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Robert W Furness - One of the best experts on this subject based on the ideXlab platform.
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change in the north sea ecosystem from the 1970s to the 2010s great skua diets reflect changing forage fish seabirds and fisheries
Ices Journal of Marine Science, 2019Co-Authors: Gabriella E Church, Robert W Furness, Glen Tyler, Lucy Gilbert, Stephen C VotierAbstract:Understanding anthropogenic impacts are crucial to maintain marine ecosystem health. The North Sea has changed in recent decades, largely due to commercial fishing and climate change. Seabirds can act as useful indicators of these changes. By analyzing n = 20 013 pellets and n = 24 993 otoliths regurgitated by great Skuas Stercorarius skua in northern Scotland over five decades from the 1970s to the 2010s (in 36 years 1973–2017), we reveal how the diet of this top predator has changed alongside the changing North Sea ecosystem. Sandeels Ammodytes spp. were the most common dietary item during the 1970s, but became virtually absent from the 1980s onward. Discarded whitefish dominated skua diets from the 1980s to the present day, despite long-term declines in North Sea discard production. However, the discarded fish eaten by great Skuas has become smaller and the species composition changed. Skua pellets only rarely contained avian prey in the 1970s but this increased during the 1980s, and fluctuated between 10% and 20% from the 1990s to 2010s. There have also been changes in the avian prey in the diet—black-legged kittiwakes Rissa tridactyla generally being replaced by auks Alcid spp. and northern fulmars Fulmarus glacialis. The Shetland marine ecosystem has experienced steep declines in sandeel stocks and in seabirds that feed on them. Great Skuas have been able to prey switch to respond to this change, supported by abundant discards, enabling them to maintain a favourable population status while other seabird species have declined.
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the effect of long range transport trophic position and diet specialization on legacy contaminant occurrence in great Skuas stercorarius skua breeding across the northeast atlantic
Environmental Pollution, 2019Co-Authors: Eliza H K Leat, Robert W Furness, Sophie Bourgeon, Sveinn Are Hanssen, Aevar Petersen, Hallvard Strom, Tor Harry Bjorn, Jan Ove Bustnes, Geir Wing Gabrielsen, Ane HaarrAbstract:Abstract High levels of halogenated organic contaminants (HOCs) have been found in the marine predatory seabird great skua (Stercorarius skua) from breeding colonies in the Northeastern Atlantic, with large unexplained inter-colony variation. The present study aimed at analyzing if the HOCs occurrence in breeding great Skuas in remote colonies was explained by local baseline food web exposure determined by long-range transport, or by ecological factors such as diet specialization and relative trophic position in the breeding area. The occurrence of organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs) was analyzed in plasma of 204 adult great Skuas collected over two years (2008 and 2009) and 5 colonies across the North-Atlantic from Shetland to Svalbard. The ΣHOCs levels in plasma ranged across two orders of magnitude, from 40 to 7600 ng/g (wet weight) and differed significantly across the great skua colonies. The variation in contaminant occurrence among colonies did not reflect long-range transport through a latitudinal or remoteness gradient, as the second northernmost colony (Bjornoya), had the highest contaminant concentrations. No latitudinal or remoteness gradient was evident in the contaminant pattern among the colonies. The contaminant levels increased significantly with increasing δ15N values, and regurgitated pellets of undigested prey suggested that great Skuas with higher δ15N values had a higher proportion of bird prey in their diet, mostly seabirds. In contrast, great Skuas from colonies with lower δ15N and lower contaminant level fed mostly on fish. The enrichment of δ13C increased with decreasing δ15N and lower contaminant levels. Therefore, individual behavior of great Skuas, such as migration strategies and diet specialization, rather than long-range transport and thus baseline food web exposure, explain among and within colony variance in contaminant occurrence.
