Diet and long-term changes in population size and productivity of brown skuas <Emphasis Type="Italic">Catharacta antarctica lonnbergi</Emphasis> at Bird Island,South Georgia

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.     

Species‐specific foraging strategies and segregation mechanisms of sympatric Antarctic fulmarine petrels throughout the annual cycle

Determining the year‐round distribution and behaviour of birds is necessary for a better understanding of their ecology and foraging strategies. Petrels form an important component of the high‐latitude seabird assemblages in terms of species and individuals. The distribution and foraging ecology of three sympatric fulmarine petrels (Southern Fulmar Fulmarus glacialoides, Cape Petrel Daption capense and Snow Petrel Pagodroma nivea) were studied at Adélie Land, East Antarctica, by combining information from miniaturized saltwater immersion geolocators and stable isotopes from feathers. During the breeding season at a large spatial scale (c. 200 km), the three species overlapped in their foraging areas located in the vicinity of the colonies but were segregated by their diet and trophic level, as indicated by the different chick δ¹⁵N values that increased in the order Cape Petrel < Southern Fulmar < Snow Petrel. During the non‐breeding season, the three fulmarines showed species‐specific migration strategies along a wide latitudinal gradient. Snow Petrels largely remained in ice‐associated Antarctic waters, Southern Fulmars targeted primarily the sub‐Antarctic zone and Cape Petrels migrated further north. Overall, birds spent less time in flight during the non‐breeding period than during the breeding season, with the highest percentage of time spent sitting on the water occurring during the breeding season and at the beginning of the non‐breeding period before migration. This activity pattern, together with the δ¹³C values of most feathers, strongly suggests that moult of the three fulmarine petrels occurred at that time in the very productive high Antarctic waters, where birds fed on a combination of crustaceans and fish. The study highlights different segregating mechanisms that allow the coexistence of closely related species, specifically, prey partitioning during the breeding season and spatial segregation at sea during the non‐breeding season.     

Diet of the brown skua Catharacta skua lönnbergi on the Kerguelen archipelago: comparisons between techniques and between islands

The diet of the brown skua Catharacta skua lönnbergi was studied on the Kerguelen archipelago, during the chick-rearing period, over four breeding seasons (1987/1988, 1990/1991, 1992/1993 and 1993/1994). Prey remains and regurgitated pellets left by the breeding pairs were analysed and compared between two nearby and similar islands that mainly differ according to the presence or absence of the rabbit, since its eradication from one of them in 1992. Proportions of prey in diets varied between sampling methods (remains/ pellets), localities, years, breeding territories, breeding pairs and non-breeding individuals. The brown skua preyed upon few species of burrowing petrels and mainly on the blue petrel. Rabbits were exploited secondarily although they apparently reduced the predation pressure on the blue petrel. The eradication of rabbits from one of the islands, during a restoration programme, has not basically changed the trophic interactions between brown skuas and petrels.     

Whisker isotopic signature depicts migration patterns and multi-year intra- and inter-individual foraging strategies in fur seals

The movement and dietary history of individuals can be studied using stable isotope records in archival keratinous tissues. Here, we present a chronology of temporally fine-scale data on the trophic niche of otariid seals by measuring the isotopic signature of serially sampled whiskers. Whiskers of male Antarctic fur seals breeding at the Crozet Islands showed synchronous and regular oscillations in both their δ¹³C and δ¹⁵N values that are likely to represent their annual migrations over the long term (mean 4.8 years). At the population level, male Antarctic fur seals showed substantial variation in both δ¹³C and δ¹⁵N values, occupying nearly all the ‘isotopic space’ created by the diversity of potential oceanic habitats (from high Antarctica to the subtropics) and prey (from Antarctic krill to subantarctic and subtropical mesopelagic fishes). At the individual level, whisker isotopic signatures depict a large diversity of foraging strategies. Some seals remained in either subantarctic or Antarctic waters, while the migratory cycle of most animals encompassed a wide latitudinal gradient where they fed on different prey. The isotopic signature of whiskers, therefore, revealed new multi-year foraging strategies of male Antarctic fur seals and is a powerful tool for investigating the ecological niche during cryptic stages of mammals'' life.     

