Although often getting a bad press from Antarctic visitors because they attack penguins, skuas are an extraordinarily interesting group for scientists, particularly those interested in behaviour and evolution. Currently of special interest to scientists is their origin and relationship to the gulls, their evolution into two or three rather different groups of species, the occurrence in at least one species of cooperative breeding, which is otherwise unknown among seabirds, and in at least one species the occurrence of siblicide, in which the older chick expels or kills the younger one.
Whereas the gulls have long had an established taxonomy, with little argument about relationships, there is not yet any consensus about how the skuas are related and how they should be named. In part this uncertainty derives from their distribution, small to moderate numbers of birds spread widely among islands and continental habitats, in part because at least for the great skuas, there has apparently been a very recent (in evolutionary terms) radiation across the Southern Hemisphere, with a single group in the Northern, so that there is not yet great distinctiveness.
With the advent of new genetic technologies and the cladistic approach to arranging systematic affinities there has been an upsurge of interest in the redefinition of species and higher classifications of the group.
In the past it was recognised that there were two groups of skuas - the great skuas, big, heavy, brown birds, with several species in the Southern Hemisphere and one on North Atlantic islands, and the small skuas or Jaegers, smaller, lighter, and often with a patterned and barred plumage, and, when breeding, long tail feathers. Some authors placed the great skuas in the genus Catharacta, the smaller ones in Stercorarius but others tended to put them all into Stercorarius. So this is the first problem. Are they a single natural group, or two groups with separate lineages? The second problem is how to arrange and group the more or less distinct species. For the jaegers this has not proved to be too difficult; there seem to be three species which are quite distinct and which do not naturally interbreed even though two, or even all three, may breed in the same place. No such certainty exists for the great skuas. They are all rather similar, and several of the more distinctive ones confuse systematic ordering by hybridising freely.
The most recent attempt to define the systematics of the entire group brought together experts from genetics, evolution, behaviour and parasitology (Cohen and others (1997), Proc. R. Soc. Lond B 264:181–190). Sad to say, this high powered attempt to resolve the problems once and for all, seems to have raised as many difficulties as it solved. The detailed genetical analysis confirmed, as behavioural ecologists had considered for some time, that the Pomarine Skua seemed to share characters of both the great skuas and jaegers. Indeed, this analysis indicated that its closest relative was the northern hemisphere Great Skua, not the other two jaegers. They were not able to explain the mix of genes they observed in this species, and since then there have been a rush of papers by other authors attempting to do so. The second feature that leaves the great skuas in uncertain relationship is that while Great Skuas and South Polar Skuas could be differentiated the other four taxa they recognised were too similar to be placed on an evolutionary sequence within their clade. Their relative lack of genetic differentiation was explained on the basis of their recent invasion of the southern region, and because there are relatively large populations of wide-ranging birds. This lack of differentiation, together with the close affinity between the northern Atlantic Great Skua and the Pomarine Skua, indicate a Northern Hemisphere origin for the great skuas, not a Southern Hemisphere one, with just one equator–crosser, as has been traditionally considered. A possible scenario is that a common population of great skuas in mid latitudes when boreal climates were inhospitable divided into northern and southern hemisphere populations, with later differentiation in the south.
How then does this analysis of translate into skua names and affinities? The analysis indicates that skuas and jaegers form a natural clade, with two subclades — one of the great skuas plus the Pomarine Skua, the other of the two smaller jaegers. But what should these two subclades be called? Two options are suggested. For those who think the great skuas are very different from the jaegers, and want to indicate this, then pomarinus would need to be placed in the genus Catharacta. For those who think that there is really only one clade, and that this is demonstrated by the intermediate status of pomarinus, then all skuas and jaegers fall within a single genus, which from priority of naming is Stercorarius. Until more research is done and a consensus about the status of pomarinus is reached it is, therefore, a matter of personal choice how these birds are named - whether the classifications shown in 1 or 2 are used. Malte Andersson, one of the most experienced and regarded skua biologists prefers (1999) 2. Bob Furness, whose book (R. W. Furness, 1987, The Skuas. Poyser) is the standard text on skuas and jaegers preferred (in 1987) two genera. I find the analyses done in the Cohen paper so compelling that after a lifetime of using two genera I am now switching to using Stercorarius for both skuas and jaegers. But it might in fact become rather more complex than this with suggestions from one recent author that there should be three genera so that the special status of pomarinus. can be recognised.
The various classifications of skuas and jaegers and preferred common names are shown in the following table.