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density dependence in seabirds great Skuas stercorarius skua start to breed at a younger age when conditions are better
Ringing and Migration, 2015Co-Authors: Robert W FurnessAbstract:Large-scale and long-term ringing and cohort colour ringing of chicks, combined with trapping of immatures on clubs and adults on nests, allows the age of first breeding by Great Skuas Stercorarius skua to be investigated. Modal and median age of first breeding of Great Skuas at their largest colony (Foula, Shetland) was seven years, but there was considerable individual variation. Birds may first breed when four to >11 years old. Compared with the situation in Foula, Great Skuas start breeding at a younger age at smaller colonies. When conditions at Foula deteriorated, resulting in reduced adult survival, reduced breeding success and population decline, age of first breeding increased. These patterns imply a density-dependent age of first breeding, but with individual variation because higher-quality birds recruit at a younger age than birds in poor body condition. Density-dependent variation in age at first breeding may have an important influence on population growth rate, and this would merit study al...
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great skua stercorarius skua movements at sea in relation to marine renewable energy developments
Marine Environmental Research, 2014Co-Authors: Helen M Wade, Chris B Thaxter, Niall H K Burton, W Bouten, Elizabeth A Masden, Angus C Jackson, Robert W FurnessAbstract:Marine renewable energy developments (MREDs) are an increasing feature of the marine environment. Owing to the relatively small number of existing developments and the early stage of their associated environmental monitoring programmes, the effects of MREDs on seabirds are not fully known. Our ability to fully predict potential effects is limited by a lack of knowledge regarding movements of seabirds at sea. We used GPS tracking to improve our understanding of the movements at sea of a protected seabird species breeding in Scotland, the great skua (Stercorarius skua), to better predict how this species may be affected by MREDs. We found that the overlap of great Skuas with leased and proposed MREDs was low; particularly with offshore wind sites, which are predicted to present a greater risk to great Skuas than wave or tidal-stream developments. Failed breeders overlapped with larger areas of MREDs than breeding birds but the overall overlap with core areas used remained low. Overlap with wave energy development sites was greater than for offshore wind and tidal-stream sites. Comparison of 2011 data with historical data indicates that distances travelled by great Skuas have likely increased over recent decades. This suggests that basing marine spatial planning decisions on short-term tracking data could be less informative than longer-term data.
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activity patterns of wintering great Skuas stercorarius skua
Bird Study, 2014Co-Authors: Ellen Magnusdottir, Eliza H K Leat, Sophie Bourgeon, Sveinn Are Hanssen, Aevar Petersen, Hallvard Strom, Jan Ove Bustnes, Jon Einar Jonsson, Richard A Phillips, Robert W FurnessAbstract:Capsule Great Skuas Stercorarius skua wintering in different areas spent different amounts of time in flight (foraging or searching for food) and so may be experiencing different feeding conditions.Aims To compare the daily percentage of time spent in flight (foraging or searching for food) between different wintering areas.Methods In 2008, loggers equipped with a saltwater sensor were deployed on adult Great Skuas at three colonies in the northeast Atlantic, and the data used to compare foraging activity between the five main wintering areas.Results The five areas used by 22 Great Skuas in winter were widely separated, from the northwest Atlantic to northwest Africa, and differ substantially in oceanography. The main difference in foraging effort among areas for individuals that were site-faithful was that the percentage of time per day spent in flight off northwest Africa was much lower than elsewhere. Among five birds that travelled between wintering areas, one reduced the percentage of time in flight ...
Stephen C Votier - One of the best experts on this subject based on the ideXlab platform.
