The effect of food availability on the foraging behaviour of breeding Great Skuas Catharacta skua and Arctic Skuas Stercorarius parasiticus

Seabirds' behavioural activity budgets have been proposed as an indicator of changes in marine food supplies. Reduced recruitment and a low biomass of sandeels Ammodytes spp. around Shetland in the late 1980s were associated with a dramatic increase in the foraging effort of breeding skuas. We present data on the territorial attendance patterns of breeding Great Skuas Catharacta skua and Arctic Skuas Stercorarius parasiticus in Shetland in 1987 and examine three aspects of the recorded increase in foraging effort.
Time budget studies indicate that recording territorial attendance by infrequent, daily spot observations provides an accurate measure of the time spent off-territory by both skua species. None of the pairs of Arctic Skua studied ever left their chicks unattended in 1987. The same was true of 10 out of 27 pairs of Great Skuas. Although the remaining pairs of Great Skuas frequently left their chicks unprotected, evidence of pair co-ordination to minimise this suggests that their foraging schedules were still constrained by the need to protect their offspring. During periods of high chick-demand, variation between pairs' foraging efforts largely reflected variation in those of the females. Thus, we suggest that increased chick neglect in years of food shortage arises as females abandon their role of guardian for that of provider. By the end of the breeding season in 1987, both male and female Great Skuas foraged from 65 to 89% of daylight hours. The lack of any further increase in the foraging effort of Great Skuas in subsequent years of poorer reproductive success was almost certainly because time and energy constraints prevented either sex from doing so. We suggest that the comparatively short amount of time spent foraging by Arctic Skuas in 1987 may reflect an advantage conferred by their role as specialist kleptoparasites.     

The effect of age and year on the survival of breeding adult Great Skuas Catharacta skua in Shetland

The survival rates of breeding adult Great Skuas Catharacta skua were examined at Foula, the largest colony in the world, where numbers have been declining since the late 1970s. Resightings of colour-ringed breeding adults over a 12-year period were analysed using Cormack-Jolly Seber models to estimate survival rates. Annual survival rates averaged 0.89 but varied among years between 0.82 and 0.93, with annual variations being temporally associated with variations in sandeel abundance during the breeding season. Most birds appeared to die outside the breeding season and so it is possible that nutritional stress and reproductive costs of breeding in years of poor food supply affect survivorship on migration or in the wintering range. Survival rates of adult Great Skuas were affected by their age according to a quadratic equation, with survival increasing significantly with age from 0.73 in 5-year-old-birds to between 0.85 and 0.96 in birds from 7 to 22 years old, with a sharp decline to between 0.75 and 0.87 in birds over 22 years old. Year effects were evident when controlling for age, indicating that annual variations in survival rates are not explained by changes in age-composition of the marked population among years.     

Chick loss in the Falkland Skua Catharacta skua antarctica

Data on reproductive success of 110 Falkland Skua Catharacta skua antarctica pairs were gathered during the austral summers of 1988–1989 and 1990–1991 on New Island, Falkland Islands. Adults laid two eggs 2–3 days apart and began incubation with the first egg. For the years combined, 1.39 chicks per nest hatched and 0.84 chicks per nest fledged (fledging was defined as surviving to 16 days of age). Brood reduction was common; 43% of the two-chick broods were reduced to one, and mortality was concentrated on the younger chick. Although asynchronous hatching and differential death are consistent with Lack's brood reduction hypothesis, application of O'Connor's quantitative criterion revealed that sibling competition may not be responsible for the observed chick mortality. Furthermore, because no aggressive interactions between chicks were observed or detected indirectly, siblicide may be absent in this population. Instead, predation modified by a variety of factors may have led to the greater mortality of the second-hatched chick.     