Long-term changes in population size,distribution and productivity of skuas (<Emphasis Type="Italic">Stercorarius</Emphasis> spp.) at Signy Island,South Orkney Islands

In this study, we investigate the numbers, productivity and territory distribution of the two species of skuas (brown Stercorarius lonnbergi and south polar Stercorarius maccormicki) breeding at Signy Island, South Orkneys, and compare the results with trends elsewhere. Comparison with previous counts indicates a biphasic increase in brown skuas at Signy Island; much faster from 1958/1959 to 1982/1983 (3.3 % per annum), than in subsequent years (0.4 % per annum from 1983/1984 to 2013/2014). Relative distribution of territories has changed little over time. The reduced rate of population growth in recent years was broadly coincident with a decrease in numbers of penguins (and therefore potential prey), which may also explain recent reductions in skua numbers at other Antarctic sites. As prey have become limiting, breeding success of brown skuas at Signy Island is now slightly lower than in the 1950s/early 1960s, but timing of breeding does not appear to have changed. Brown skuas at Signy Island may still have enough resources to start breeding, but as the season progresses and availability of resources declines, chick survival is reduced. South polar skuas have declined from ten pairs in 1982/1983 to one pair in 2013/2014, and mixed pairs have increased from one to three pairs. A review of the literature indicated that although population trend data are available for relatively few sites elsewhere in the subantarctic and Antarctic, numbers of brown skuas appear to be generally decreasing or stable, and of south polar skuas to be stable or increasing.     

Stable isotopes document the winter foraging ecology of king penguins and highlight connectivity between subantarctic and Antarctic ecosystems

The poorly known winter foraging ecology of the king penguin, a major Southern Ocean consumer, was investigated at the subantarctic Crozet Islands where the largest global population breeds. Blood δ¹³C and δ¹⁵N values were used as proxies of the birds’ foraging habitat and diet, respectively, and circulating prolactin levels helped in determining the birds’ reproductive status. Plasma prolactin concentrations showed that king penguin adults of unknown breeding status (n = 52) that were present at the colony in winter were in fact breeders and failed breeders, but were not non ‐breeders. Circulating prolactin was neither related to δ¹³C nor δ¹⁵N values, thus suggesting that both breeders and failed breeders used the same foraging habitats and fed on the same prey. Plasma and blood cell isotopic values depicted four new relevant biological features on the feeding strategies of king penguins during the critical winter period: (1) 42% of the birds foraged in the distant Antarctic Zone, but 58% fed primarily in subantarctic waters (δ¹³C), (2) they preyed upon myctophids in both zones (δ¹⁵N), (3) individuals were consistent in their foraging strategies over the winter months (δ¹³C and δ¹⁵N), and (4) a higher proportion of females (77%–80%) than males (27%–31%) favored feeding in distant Antarctic waters (δ¹³C). This study highlights trophic connectivity between subantarctic and Antarctic ecosystems and hence the key role of energy export from Antarctic waters to sustain breeding populations of subantarctic predators, including during the Austral winter.     

Changes in diets of individual Baltic ringed seals (Phoca hispida botnica) during their breeding season inferred from stable isotope analysis of multiple tissues

The stable isotope ratios (δ¹³C and δ¹⁵N) of three tissues with different metabolic rates (plasma, liver, and muscle) were used to investigate temporal variation in diet among nine individual Baltic ringed seals (Phoca hispida botnica Gmelin) from the Bothnian Bay, northeast Baltic Sea. The isotope values from plasma should reflect the most recent diet, values from liver the diet of the past weeks prior to sampling, and values from muscle should integrate diet over almost the entire breeding season of the ringed seals. In general, δ¹³C values of liver were more enriched in ¹³C than were those of either muscle or plasma, suggesting that the diet of the seals may have included a higher proportion of ¹³C‐enriched benthic prey in April. Females showed more variable δ¹³C values than males, suggesting possible gender differences in diet or in foraging locations. The differences that were apparent between females possibly reflect individual variation in the onset and duration of parturition and lactation, both of which likely restrict female foraging. Previous data from parasite infections and from alimentary tract contents of the same seals were linked to the isotope data to assist in drawing inferences about changes in the diets of individual seals.     