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change in the north sea ecosystem from the 1970s to the 2010s great skua diets reflect changing forage fish seabirds and fisheries
Ices Journal of Marine Science, 2019Co-Authors: Gabriella E Church, Robert W Furness, Glen Tyler, Lucy Gilbert, Stephen C VotierAbstract:Understanding anthropogenic impacts are crucial to maintain marine ecosystem health. The North Sea has changed in recent decades, largely due to commercial fishing and climate change. Seabirds can act as useful indicators of these changes. By analyzing n = 20 013 pellets and n = 24 993 otoliths regurgitated by great Skuas Stercorarius skua in northern Scotland over five decades from the 1970s to the 2010s (in 36 years 1973–2017), we reveal how the diet of this top predator has changed alongside the changing North Sea ecosystem. Sandeels Ammodytes spp. were the most common dietary item during the 1970s, but became virtually absent from the 1980s onward. Discarded whitefish dominated skua diets from the 1980s to the present day, despite long-term declines in North Sea discard production. However, the discarded fish eaten by great Skuas has become smaller and the species composition changed. Skua pellets only rarely contained avian prey in the 1970s but this increased during the 1980s, and fluctuated between 10% and 20% from the 1990s to 2010s. There have also been changes in the avian prey in the diet—black-legged kittiwakes Rissa tridactyla generally being replaced by auks Alcid spp. and northern fulmars Fulmarus glacialis. The Shetland marine ecosystem has experienced steep declines in sandeel stocks and in seabirds that feed on them. Great Skuas have been able to prey switch to respond to this change, supported by abundant discards, enabling them to maintain a favourable population status while other seabird species have declined.
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seabird predation by great Skuas stercorarius skua intra specific competition for food
Journal of Avian Biology, 2007Co-Authors: Stephen C Votier, Jonathan E. Crane, Jose Manuel Arcos, Stuart Bearhop, Robert W FurnessAbstract: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.
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Seabird predation by great Skuas Stercorarius skua– intra‐specific competition for food?
Journal of Avian Biology, 2007Co-Authors: Stephen C Votier, Jonathan E. Crane, Jose Manuel Arcos, Stuart Bearhop, Robert W FurnessAbstract: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.
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fatty acid signatures as indicators of diet in great Skuas stercorarius skua shetland
Marine Ecology Progress Series, 2006Co-Authors: Anne Kakela, Jonathan E. Crane, Robert W Furness, Stephen C Votier, Reijo KakelaAbstract:Fatty acid signatures (FAS) were determined in plasma and adipose tissue of great Skuas Stercorarius skua from Shetland in order to test the applicability of this biomarker in estimat- ing diets of wild scavenging seabirds. The plasma FAS were compared with those of captive herring gulls Larus argentatus, which were fed typical Northeast Atlantic demersal and pelagic fish. The individual fatty acids that showed the largest proportional changes in FAS due to changes of dietary fish in herring gulls also varied the most in wild great Skuas, suggesting a dietary origin of these changes in FAS. Thus, great Skuas that had recently been feeding largely on a demersal or terrestrial diet were distinguished from individuals feeding mainly on a pelagic diet. Pronounced variation in the plasma FAS of great Skuas suggests either very flexible feeding behaviour or individual dietary specialisation, which concurs with previous studies based on pellets. Individuals that regurgitated pellets consisting only of demersal fish, bird or rabbit showed larger values of a specific polyunsatu- rated fatty acid ratio (20:4n-6/18:3n-3+18:4n-3+20:5n-3) than birds whose pellets also contained remains of pelagic fish. Although pellet data showed a clear dominance of demersal fish in the sum- mer diet of the great skua, the large proportions of long-chain monounsaturated fatty acids (e.g. 20:1n-9 and 22:1n-11) in adipose tissue of the great skua suggest that a considerable part of fat accumulated outside the breeding season comes from fatty pelagic fish.
