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.     

Heat stress in a high-latitude seabird: effects of temperature and food supply on bathing and nest attendance of great skuas Catharacta skua

Birds such as great skuas Catharacta skua adapted for successful breeding at high latitudes may experience problems of heat dissipation in mild climates. Great skuas spend time bathing at freshwater sites close to breeding territories and here, we examine impacts of heat stress on bathing, foraging and nest attendance of adults during three breeding seasons with marked variation in the availability of prey (1-group sandeels Ammodytes marinus ). Adults exhibited diurnal variation in bathing activity that matched heat-stress conditions. Moreover more birds bathed on days of higher average heat stress, suggesting that bathing plays a role in thermoregulation. Bathing numbers were lower in years of poor food availability, when adult attendance at territories was low, probably because lower attendance reduced the opportunity for parents to bathe without leaving chicks unattended. Chicks are normally guarded by female parents and fed by males but under conditions of low food availability territorial attendance of breeding pairs was particularly low on days of high heat stress, with chicks regularly left unattended at air temperatures exceeding 14°C. Unattended chicks are at risk of being killed by neighbouring conspecifics and survival of chicks to fledging was low in the two years of low sandeel stocks. Our study indicates that for great skuas, indirect effects of climate change on prey stocks and direct effects on heat stress experienced by adults may be additive.     

Effect of predation by skuas on breeding success of the Cape petrel Daption capense at Nelson Island, Antarctica

Breeding success of Cape petrels at Nelson Island (South Shetland Islands) in 1991/1992 averaged 29%. Predation by skuas accounted for 76% of all nesting failures. Direct attacks prevailed over opportunistic predation. Intensity of observer disturbance alone could not account for the inter-colony variation in predation rate, which tended to increase with colony size, presence of nearby breeding skuas and local availability of other prey species (penguins). Predation was significantly lower on nests that could be attacked only, or more easily, from the air and nests with close neighbours. Selective predation on nests in which relatively narrower eggs were laid was independent of nest-site characteristics and laying date, suggesting an effect of parental age/experience on breeding success through antipredator behaviour. Development of oil-spitting behaviour in growing chicks balanced the parental attendance and effort in nest defence, which declined as chick age increased. Increased predation on lighter but not smaller chicks may reflect the lower ability of such chicks to defend themselves by spitting stomach oil. Seasonal variations of predation rate on Cape petrel nests did not result from predator swamping by numbers of simultaneously active nests, but rather reflected temporal availability of alternative prey. Success of all phases of nesting declined with later laying date. Received: 15 December 1997 / Accepted: 11 April 1998     

The diet of black guillemots Cepphus grylle in Shetland

The diet of black guillemots Cepphus grylle (L.) was studied in Shetland between 1982 and 1985, both during and outside the breeding season. Full-grown birds consumed a wide variety of fish and invertebrate species in summer, broadly reflecting the diversity of the inshore benthic fauna. In winter a greater diversity of invertebrates was taken, which assumed increased importance in the diet as the availability of benthic fish probably declined. Chicks were fed a diet of fish, with 70–80% being sandeels Ammodytes marinus and butterfish Pholis gunnellus on the island of Mousa. Sandeels were delivered more frequently in the early morning, coincident with a peak in their feeding activity. Older chicks were fed significantly longer fish. The composition of chick diet varied significantly with date, not chick age. A seasonal decline in the proportion of sandeels in the chick diet may have reflected changes in Zooplankton abundance, but the possibility that intensive industrial fishing of sandeels had adversely affected local stocks can not be ruled out.     

Aspects of the foraging behaviour of the Antarctic Tern Sterna vittata gaini at Harmony Point, South Shetland Islands

During January and February of 2002 and 2003, we studied the diet of the Antarctic Tern Sterna vittata gaini at two colonies in Nelson Island, South Shetland Islands, by identifying the prey fed to chicks by breeders. The fish Notothenia coriiceps was the main prey in both seasons, followed by the myctophid Electrona antarctica, Antarctic krill Euphausia superba and gammarid amphipods. The contribution of fish to the diet increased as chicks grew older. Fish and amphipods were brought to chicks during the day, whereas adults brought Antarctic krill at sunrise and sunset. Both the duration of the feeding trips and the number of trips per foraging bout varied according to the type of prey caught. Preliminary information suggests that, among other causes, the foraging strategy is strongly influenced by the predation pressure of skuas on chicks. Results are compared with the only two previous study on the diet of the Antarctic Tern at the South Shetland Islands.     

