. skua and differs sharply from that of S. maccormicki.

The Chilean Skua, studied in the Strait of Magellan, southern Tierra del Fuego, and the Chilean channels is also rather variable in adult plumage, though the specific traits of the species, cap, red or pale underwing coverts and throat, and bicolored bill are found in all types. Juveniles tend to be brighter, redder, than adults and do or do not have rufous edges to the feathers of the upperparts. The species is considerably less aggressive at the nest than other large skuas.

The Chilean Skua has been occasionally seen in the Falklands, a new occurrence being reported in this study. On the coast of Patagonia, Falkland Skuas breed alone at Punta Tombo and at least as far south as Camarones (Chubut). The characters of this population are very similar, though not quite identical, to those of Falkland antarcticus. At Puerto Deseado, Santa Cruz, breeds a mixed skua population, including S. antarcticus, S. chilensis and hybrids. Both parental forms occur side by side and apparently retain their specific behavior at the nest, antarcticus aggressive, chilensis not.

It is proposed that S. antarcticus and S. chilensis be considered distinct species on the basis of clear-cut morphological differences in all plumages, involving signal-characters, limited hybridization in the zone of contact, and very restricted, non-expanding, area of contact. This type of interaction is typical of many species of larids. S. chilensis is monotypic. S. antarcticus is very closely allied to S. lonnbergi, S. hamiltoni and S. skua. In particular, adult plumage of some individuals of the first three are practically indistinguishable, as are some juveniles of antarcticus, hamiltoni and skua. Some adult skua differ very little from hamiltoni. The four forms are probably best considered conspecific.

The evolutionary history of the large skuas is discussed. An orderly variation in amount of capping, and presence of red in the plumage, can be observed in the South American quadrant from light S. maccormicki through dark S. maccormicki, S. skua lonnbergi, S. s. antarcticus to S. chilensis. S. s. hamiltoni is related to both S. s. lonnbergi and S. s. antarcticus, S. s. skua to S. s. hamiltoni and/or S. s. antarcticus. A parallel is drawn with the distribution and history of the Imperial Shags, Phalacrocorax atriceps. It is suggested that the Atlantic coast of Patagonia was colonized overland by S. chilensis from the Chilean Fjords, while the presence of S. s. antarcticus in Chubut could be either a relict from glacial times or a recent recolonization from the Falklands.