Several different techniques have been used to study the diet composition of skuas and gulls in polar regions. In this study, we assessed the diet of great skuas using five techniques (pellets, prey remains, spontaneous regurgitates, observed feeds and water off-loading). The estimates of diet derived from each sampling technique during a single breeding season were generally similar. Although it is easy to collect large samples of pellets and remains, these over-estimate the amount of indigestible material in the diet. Prey remains provide a high degree of taxonomic accuracy, but may under-estimate prey swallowed whole. Water off-loading provides unbiased estimates of chick diet but only during a short period in the breeding season. It may also stress birds, and is labour-intensive. Spontaneous regurgitates are useful for assessing chick and adult diet but are difficult to collect systematically, and differing prey digestibility may bias results. Observed feeds are time consuming to record and over-represent easily identifiable prey. Technique-dependent biases highlight that sampling methods should be selected on the basis of sample sizes, time, taxonomic detail and age of study birds. Biomass may be estimated using pellets with correction factors, and also spontaneous regurgitates and water off-loading, but prey remains and observed feeds may be more inaccurate.

Assessing the diet of great skuas, Catharacta skua, using five different techniques

Breeding ecology of brown skuas (Catharacta antarctica lonnbergi) was studied at Bird Island, South Georgia in the austral summers of 2000/2001–2003/2004. A complete census recorded 467 breeding pairs in 3.55 km2 of suitable habitat (132 pairs per km2), and an additional 312 nonbreeders at club-sites. Comparison with previous counts indicates two phases of population change: an initial rapid increase (3.6% per annum) from the late 1950s to early 1980s, probably attributable to increased carrion availability from the expanding Antarctic fur seal (Arctocephalus gazella) population, followed by slower growth (0.9% p.a.). Currently, seal carrion dominates the diet of skuas during incubation, with a switch to seabird prey during chick-rearing. Breeding is now later, chick growth poorer, and productivity significantly lower than in the early 1980s. There is also a strong seasonal decline in adult attendance, and chicks that hatch later and are in poorer condition are less likely to fledge. These results suggest a long-term increase in competition for carrion that is particularly apparent once fur seal pupping has ceased.

Diet and long-term changes in population size and productivity of brown skuas Catharacta antarctica lonnbergi at Bird Island, South Georgia

Different organochlorine compounds (OCs) were measured in the blood of breeding south polar skuas (Catharacta maccormicki) at Svarthamaren, Dronning Maud Land (Antarctica) and compared to those in two species of northern hemisphere gulls:  the Arctic glaucous gull (Larus hyperboreus) and the subarctic great black-backed gull (Larus marinus). The skuas had 8% and 29% of the ∑OC levels (45 ng/g, wet weight) of glaucous gulls (591 ng/g) and great black-backed gulls (158 ng/g), respectively. Polychlorinated biphenyls (PCBs) and p,p‘-dichlorodiphenyldichloroethylene (p,p‘-DDE) were very low in skuas compared to northern gulls, but the mean hexachlorobenzene (HCB) level was 1.7 times higher than in great black-backed gulls and one-third of the glaucous gull level. Mirex levels in skuas were among the highest reported in birds, the mean level being 3 and 26 times higher than those in glaucous gull and great black-backed gulls, respectively. In skuas, the mean levels of HCB, oxychlordane, p,p‘-DDE, and PCBs incre...

Organochlorines in antarctic and arctic avian top predators: a comparison between the South Polar Skua and two species of northern hemisphere gulls.

Hybridisation between South polar skua (C. maccormicki) and Brown skua (C. antarctica lonnbergi )i n the area of the Antarctic Peninsula is known at least since the beginning of the last century but no survey has been done so far. This paper reviews information on the species composition of skua colonies of more than 10 pairs in the Antarctic Peninsula region, and the incidence of mixed pairs. Morphometrics, population size and breeding suc- cess were examined in detail at King George Island. The northward distribution of South polar skuas extended to King George Island (62� 11¢ S5 9� 00¢ W), with a small outlying population on Signy Island (60� 45¢ S4 5� 36¢ W), whereas Brown skuas did not breed further south than Anvers Island archipelago (64� 46¢ S6 4� 03¢ W). The proportion of mixed pairs was highest at the northern end of the 500-km-wide hybrid zone. Body size distribution of sympatric skuas from King George Island is clearly bimodal but overlaps considerably and hybrids cannot be identified. Skua population sizes at Potter Peninsula/King George Island remained stable between 1994 and 2004. Numbers of mixed breeding pairs fluctuated more strongly than those of pure species pairs. Breeding success of Brown skuas varied the least.

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Hybridisation between South polar skua ( Catharacta maccormicki ) and Brown skua ( C. antarctica lonnbergi ) in the Antarctic Peninsula region

South polar skuas, SPS, (Catharacta maccormicki) and brown skuas, BS, (C. antarctica lonnbergi) are regarded as opportunistic predators. When breeding in sympatry, BS feed mainly on penguin eggs and chicks, while SPS forage almost exclusively at sea. The objective of this study was to determine the diet composition of adult SPS and BS breeding in sympatry, in order to assess food resource partitioning between these species. The total number of food items consumed was 375 for BS and 682 for SPS in 1992–93, and 427 for BS and 579 for SPS in 1995–96. The pellets composition was significantly correlated between skua species for the same breeding season (rs = 0.67, p = 0.0062 and rs = 0.81, p < 0.001, for 1992–93 and 1995–96, respectively), and between breeding seasons for the same skua species (rs = 0.71, p = 0.001 and rs = 0.81, p < 0.001, for SPS and BS, respectively). Trophic niche breadth of BS was wider than that of SPS (BA(BS) = 0.28 and BA(SPS) = 0.24; Z = 7.67; p < 0.001). The trophic niche overlap between BS and SPS was over 65% in both breeding seasons. In agreement with other studies on the diet of these skua species in situations of sympatry, SPS consumed more fish and BS consumed more birds.

Diet of the south polar skua Catharacta maccormicki and the brown skua C. antarctica lonnbergi at Cierva Point, Antarctic Peninsula

South Polar skuas (Catharacta maccormicki) breed on Ardery Island in the absence of a local breeding population of Adelie penguins (Pygoscelis adeliae). Assessment was made of the food remains in skua feeding territories in 1995/1996. The diet of South Polar skuas largely consisted of fulmarine petrel species which bred on Ardery Island. Southern fulmar (Fulmarus glacialoides) remains were the predominant prey items found, and skuas appeared to feed preferentially on this species.

C. Barbraud

Papers

Foods of the South Polar skua Catharacta maccormicki at Ardery Island, Windmill Islands, Antarctica