Isotopic tracking of foraging and long-distance migration in northeastern Pacific pinnipeds

We investigated the impact of foraging location (nearshore vs offshore) and foraging latitude (high vs middle) on the carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope compositions of bone collagen of northern fur seals (Callorhinus ursinus), harbor seals (Phoca vitulina), California sea lions (Zalophus californianus), and northern elephant seals (Mirounga angustirostris). Nearshore-foraging harbor seals from California had δ¹³C values 2.0‰ higher than female northern elephant seals foraging offshore at similar latitudes. Likewise, nearshore-foraging harbor seals from Alaska had values 1.7‰ higher than male northern fur seals, which forage offshore at high latitudes. Middle-latitude pinnipeds foraging in either the nearshore or offshore were ¹³C enriched by ∼1.0‰ over similar populations from high latitudes. Male northern elephant seals migrate between middle and high latitudes, but they had δ¹³C values similar to high-latitude, nearshore foragers. Female northern fur seal δ¹³C values were intermediate between those of high- and middle-latitude offshore foragers, reflecting their migration between high- and middle-latitude waters. The δ¹³C values of California sea lions were intermediate between nearshore- and offshore-foraging pinnipeds at middle latitudes, yet there was no observational support for the suggestion that they use offshore food webs. We suggest that their “intermediate” values reflect migration between highly productive and less-productive, nearshore ecosystems on the Pacific coasts of California and Mexico. The relative uniformity among all of these pinnipeds in δ¹⁵N values, which are strongly sensitive to trophic level, reveals that the carbon isotope patterns result from differences in the δ¹³C of organic carbon at the base of the food web, rather than differences in trophic structure, among these regions. Finally, the magnitude and direction of the observed nearshore-offshore and high-to middle-latitude differences in δ¹³C values suggest that these gradients may chiefly reflect differences in rates and magnitudes of phytoplankton production as well as the δ¹³C value of inorganic carbon available for photosynthesis, rather than the input of ¹³C-enriched macroalgal carbon to nearshore food webs. Received: 8 September 1998 / Accepted: 24 February 1999     

Prevalence of <Emphasis Type="Italic">Edwardsiella tarda</Emphasis> in Antarctic wildlife

For many years, the Antarctic region has been isolated from human activity. However, there is little data available regarding endemic and exotic diseases. The purpose of this work was to determine the prevalence of Edwardsiella tarda in Antarctic wildlife, including birds, mammals and fish. During the summer of 2000 and 2002 in the Potter Peninsula, and during the summer of 2001 and 2003 in Hope Bay, a total of 1,805 faecal samples from Antarctic animals and 50 infertile eggs of Adelie penguins (Pygoscelis adeliae) were collected in order to isolate E. tarda. The classic Edwardsiella tarda was isolated from 281 (15.1%) of the 1,855 Antarctic wildlife samples. This is the first report of E. tarda isolation from southern giant petrels (Macronectes giganteus), brown skuas (Stercorarius lonnbergi), south polar skuas (Stercorarius maccormicki), kelp gulls (Larus dominicanus), greater sheathbills (Chionis albus), chinstrap penguins (Pygoscelis antarctica), eggs of Adelie penguins and Weddell seals (Leptonychotes weddelli). None of the evaluated animals showed clinical signs of disease. Our results suggest that E. tarda is a common bacterium amongst Antarctic birds and mammals.     

One House Two Families: Petrel Squatters Get a Sniff of Low‐Cost Breeding Opportunities

Burrowing is a widespread nesting behaviour, found in vertebrates and invertebrates. It is particularly common in small procellariiform seabirds such as blue petrels (Halobaena caerulea) and Antarctic prions (Pachyptila desolata), two closely related petrel species. However, digging a burrow is costly and alternative strategies may evolve. Accordingly, blue petrel males can adopt two alternative nesting strategies: digging a new burrow or squatting in an empty one. Importantly, a blue petrel squatter arriving at the colony to breed is more likely to find empty Antarctic prion burrows than empty blue petrel burrows, since the former species only start breeding a month later. However, squatting in a prion’s burrow is risky for blue petrels as the legitimate owner very often returns and claims the burrow back, thus ruining the squatter’s breeding attempt. We present here results of a survey of two sympatric colonies of blue petrels and Antarctic prions on Kerguelen Island. Our data show that blue petrel squatters preferentially occupy blue petrel empty burrows. To investigate potential underlying mechanisms behind this preference, we used a simple Y‐maze design to show that blue petrels can discriminate and prefer their specific odour over the prion odour. Our results confirm the existence of alternative burrowing strategies in blue petrels and suggest that squatters could use olfaction to avoid the less suitable Antarctic prion burrows.     