Influences of adult age and experience, nest location, clutch size and laying sequence on the breeding success of the Great skua Catharacta skua

In 881 nesting attempts by Great skuas on Foula during 1975 and 1976, 1.24 chicks were fledged per pair. Addled eggs and egg predation were the main causes of losses. Eggs rolled out of the nest were not retrieved. Almost all chicks which hatched also fledged. Young or inexperienced breeders were almost as successful as older or experienced pairs. Birds producing single egg clutches and addled eggs usually also showed low aggression and held small territories. Egg predation was greatest at high nest density, when food availability was low, and when adult aggression score was low. High density nesting seems to have been imposed by the limited size of the island and prolonged population increase.     

Interactions with Great Skuas Stercorarius skua as a factor in the long‐term decline of an Arctic Skua Stercorarius parasiticus population

Many of the UK’s seabird species have displayed high variation in breeding success since the 1980s, largely due to changes in the availability of Lesser Sandeels Ammodytes marinus, their main prey. During this time, Arctic Skuas Stercorarius parasiticus experienced a rapid decline in the UK and the species has subsequently been placed on the Red List of birds of conservation concern. Although shortage of Lesser Sandeels is likely to be an influential factor, the Arctic Skua’s breeding range overlaps with that of the Great Skua Stercorarius skua, a larger bird with a more varied diet, and interspecific interactions for nesting habitat may exert an additional pressure on Arctic Skua breeding populations. Results from four censuses, spanning 21 years, were used to model habitat use and analyse distributional change in nesting Arctic Skuas at a major colony located on Fetlar, Shetland, Scotland. The decline in Arctic Skuas was not uniform across the island and competition with Great Skuas for nest‐sites appears to have influenced localized breeding distribution. By 2006, Arctic Skuas had been almost entirely excluded from shrub heath, blanket bog and coastal heath habitats, which were identified as preferred habitat in 1986. In 2006, Arctic Skua breeding territories were mainly restricted to one core area of preferred habitat where over 90% nested in high density as this habitat became increasingly occupied by Great Skuas. The more generalist foraging habit of the Great Skua allowed the population to grow rapidly as numbers of the more specialist Arctic Skua decreased during times of low sandeel availability. Our model suggests that both interspecific competition for territories with Great Skuas and food limitation have played important roles in the decline of Arctic Skuas on Fetlar.     

Variations in size and growth of Great skua Catharacta skua chicks in relation to adult age, hatching date, egg volume, brood size and hatching sequence

The paper describes the relationships between adult age, hatching date, egg volume, brood size, hatching sequence, chick weight at hatching, chick growth rate and chick body size for a population of Great skuas in Shetland. The relative importance of these factors, and food availability, is assessed in determining chick size, weight, and growth rate.     

Chlamydophila abortus in a Brown skua (Catharacta antarctica lonnbergi) from a subantarctic island

On Bird Island, South Georgia, a new strain of Chlamydophila abortus was detected in one Brown skua out of 37 specimens from six different seabird species. Phylogenetic analysis of the rnpB and omp1 genes indicated the strain to be more closely related to C. abortus than to 6BC, the type strain of Chlamydophila psittaci.     

Individual identity coding depends on call type in the South Polar skua Catharacta maccormicki

In colonial birds, acoustic communication is essential for mate recognition. The South Polar skua (Catharacta maccormicki) lives in loose colonies and is highly territorial for feeding and breeding. We studied the potential of individual identity coding in the three main calls of the South Polar skua repertoire: the courtship, the contact and the alarm calls. We investigated parameters in both temporal and frequencial domains, i.e. amplitude modulation, frequency modulation and power spectrum density. For each parameter, the intra- and inter-individual variabilities were calculated. The ratio between these values represents the potential of individuality coding (PIC) of the considered feature. Low values of PICs for amplitude and frequency modulations show that both parameters may not be used for individual recognition. In contrast, high values of PIC for the power spectrum density indicate that the energy distribution among the frequency spectrum is likely to be an individual marker. PIC also varies according to the call type. Both courtship and contact calls have a higher potentiality of individual identity coding than the alarm call. The two former calls may allow individual recognition whereas the latter may not, and this last result can be extrapolated to many other species.     