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Nocturnal foraging by great Skuas Stercorarius skua: implications for conservation of storm-petrel populations
Journal of Ornithology, 2006Co-Authors: Stephen C Votier, Jonathan E. Crane, Ana León, Claire A. Mcsorley, Eduardo Mínguez, Ian P. Mitchell, Matthew Parsons, Richard A Phillips, Stuart Bearhop, Robert W FurnessAbstract:At St Kilda, Outer Hebrides, a large colony of great Skuas Stercorarius skua feed extensively on one of the largest colonies of Leach’s storm-petrels Oceanodroma leucorhoa in Europe, but little is known about the dynamics of this predator–prey system. Recently published population estimates of storm-petrels make it possible to estimate the impact of skua predation for the first time. Although Skuas in the southern hemisphere catch petrels attending breeding colonies at night, it is not known whether congeners in the northern hemisphere also forage during the hours of darkness. We found (using radio-transmitters) that Skuas regularly forage at night and (using light intensifying equipment) observed them catching storm-petrels at night. However, Skuas also foraged during daylight hours, and it is unknown whether they might also catch storm-petrels at sea. Data on diet composition reveals that the proportion of storm-petrels in skua diet declined between 1996 and 1997, but remained constant thereafter. Although a large proportion of the storm-petrel prey is likely to consist of non-breeders, numbers consumed suggest that breeders and an unknown quantity of transients may also been eaten. The numbers of storm-petrels eaten are not sustainable and may result in substantial long-term population declines. Under current conditions, maintenance of large populations of both Leach’s storm-petrels and great Skuas at St Kilda appears to be mutually exclusive.
Richard A Phillips - One of the best experts on this subject based on the ideXlab platform.
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Long-term changes in population size, distribution and productivity of Skuas (Stercorarius spp.) at Signy Island, South Orkney Islands
Polar Biology, 2016Co-Authors: Ana P. B. Carneiro, Andrea Manica, Richard A PhillipsAbstract: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.
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activity patterns of wintering great Skuas stercorarius skua
Bird Study, 2014Co-Authors: Ellen Magnusdottir, Eliza H K Leat, Sophie Bourgeon, Sveinn Are Hanssen, Aevar Petersen, Hallvard Strom, Jan Ove Bustnes, Jon Einar Jonsson, Richard A Phillips, Robert W FurnessAbstract:Capsule Great Skuas Stercorarius skua wintering in different areas spent different amounts of time in flight (foraging or searching for food) and so may be experiencing different feeding conditions.Aims To compare the daily percentage of time spent in flight (foraging or searching for food) between different wintering areas.Methods In 2008, loggers equipped with a saltwater sensor were deployed on adult Great Skuas at three colonies in the northeast Atlantic, and the data used to compare foraging activity between the five main wintering areas.Results The five areas used by 22 Great Skuas in winter were widely separated, from the northwest Atlantic to northwest Africa, and differ substantially in oceanography. The main difference in foraging effort among areas for individuals that were site-faithful was that the percentage of time per day spent in flight off northwest Africa was much lower than elsewhere. Among five birds that travelled between wintering areas, one reduced the percentage of time in flight ...
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wintering areas of great Skuas stercorarius skua breeding in scotland iceland and norway
Bird Study, 2012Co-Authors: Ellen Magnusdottir, Eliza H K Leat, Sophie Bourgeon, Sveinn Are Hanssen, Aevar Petersen, Hallvard Strom, Jan Ove Bustnes, Pall Hersteinsson, Richard A Phillips, Robert W FurnessAbstract:Capsule Great Skuas Stercorarius skua breeding in Scotland, Iceland, and Norway, winter in different areas. Aims To assess the winter distribution of adult Great Skuas breeding in different countries. Methods Geolocation data-loggers were deployed on breeding adults at colonies in Shetland (Scotland), southeast Iceland, and Bjornoya (Norway) in 2008. Loggers were recovered when birds returned to breed the next year and downloaded data were processed to indicate the location of each individual throughout the winter period. Results Adult Great Skuas from Scotland wintered off northwest Africa and southern Europe. Adults from Iceland mostly wintered off Canada, with small numbers visiting northwest Africa and Europe. Although adults from Bjornoya (Norway) migrated to similar areas to birds from Iceland, a slightly greater proportion wintered off Europe, and most used areas further north than birds from Scotland. Although three birds studied over consecutive winters used the same small area in consecutive years, four moved between different areas within one winter. Conclusion Great Skuas show clear variation in migrations among breeding regions, and some evidence of individual consistency.