Skuas (Stercorarius spp.) moult body feathers during both the breeding and inter‐breeding periods: implications for stable isotope investigations in seabirds

Seabirds are mostly thought to moult during the inter‐breeding period and the isotopic values of their feathers are often therefore assumed to relate to their assimilated diet during such periods. We observed Brown Skuas Stercorarius antarcticus lonnbergi and South Polar Skuas Stercorarius maccormicki moulting on a breeding site at King George Island, Antarctica. This raises concerns about the reliability of using stable isotopes in feathers to infer feeding localities of birds during the inter‐breeding period. We analysed the δ¹³C and δ¹⁵N values of growing and fully grown body feathers collected from the same individuals. For both species, δ¹³C values of growing feathers indicated feeding areas in the Antarctic zone (breeding grounds), whereas most fully grown feathers (100% for South Polar Skuas and 93.3% for Brown Skuas) could be assigned to northern latitudes (non‐breeding grounds). However, a few fully grown body feathers of Brown Skuas (6.7% of the feathers, belonging to two birds) showed isotopic values that indicated moult in the Antarctic zone. As the growth period of those feathers was unknown, they could not be used with confidence to depict the foraging behaviour of the birds during the non‐breeding period. Although precautions must be taken when inferring dietary information from feathers in seabirds where the moulting pattern is unknown, this study shows that if the development stage of a feather (growing/fully grown) is identified, then dietary information from both breeding and non‐breeding seasons can be obtained on the same individual birds.     

Ecological drivers of change at South Georgia: the krill surplus,or climate variability

Macaroni penguins Eudyptes chrysolophus are thought to be one of the most important mesopredators in the Southern Ocean having a greater impact on prey availability and abundance than any other seabird species. Their population centre has long been held to be South Georgia where populations were thought to comprise many million animals. Here we report the results of a recent census of the macaroni population at South Georgia undertaken using aerial survey methods. We report dramatic declines in numbers (～1.0 million breeding pairs) compared to numbers observed in the late 1970s (～5.4 million pairs), but show that these reductions have occurred principally at sites where numbers had previously been very large. During the breeding season, the main foraging grounds of birds from these sites overlap with the foraging grounds of Antarctic fur seals Arctocephalus gazella, a major competitor for their principal prey, Antarctic krill Euphausia superba. We suggest that the redistribution of the macaroni penguin population at South Georgia reflects the recent recovery of fur seal populations and thus the ongoing consequences of human intervention at South Georgia, a process which started more than 2 centuries previously. The implied resource competition and the observed population changes may also be exacerbated by recent reductions in Antarctic krill abundance which have been linked with reductions in seasonal sea ice following recent, rapid, regional warming in the Antarctic; however, the recovery of fur seal populations, and the ongoing recovery of krill‐eating whale populations argues that tropho‐dynamic interactions may be sufficient to explain the observed changes.     

Penguins, fur seals, and fishing: prey requirements and potential competition in the South Shetland Islands, Antarctica

Antarctic and sub-Antarctic seabirds, marine mammals, and human fisheries concentrate their foraging efforts on a single species, Antarctic krill (Euphausiasuperba). Because these predators may have a significant effect on krill abundance, we estimated the energy and prey requirements of Adelie (Pygoscelisadeliae), chinstrap (Pygoscelisantarctica), and gentoo (Pygoscelispapua) penguins and female Antarctic fur seals (Arctocephalusgazella) breeding on the South Shetland Islands, Antarctica and compared these estimates with catch statistics from the Antarctic krill fishery. Published data on field metabolic rate, population size, diet, prey energy content, and metabolic efficiency were used to estimate prey requirements of these breeding, adult, land-based predators and their dependent offspring. Due to their large population size, chinstrap penguins were the most significant krill predators during the period examined, consuming an estimated 7.8 × 10⁸ kg krill, followed by Adelie penguins (3.1 × 10⁷ kg), gentoo penguins (1.2 × 10⁷ kg), and Antarctic fur seals (3.6 × 10⁶ kg). Total consumption of all land-based predators on the South Shetland Islands was estimated at 8.3 × 10⁸ kg krill. The commercial krill fishery harvest in the South Shetland Island region (1.0 × 10⁸ kg) was approximately 12% of this. Commercial harvest coincides seasonally and spatially with peak penguin and fur seal prey demands, and may affect prey availability to penguins and fur seals. This differs from the conclusions of Ichii et al. who asserted that the potential for competition between South Shetland predators and the commercial krill fishery is low. Received: 26 August 1997 / Accepted: 16 December 1997     

Year‐round movements of sympatric Fork‐tailed (Oceanodroma furcata) and Leach's (O. leucorhoa) storm‐petrels