The influence of food supply on the breeding ecology of Kittiwakes Rissa tridactyla in Shetland

We measured the breeding performance, body condition, time budgets and foraging ranges of Kittiwakes Rissa tridactyla at Sumburgh Head, Shetland, in two years of contrasting food availability. Kittiwakes in Shetland generally feed their young almost entirely on sandeels, and fisheries data indicated that stocks of sandeels in Shetland waters were at least ten times higher in 1991 than in 1990. Fledging success of Kittiwakes was nil in 1990 and 68% of eggs laid in 1991, although clutch-size and hatching success were no different between years. Post-hatching foraging trips in 1991 were of comparable duration to those recorded at other colonies in conditions of good food supply (2–3 h), while trips recorded during incubation or post-hatching in 1990 were approximately three times longer on average than at corresponding stages of the breeding season in 1991. Radio-tracking data indicated that adults generally stayed within 5 km of the colony in 1991 but flew more than 40 km from the colony on each trip in 1990. Eggs were apparently not left unattended in either year, despite the fact that this required adults to incubate for periods in excess of 44 h in 1990. The extent to which adults were able to increase trip durations, foraging ranges and incubation shift lengths between years, while maintaining hatching success, indicates the degree to which Kittiwakes are normally buffered against adverse feeding conditions during incubation. Reduced nest attendance and lower body-condition of adults post-hatching in 1990, in conjunction with complete post-hatching breeding failure, indicate that adults were beyond the limits of their buffering capacity during chick-rearing in 1990.     

Seabird predation by great skuas Stercorarius skua– intra‐specific competition for food?

Competition for food is widely cited as an important cost of coloniality among birds and much of the evidence in support of this hypothesis comes from studies of colonial piscivorous seabirds. However, for generalist seabirds able to switch between different prey types, the role of food availability in relation to colony size is unclear. Here we investigate patterns of the consumption of seabird prey in relation to colony size in a generalist seabird, the great skua Stercorarius skua, in Shetland, UK. At the population level skuas feed mainly on sandeels Ammodytes marinus and fishery discards, but respond to declines in fish availability to facultatively prey on other seabirds. By comparing the consumption of seabirds among seven different sized colonies, including one colony with artificially reduced numbers of skuas (Fair Isle), we investigate whether consumption of seabird prey is influenced by skua population size, while simultaneously measuring seabird prey availability. Data from five years also enables us to investigate the influence of annual variation in environmental conditions on seabird consumption. Using measures of body condition and reproductive performance we investigate the consequences of living in different sized colonies, which may provide insight into ultimate costs of nesting at high population density. Skua diets varied among colonies and the proportion of seabird prey in the diet was inversely related to skua colony size, despite similar per capita numbers of seabirds across colonies. At the colony where their numbers were artificially suppressed, skuas consumed a greater proportion of seabirds per capita. Highly significant year effects in seabird predation were observed but the pattern among colonies remained consistent over time. Two measures of adult body condition (pectoral muscle index and mean corpuscular volume) revealed that adult great skuas were in poorer condition at the largest colony (Foula), but reproductive performance did not alter significantly among colonies. This study provides evidence that intra‐specific competition among skuas may limit opportunities for obtaining seabird prey, which may be particularly important during periods of poor availability of sandeels and fishery discards, and has implications for assessing the impact of skuas on seabird populations.     