Hybridisation between lonnbergi and maccormicki occurs along the Antarctic Peninsula and islands, between chilensis and antarctica in Patagonia, and between chilensis and maccormicki on the South Shetland Islands.
As always in ornithology care needs to taken in the use of common names, particularly of the use of Antarctic and Subantarctic, and neither of these names should be used without their scientific ones. The common names shown in the table are those currently most in use. There is some confusion as well in the literature about whether some of the great skuas should be grouped together within the subgenus skua but this seems over elaborate at the present time when the taxonomy overall is in such flux.
The skuas seen in New Zealand
Paradoxically the most commonly seen skua around the coasts and harbours of the New Zealand mainland is the Arctic Skua from Northern Hemisphere. This is a common bird in our harbours during summer and usually comes to notice while harrying gulls and terns. The only skua breeding on New Zealand islands is the Brown Skua, but this is rarely seen around the mainland, indeed the South Polar Skua is more likely to be seen during its annual migration overwinter between Antarctica and the northern Pacific Ocean, following a similar path to the Sooty Shearwaters. Of the several hundred breeding Brown Skuas and 1200 fledglings banded on the Chatham Island in the past 25 years only one has been recorded subsequently from the New Zealand mainland. Brown Skuas on temperate islands apparently do not move far from their breeding grounds.
Siblicide in Skuas
In the South Polar Skua, uniquely within the family, and almost without precedent within sea birds, lethal sibling aggression is common and results in brood reduction from two hatchlings to a single chick. In this siblicide the first hatched chick attacks the second one within a day or two of its hatching and either chases it from the nest area (the most common result) or maims it so severly that it dies. Parents may or may not intervene. Siblicide is common in boobies, herons, and raptors and is considered in these groups to be an evolutionary response to predictable food shortage later in the chick rearing period. Its common occurrence among Antarctic skua populations suggests a similar selective factor is operating, with attacks triggered by hunger or malnutrition. Much research remains to be done on this phenomenon, especially of chick behaviour, parental response, and incidence in relation to food stress through the season.
Distribution and abundance
Surveys need to be carried out of skua abundance and distribution on all New Zealand southern islands. At the same time as this is being done an attempt should be made to assess diet — in most places this skua relies on burrowing petrels.
Cooperative breeding in lonnbergi skuas
Also uniquely for the family, and again without precedent for sea birds, a fair proportion of the skuas on Chathams, Snares and Stewart islands breed in cooperative groups. Long term research of the skuas on Mangere and Rangatira islands in the Chathams has found that these groups are very stable entities, lasting for many years with the same members, and comprise a single female and two to several males. The males share offspring paternity, both between and within years. These groups are not made up of family members so that kin-selection, which is the basis for coooperation in other cooperative species where parents are assisted by offspring to raise following broods, cannot be implicated in this unusual breeding system.
It was expected that because of the assistance that the extra males could provide that groups would be more successful in raising chicks than would pairs and that their chicks would grow faster, be more robust and more likely to survive to breeding. Research so far has not been able to establish either of these predictions. Nor has it been possible to show that the groups form because of lack of breeding space or breeding partners.
Although this long term study has finished it is worth recording that there is still a great deal to learn about this breeding system. Comparative study of natural populations in natural habitats would be of considerable value.
In summary, skuas are big robust birds occurring in a wide range of habitats and exploiting diverse foods. They are tough, totally admirable, intelligent and opportunistic birds, proving excellent subjects for professional research and bird watching where ever they occur. Only those who consider nature benign, or want it sanitised, would find them despicable and villanous. They should be judged as we judge the hawks and eagles, which after all live and forage in much the same way, but whose killing and scavenging is less obvious. The great thing about skuas is that they are very noisy and always easy to see.
Other common names: —
Antarctic skua, MacCormick’s skua, Catharacta maccormicki.
59 cm., male 1275 g., female 1325 g., like brown skua but smaller, shorter black bill, yellow on hind neck, variable plumage from pale to all dark, all have prominent white wing flashes, legs and feet black.
Where to find: —
Breeds Antarctica, usually near seabird colonies; Ooccasionally seen of the New Zealand coast while migrating to the north Pacific from January to April.
Credit for the photograph: —
E.C. Young, Emeritus Professor of Zoology, Auckland University.
Illustration description: —
E.C. Young, The Millennium Bird.
Heather, B., & Robertson, H., Field Guide to the Birds of New Zealand, 2000.
E.C. Young, Emeritus Professor of Zoology, Auckland University. email@example.com
Page date & version: —
Wednesday, 4 June 2014; ver2009v1