Long‐distance movements are characteristic of most seabirds in the order Procellariiformes. However, little is known about the migration and foraging ranges of many of the smaller species in this order, especially storm‐petrels (Hydrobatidae). We used Global Location Sensors to document the year‐round movements of sympatrically breeding Fork‐tailed Storm‐Petrels (Oceanodroma furcata) and Leach's Storm‐Petrels (O. leucorhoa) from the Gillam Islands located northwest of Vancouver Island, British Columbia, Canada. In 2016, breeding Fork‐tailed (N = 5) and Leach's (N = 2) storm‐petrels traveled maximum distances of ~1550–1600 km from their colony to a region that has a wide shelf with major canyons creating a highly productive foraging area. After the breeding season, Fork‐tailed Storm‐Petrels (N = 2) traveled to similar areas west of the Gillam Islands, a maximum distance of ~3600 km from the breeding colony, and remained in the North Pacific Ocean and north of the Subarctic Boundary for an average of 5.4 mo. Post‐breeding Leach's Storm‐Petrels (N = 2) moved south to the Eastern Tropical Pacific, west of central Mexico, Ecuador, and northern Peru, an estimated maximum distance of ~6700 km from their breeding colony, and remained there for an average of 7.2 mo. Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analyses of feathers revealed niche separation between Fork‐tailed (N = 21) and Leach's (N = 53) storm‐petrels. The wide range of δ¹⁵N values in the feathers of Leach's Storm‐Petrels (N = 53) suggests that they foraged at a variety of trophic levels during the non‐breeding season. Our results demonstrate that storm‐petrels have large core foraging areas and occupy vast oceanic areas in the Pacific during their annual cycle. However, given the coarse precision of Global Location Sensors, additional study is needed to identify the specific areas used by each species during both breeding and non‐breeding periods.     

Feeding habits of three seal species at the Danco Coast, Antarctica: a re-assessment

The analysis of prey overlap among Weddell, Antarctic fur and leopard seals was conducted using fecal samples collected at the Danco Coast, Antarctic Peninsula, in 1998 and 2000. The re-occurrence of prey species was moderate in samples collected in 1998, and low in 2000, and reflects resource partitioning among seal species. Prey species that mostly co-occurred in seals’ diet were the Antarctic krill Euphausia superba, bivalves, and the myctophids Gymnoscopelus nicholsi and Electrona antarctica. A dietary similarity index of prey overlap has been calculated and demonstrates evident fluctuations in pairwise comparisons between the seal species. The highest and lowest values of prey overlap were observed between Antarctic fur seals and leopard seals, and between Weddell seals and leopard seals, respectively. Prey overlap between Antarctic fur seals and Weddell seals was moderate in both seasons.     

The diet of the Antarctic fur seal <Emphasis Type="Italic">Arctocephalus gazella</Emphasis> at the South Orkney Islands in ten consecutive years

To improve the knowledge on the Antarctic fur seal foraging behavior, evaluate whether changes in its diet are associated with changes in prey availability, and evaluate whether fisheries had negative impacts on Antarctic fur seal populations, a total of 1359 scats of non-breeding males were collected in ten consecutive sampling periods between 1994 and 2003 at Laurie Island, South Orkney Islands. Antarctic krill was the most numerous prey throughout the sampling period followed in importance by fish prey. Antarctic krill also predominated by reconstructed mass, except during 1994 and 1998 when penguins were the most important prey, and during 1995 when fish dominated. Among fish, demersal-benthic species (mainly Gobionotothen gibberifrons, Chaenocephalus aceratus and Chionodraco rastrospinosus) dominated the diet except in 1997, 1998, 1999 and 2002 when myctophids (mainly Gymnoscopelus nicholsi and Electrona antarctica) were the most important fish prey. In seasons when fish dominated the diet, the diet was primarily comprised of demersal species. Although our results are in overall agreement with previous studies, the contribution to the diet of the main preys changed throughout the sampling period, probably in parallel with changes in prey abundance. According to historical results on fish consumption, the fisheries carried out in the past have had negative impacts on seal populations from the South Orkney Islands.     