Dispersion and site fidelity of breeding female grey seals (Halichoerus grypus) on North Rona, Scotland

Female grey seals ( Halichoerus grypus ) formed breeding aggregations on the island of North Rona, Scotland. Aggregations of females were associated particularly with gullies leading from the sea, leaving large areas of available space unoccupied. Changes in the degree of aggregation of females during the breeding season were similar in 1987, 1988 and 1989. Pronounced aggregations occurred in the early and late parts of each breeding season.
Of 67 breeding females marked in 1985, 62 (93%)) returned to N. Rona to breed in at least one season up to 1989, but 18 (27%) were present in all five years. Females came ashore up to 14 days before giving birth and 82% were observed first in the vicinity of their subsequent pupping site. Between 1985 and 1989, marked females which returned were faithful to their previous pupping sites, even when the previous pup had died. There was no evidence of a gradual change in the location of individual pupping sites over time. This pupping site fidelity may generate aggregations whose location, timing and composition is predictable.     

Nesting densities,productivity and survival of breeding Whimbrel Numenius phaeopus in Shetland

Between 1986 and 1988 data on the nesting densities, productivity, and survival of breeding Whimbrel Numenius phaeopus were collected from 5 study sites in the Northern Isles of Shetland. Nesting densities on sites varied between 11 and 21 pairs per 100 ha. Few studies elsewhere have recorded such high densities of nesting Whimbrel as those on certain sites used in the present study. Each breeding pair produced an average of between 0.75 and 0.91 fledglings per year. Productivity varied considerably between different study sites however, primarily because of differences in the survival rate of chicks between hatching and fledging (ranging from 3% to 55% in one year). The annual survival rate of breeding adults was at least 89%. Considering adult survival in relation to overall productivity, it appears likely that the production of fledglings from study sites is in excess of that required to balance adult mortality. This result is consistent with the current increase of the Whimbrel population in Shetland.     

Trophic interactions between brown and south polar skuas at Deception Island,Antarctica

It is broadly accepted that the brown skua (Stercorarius antarcticus lonnbergi) competitively excludes the south polar skua (S. maccormicki) from penguin colonies when breeding sympatrically, forcing the latter to feed on marine resources. The purpose of this work was to examine the diets and trophic niche breadths of each species where they co-occur and to determine the degree of overlap. To this end, we analyzed 169 pellets of brown skuas, collected in two different areas (20 individuals), and 152 of south polar skuas, collected in three different areas (18 individuals), on Deception Island, South Shetland Islands, Antarctica, during the austral summer 2000. Pellet analysis often underestimates the amount of easily digestible prey, but allows for comparisons of the relative contributions of different items in the diet. South polar skuas at our study locations consumed seven different food items and had a trophic niche breadth of 0.133 compared to brown skuas that fed on 10 different items and had a trophic niche breadth of 0.078. The niche overlap between the species was 82.1%. Penguins were the principal food source of both species, however, brown skuas fed mostly on chicks, while south polar skuas fed on adults (carcasses). The use of different age classes of penguins as a food source offers an alternative to competitive exclusion, allowing the coexistence of these species on Deception Island.     

A preliminary assessment of the quantities of Shetland sandeels taken by seabirds, seals, predatory fish and the industrial fishery in 1981–83

Shetland holds a very high concentration of seabirds that feed to a large extent on sandeels Ammodytes marinus . The available data allow an assessment of the quantity of sandeels consumed by seabirds each year 1981–83, and sensitivity analysis suggests that this figure is accurate to about ±30%. Only very approximate estimates of sandeel consumption by seals and by predatory fish can be made.
From 1981 to 1983 the Shetland fishery catch averaged 45,000 tonnes of sandeels, with an estimated natural consumption of 47,000 tonnes by seabirds (predominantly Guillemots Uria aalge and Fulmars Fulmarus glacialis ). Based on sensible guesses and the little data available, consumption may have been about 25,000 tonnes by predatory fish and 9,000 tonnes by seals. The figures suggest that natural mortality of the Shetland sandeel stock in 1981–83 was considerably in excess of the figure used in VPA stock assessments. Further information is required for key variables in order to permit better estimates of sandeel stock dynamics. In particular, we need better data on numbers and diets of seals in Shetland, numbers and diets of predatory fish, diets and foraging distributions of Fulmars, especially out with the chick-rearing period. In future, sandeel biomass assessments for Shetland should incorporate estimates of predation by seabirds as this is clearly an important part of total mortality for the stock.     