Contrasting effects of GPS device and harness attachment on adult survival of Lesser Black‐backed Gulls Larus fuscus and Great Skuas Stercorarius skua

Telemetry has become an important method for studying the biology and ecology of animals. However, the impact of tracking devices and their method of attachment on different species across multiple temporal scales has seldom been assessed. We compared the behavioural and demographic responses of two species of seabird, Lesser Black‐backed Gull Larus fuscus and Great Skua Stercorarius skua, to a GPS device attached using a crossover wing harness. We used telemetry information and monitoring of breeding colonies to compare birds equipped with a device and harness, and control birds without an attachment. We assessed whether tagged birds have lower short‐term breeding productivity or lower longer term overwinter return rates (indicative of overwinter survival) than controls. For Great Skua, we also assessed whether territory attendance within the breeding season differed between tagged and control birds. As with previous studies on Lesser Black‐backed Gull, we found no short‐term impacts on breeding productivity or long‐term impacts on overwinter return rates. For Great Skua, there was no evidence for impacts of the device and harness on territory attendance or breeding productivity. However, as found by a previous study of Great Skuas using a different (body) harness design, there was strong evidence of reduced overwinter return rates. Consequently, a device attached using a wing harness was considered suitable for long‐term deployment on Lesser Black‐backed Gulls, but not on Great Skuas. These findings will inform the planning of future tracking studies.     

Survival, site and mate fidelity in South Polar Skuas Catharacta maccormicki at Anvers Island, Antarctica

In 1974–1975, 34 adult South Polar Skuas Catharacta maccormicki were colour-ringed on 18 nest territories at Bonaparte Point, Anvers Island, near Palmer Station along the Antarctic Peninsula. Subsequently, the area was searched for these birds during the austral summers of 1975–1976 to 1984–1985 and in 1987–1988 and 1989–1990. Fifty-three percent were seen in 1984–1985, 32% in 1987–1988 and 21% in 1989–1990. Annual survival rate averaged 95% from 1974–1975 to 1984–1985; no sexual differences were detected (n = 28 of known sex). Strong territory and mate fidelity were apparent: 34 skuas averaged 1.1 nest territories and 1.7 mates each in 16 years. Only 4 of 34 individuals (all females) were known to change territories, and each territory change involved a change of mates. Although males showed higher territory fidelity than females (P < 0.01), most females (four of five) retained their territories when previous mates failed to return. Seventeen of 34 birds changed mates a total of 24 times; at least 20 mate changes followed the death or disappearance of the former mate. Males showed slightly higher mate fidelity than females (P < 0.04). Female South Polar and Brown Skuas Catharacta lonnbergi did not differ in territory or mate fidelity. From 1974–1975 to 1984–1985, 120 South Polar Skua chicks were ringed on 18 nest territories on Bonaparte Point: 17 were resighted in the Palmer area when they were 3–10 years old. All 15 returnecs were found within 3 km of their natal nest sites, and four of them occupied nest territories on Bonaparte Point.     

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

Hybridisation between South polar skua (C. maccormicki) and Brown skua (C. antarctica lonnbergi) in 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 success were examined in detail at King George Island. The northward distribution of South polar skuas extended to King George Island (62°11′ S 59°00′ W), with a small outlying population on Signy Island (60°45′ S 45°36′ W), whereas Brown skuas did not breed further south than Anvers Island archipelago (64°46′ S 64°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.     

The effects of changes in food availability on the breeding ecology of great skuas Catharacta skua in Shetland