Feeding territoriality and the reproductive consequences in brown skuas<Emphasis Type="Italic"> Catharacta antarctica lonnbergi</Emphasis>

In the maritime Antarctic, brown skuas (Catharacta antarctica lonnbergi) show two foraging strategies: some pairs occupy feeding territories in penguin colonies, while others can only feed in unoccupied areas of a penguin colony without defending a feeding territory. One-third of the studied breeding skua population in the South Shetlands occupied territories of varying size (48 to >3,000 penguin nests) and monopolised 93% of all penguin nests in sub-colonies. Skuas without feeding territories foraged in only 7% of penguin sub-colonies and in part of the main colony. Females owning feeding territories were larger in body size than females without feeding territories; no differences in size were found in males. Territory holders permanently controlled their resources but defence power diminished towards the end of the reproductive season. Territory ownership guaranteed sufficient food supply and led to a 5.5 days earlier egg-laying and chick-hatching. Short distances between nest and foraging site allowed territorial pairs a higher nest-attendance rate such that their chicks survived better (71%) than chicks from skua pairs without feeding territories (45%). Due to lower hatching success in territorial pairs, no difference in breeding success of pairs with and without feeding territories was found in 3 years. We conclude that skuas owning feeding territories in penguin colonies benefit from the predictable and stable food resource by an earlier termination of the annual breeding cycle and higher offspring survivorship.Research licence: Umweltbundesamt Bonn 13.4-94003-1/5-7.     

Fur seals at Macquarie Island: post-sealing colonisation,trends in abundance and hybridisation of three species

Commercial sealers exterminated the original fur seal population at Macquarie Island in the early 1800s. The first breeding record since the sealing era was not reported until March 1955. Three species of fur seal now occur at Macquarie Island, the Antarctic (Arctocephalus gazella), subantarctic (A. tropicalis) and New Zealand (A. forsteri) fur seal. Census data from 54 breeding seasons in the period 1954–2007 were used to estimate population status and growth for each species. Between the 1950s and 1970s, annual increases in pup production for the species aggregate were low. Between 1986 and 2007, pup production of Antarctic fur seals increased by about 8.8% per year and subantarctic fur seals by 6.8% per year. The New Zealand fur seal, although the most numerous fur seal species on Macquarie Island, has yet to establish a breeding population, due to the absence of reproductively mature females. Hybridisation among species is significant, but appears to be declining. The slow establishment and growth of fur seal populations on Macquarie Island appears to have been affected by its distance from major population centres and hence low immigration rates, asynchronous colonisation times of males and females of each species, and extensive hybridisation.     

Marine foraging and annual fish consumption of a south polar skua population in the maritime Antarctic

Pelagic fish are an important component of Antarctic food webs but few quantitative data exist on energy transfer from fish to seabirds for the Seasonal Pack-ice Zone. We studied a local population of south polar, skuas Catharacta maccormicki during a whole breeding cycle and estimated its entire annual food consumption. The lengths of foraging trips suggested that skuas foraged in an area of 817 km² of coastal waters around the breeding site. Their fish prey consisted almost entirely of two pelagic species, Electrona antarctica and Pleuragramma antarcticum, with individual mean energy contents of 28.62 and 30.26 kJ/g dry weight and body masses of 4.6 and 10.9 g, respectively. Total energy budget estimates of the entire south polar skua population resulted in 3 and 5 tons of pelagic fish caught per season (1994 and 2001, respectively), wherein a single breeding pair raising two chicks requires approximately 115.7 kg E. antarctica and 24.4 kg P. antarcticum. Our study suggests that the pelagic fish in coastal areas are highly important for surface feeding seabirds in the maritime Antarctic.     

Exploitation of inland-breeding Antarctic petrels by south polar skuas

During the austral summer of 1996/1997 we studied south polar skuas at Svarthamaren, Dronning Maud Land, Antarctica, where the world's largest known colony of Antarctic petrels is found. Our censuses suggested approximately 250 full-grown skuas and 140,000 breeding pairs of petrels were present. During their breeding season, skuas did not visit the open sea at least 200 km from the site; they relied entirely on prey caught and scavenged from the petrel colony. Because the site is so isolated, we asked whether the prey (petrels) had swamped the predators (skuas), or whether there was evidence that predator numbers were limited by the size of the prey population. Particularly at the end of the petrel incubation period, we found a close correspondence between the energy required by adult skuas and their chicks, ascertained from time budget studies, and the rate at which petrel eggs disappeared from the colony. This suggests that, in this closed system, the predator population was limited by the prey population, and that predator swamping was not an advantage that petrels gained by nesting in this remote location. Received: 12 April 1999 / Accepted: 30 June 1999