Diets and feeding of Fulmars Fulmarus glacialis during the breeding season:a comparison between St Kilda and Shetland colonies

Diets and nest attendance of Fulmars at Foula, Shetland and St Kilda, Outer Hebrides were examined during the breeding season. At Foula, sandeels formed the bulk of the diet, with fish offal and pelagic zooplankton of minor importance. At St Kilda, 71% of regurgitates consisted of pelagic zooplankton, probably captured at night. Dietary overlap between the two colonies was 14% by species composition. Foraging trips from Foula shortly after chick hatching generally lasted for less than 10 h, while trips from St Kilda late in chick rearing often lasted more than 24 h. The diurnal pattern of feeding suggested mainly nocturnal foraging from St Kilda but both diurnal and nocturnal foraging from Foula.     

Impact of predation on king penguin Aptenodytes patagonicus in Crozet Archipelago

Predation can have major effects on population dynamics, but predator–prey interactions in marine ecosystems have rarely been studied. While the king penguin is one of the most studied seabirds, little is known about the impact of predation on its population dynamics. Here, we determine the impact of the main predators (giant petrels and skuas) on king penguin breeding success taking into account the nocturnal predation of petrels. We found that predation is the most important source of breeding failure for king penguins. The smallest chicks within crèches are the most hunted. The periphery of the colony suffers the highest risk of predation during summer. Our study shows the unequal quality of some areas inside the colony in terms of predation risk and breeding success, and points out the importance of timing in successful breeding.     

Status of breeding Dotterel Charadrius morinellus in Britain in 1999

Capsule A minimum of 220 male Dotterel nested in Britain in 1999, which after correction for missed birds led to an estimated 630 breeding males.

Aims To provide a reliable population estimate of Dotterel against which past and future estimates could be compared.

Methods Over half of the potential breeding habitat in Britain, selected randomly and to cover protected sites and former breeding sites for Dotterel, was surveyed through a single visit when Dotterel had young chicks.

Results After correcting for missed birds, 750 males were estimated to be breeding in Britain in 1999. An alternative method led to an estimated 510 males. Both methods had their drawbacks – 630 males probably represented the best estimate. Almost 93% of males bred in three regions: central, east and north Highlands. Around a third of surveyed sites were occupied by breeding birds. The national population estimate was significantly higher in 1987/88 but numbers were probably greatest in 1989 and declined in the 1990s. In 1999 Dotterel distribution had contracted to those sites with the highest densities in the late 1980s.

Conclusion It is unlikely that changes in numbers between surveys were due to changes in the British breeding grounds. They may be due to adverse influences away from the breeding grounds or, because Dotterels can move across large distances between breeding attempts, a redistribution of birds away from Scotland to breeding sites elsewhere in the Palearctic.     

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.     

Seabirds and sandeels: the conflict between exploitation and conservation in the northern North Sea

Sandeels, especiallyAmmodytes marinus are a major component of the diet of many predatory fish, seabirds and seals. The industrial fishery for sandeels is now the largest of the North Sea fisheries. A sandeel fishery in the Shetland area began in 1974 but has recently declined. This change was accompanied by dramatic declines in the breeding success of certain seabirds, particularly Arctic terns and kittiwakes. Current information on seabirds and sandeels in the Shetland area is reviewed and areas where further research is needed, highlighted. The Shetland problem illustrates the difficulties of reconciling conservation and exploitation when fundamental ecological and behavioural knowledge is lacking, and also the need to obtain further information on the ecological impact of industrial fisheries.     

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.     

Behavioural ecology of lonnbergi skuas in relation to environment on the Chatham Islands,New Zealand