Great skuas on Foula, Shetland have responded to a decline in the availability of sandeels since the late 1970s by increasing the proportion of other items in their diets. This change is correlated with the annual recruitment of sandeels in Shetland waters. Since 1983 there has been a 10-fold increase in predation by great skuas upon other seabirds, as Furness & Hislop (1981) suggested might occur in response to a low availability of sandeels. Changes in diet have been accompanied by a 50% reduction in adult territorial attendance as adults increased their foraging effort, such that between 1987 and 1989 breeding adults were probably working as hard as they were able to. Despite this, breeding success was less than 40% in 1987 and less than 15% in 1988 and 1989. The major cause of breeding failure was predation of unguarded chicks by adults from neighbouring territories. The willingness of adults to expose their chicks to high predation risk is probably maintained because of a positive correlation between chick pre-fledging growth and post-fledging survival, which is expressed up to the age of two years and which will place a strong pressure upon adults to feed their chicks as well as possible. The high expenditure of effort by adults in 1987 and 1988 did not affect the weights of those birds incubating eggs in 1988 and 1989, but there was a slight (3%) decrease in egg size between the late 1970s and the late 1980s. Changes in the age structure of the breeding population and the absence in 1989 of 28% of adults colour-ringed during incubation in 1988 suggest an increase in the rate of egress since the 1970s. These changes probably represent an increase in the long-term costs of reproduction to adults at this colony.     

Diet of the south polar skua Catharacta maccormicki at Cape Bird,Ross Island,Antarctica

Regurgitated pellets and fecal samples from south polar skuas Catharacta maccormicki breeding on Ross Island were analyzed for diet items. Adult penguin feathers were found in 79% of all the pellets and 42% of the fecal samples. Evidence of fish was found in 26% of the pellets and 39% of the fecal samples. The amount of refuse in the diet of skuas on Ross Island was directly related to the distance from McMurdo Station.     

Thelebolus microsporus mycelial mats in the trachea of wild brown skua (Catharacta antarctica lonnbergi) and South Polar skua (C. maccormicki) carcasses

Sixteen brown skuas (Catharacta antarctica lonnbergi) and seven South Polar skuas (C. maccormicki) were found dead near Boekella Lake, Hope Bay, Antarctica, in February 1997. Postmortem examination revealed conspicuous caseous, deep yellow fungal/mycelial mats or cores in the trachea of nine of 19 carcasses that were examined. These mycelial cores, highly suggestive of aspergillomas, completely occluded the tracheal lumen in four of these nine carcasses. Thelebolus microsporus, a psychrophilic ascomycetous fungus commonly isolated from skua dung and skua nesting material, was isolated in pure culture from these tracheal plugs. Awareness of pseudolesions resulting from Thelebolus microsporus profuse postmortem growth in the trachea of dead skuas will minimize potential confusion with aspergillosis when investigating causes of epornithics in Antarctica.     

Observations on the food of the south polar skua,Catharacta maccormicki near Davis,Antarctica

Summary Regurgitated boluses of undigested material were examined from six sites in East Antarctica. Skuas nesting near Adelie penguin, southern fulmar and Antarctic petrel colonies showed greater site fidelity in their feeding than did skuas nesting close to cape petrels or a Weddell seal pupping site.     

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

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. Accepted: 3 June 2000     

Foods of the south polar skua Catharacta maccormicki in the eastern Larsemann Hills,Princess Elizabeth Land,East Antarctica

Regurgitated pellets and food remains were collected near nest sites, and from a club site, of south polar skuas Catharacta maccormicki in the eastern Larsemann Hills, Princess Elizabeth Land, East Antarctica, during the skuas' presence in the area. The samples indicated that the snow petrel Pagodroma nivea, the most abundant seabird species breeding locally, formed the major dietary component, comprising some 66% of food items identified in pellets and 80% of the food remains obtained. Adélie penguins Pygoscelis adeliae (which do not breed in the Larsemann Hills), other seabirds, fish and marine foods were rarely found as remains or in pellets. However, refuse (meat, fish and vegetable remains) taken as food by skuas from nearby stations occurred in pellets at all sites and formed about 12% of the food remains collected and identified. In this study, foods taken by skuas were related both to the local breeding distribution of snow petrels, and to the possession of a feeding territory. Birds without feeding territories took somewhat fewer snow petrels and included more refuse from local stations in their diet, as did those at the club site. Future monitoring of the influence of anthropogenic foods (and inedible waste materials) on the species' ecology is considered important.