Abstract

A population of lonnbergi skuas (family Stercorariidae) was studied over December and January 1974–75 on Rangatira Island, Chatham group (44°22′S,176°11′W). The main aim was to compare the behavioural ecology of these birds with that of maccormicki skuas of Antarctica, and to assess the moderating effect of environment on habits. The skuas held extensive territories along the coast and in inland areas clear of forest and bracken. Five of the 11 study territories were occupied by trios of adult birds. A major effect of the vegetation was to reduce contact between neighbours on the ground at territorial boundaries, and most territorial defence was by aerial display and attack. Eggs were laid from late September, and chicks began flying in late December and January. This breeding schedule is about 2 months earlier than for lonnbergi at Signy Island or for skuas in Antarctica, but falls on a trend line of latitude against egg-laying when all populations are considered. Seventeen chicks were reared on the 11 territories, a breeding success of 1.55 chicks per nest. This high success rate was attributable largely to success in rearing both chicks when two were hatched. This is seldom achieved in more extreme climates, and is apparently determined by the intensity of sibling competitiveness. It is hypothesised that this intensity is graded, and aggression by the older chick of the pair is provoked more easily by food shortage as environmental conditions become increasingly severe. The most important prey items recorded were adult petrels, especially broad-billed prion (Pachyptila vittata vittata) and white-faced storm petrel (Pelagodroma marina maoriana) captured on the ground at night. The skuas were mostly inactive during the day, and were not seen to fish or to chase or harass any other bird on or about the island. Southern blue penguins (Eudyptula minor minor), though very common on the island, were not attacked by the skuas. Skuas were not seen to attempt to dig out petrel or penguin nesting burrows. Of the differences in ecology recorded, only the presence of three adults on territories appears to be characteristic of lonnbergi; it has not been recorded for maccormicki or hamiltoni, and is either unknown or very rare in skua.     

PREDATION AND KLEPTOPARASITISM BY SKUAS IN A SHETLAND SEABIRD COLONY

Feeding methods and relations of Great Skuas and Arctic Skuas to prey were studied in a seabird colony at Hermaness, Shetland. Great Skuas obtained food by kleptoparasitism, predation and scavenging. They induced Gannets to regurgitate by interfering with their flight; grasping the Gannet by the wing or tail or pushing it down with the feet on its back. Gannets tried to escape by descending to the surface, and regurgitated during 12% of the chases, most frequently when pursued by several birds. Great Skuas caught Puffins by swooping at flocks in the colony. Puffins flying with fish to their young were also chased, releasing food on one fifth of the attacks, or escaping down to the sea and diving. Great Skuas also took Kittiwake nestlings by hovering and grasping the chick with the bill, killing and eating it on the surface. Adult Kittiwakes from nearby nests took to the air, mobbing the predator. More Kittiwakes were engaged in mobbing at unsuccessful than at successful predation attempts, indicating that colonial breeding may be of selective value under such predation. Two different estimates pointed to a Kittiwake nestling predation of 0–12 and 014 young per pair. Fledging success of Kittiwakes was estimated at 0–87-1-06 young per pair, considerably lower than at English colonies where predators are absent. In spite of the predation, the Kittiwake colony showed no signs of decrease. Agonistic behaviour and other evidence indicate that Great Skuas defend feeding territories at the seabird colony. Skuas, gulls and Fulmars competed for food at carcasses. Fulmars dominated and chased away skuas. Arctic Skuas deprived Puffins of food. They patrolled the cliff, intercepting Puffins arriving with fish, snatching it from their victim's bill, or inducing them to release fish. Puffins continuing their inward flight lost food more often (30%) than birds descending to the sea (15%)—sometimes diving below. This opportunity to escape may explain the lower success of skuas at Hermaness than at a Puffin colony farther inland from the shore (Grant 1971). Other factors being equal, proximity to the sea may thus reduce the risk of kleptoparasitism.     

Breeding performance of the red-throated diver Gavia stellata in Shetland

Breeding red-throated divers Gavia stellata were studied in Shetland during 1981-1983. The overall probabilities of nest success were 0.35 in 1981 and 0.25 in 1982, as calculated by the Mayfield method. Productivity was shown to be at least 0.51 chicks fledged per breeding pair in 1981 (n = 100) and 0.36 in 1982 (n = 91), and a review of all field studies for Shetland gave a mean value of 0.45 per year (n = 1104) with no detectable trends in productivity since 1918; this was considered probably sufficient to maintain a stable population. Nests at lochs smaller than one hectare experienced the greatest hatching success. The majority of known breeding lochs wore used in three consecutive years, with a significantly greater re-use of lochs which had previously supported successful pairs. Breeding success declined with season in 1981 and 1982 largely due to reduced hatching success of late nesters, and a slight seasonal decline in egg volume was found in 1983.