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the influence of food availability on the breeding effort and reproductive success of arctic Skuas stercorarius parasiticus
Ibis, 2008Co-Authors: Richard A Phillips, R W G Caldow, Robert W FurnessAbstract:Territorial attendance, chick growth rate and breeding success of Arctic Skuas Stercorarius parasiticus in Shetland were lowest in the late 1980s when recruitment of Sandeels Am-modytes marinus in the surrounding waters was poor. The relationships between both fisheries-based and avian indices of food availability and annual variation in Arctic Skua chick growth and breeding success between 1976 and 1994 were better described by a threshold effect rather than linear functions. Arctic Skuas conform to the model proposed by Cairns, which predicts the responses of seabirds to changes in prey availability. Skua clutch size, egg volume, hatching success and hatching date were not reliable indices of Sandeel availability. However, annual fluctuations in Arctic Skua breeding numbers may be a useful indicator of changes in prey abundance.
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Movements, winter distribution and activity patterns of Falkland and brown Skuas: Insights from loggers and isotopes
Marine Ecology Progress Series, 2007Co-Authors: Richard A Phillips, Paulo Catry, Stuart Bearhop, Janet R. D. Silk, Rona A. R. Mcgill, Vsevolod Afanasyev, Ian J. StrangeAbstract:In the first published study of the wintering ranges and activity patterns of Skuas from any colony, we combined tracking (geolocator) and stable isotope analysis in a comparison of migra- tion behaviour of brown Skuas Catharacta lonnbergi and Falkland Skuas C. antarctica from South Georgia and the Falkland Islands, respectively. Brown Skuas, particularly failed breeders, departed and returned to the colony much earlier than Falkland Skuas, and 2 of 3 brown Skuas performed a pre-laying exodus. During winter, brown Skuas were distributed widely over deep, oceanic water within the Argentine Basin (37 to 52° S) between the Antarctic Polar Front and the northern sub-trop- ical Front. Falkland Skuas, by comparison, wintered mainly in subantarctic waters around the central Patagonian shelf-break (40 to 52° S). Much greater overlap existed among core areas within than between species, and sex did not influence distribution. The partial inter-specific spatial segregation was also reflected in a divergence in activity patterns, with brown Skuas in flight for a greater propor- tion, and more time on average, during both daylight and darkness. Both species of skua spent far more time on the water than do foraging albatrosses, and there was limited overlap between their nonbreeding distributions and those of large procellariids from the same archipelagos. Stable isotope signatures of brown skua feathers indicated that distributions of tracked birds were typical of most or all of the breeding population, and were consistent from year to year. None was characteristic of species that winter on adjacent continental shelves or off south-west Europe. Isotope values also sug- gested a mixed diet for brown Skuas of zooplankton, low trophic-level squid and fish, with little or no reliance on seabird predation or fisheries.
Norman Ratcliffe - One of the best experts on this subject based on the ideXlab platform.
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Combined bottom‐up and top‐down pressures drive catastrophic population declines of Arctic Skuas in Scotland
The Journal of animal ecology, 2018Co-Authors: Allan J. Perkins, Norman Ratcliffe, Claire Smith, Dave Suddaby, Brian Ribbands, Pete Ellis, Eric Meek, Mark BoltonAbstract:Understanding drivers of population change is critical for effective species conservation. In the northeast Atlantic Ocean, recent changes amongst seabird communities are linked to human and climate change impacts on food webs. Many species have declined severely, with food shortages, and increased predation reducing productivity. Arctic skua Stercorarius parasiticus, a kleptoparasite of other seabirds, is one such species. The aim of the study was to determine relative effects of bottom-up and top-down pressures on Arctic Skuas across multiple colonies in a rapidly declining national population. Long-term monitoring data were used to quantify changes in population size and productivity of Arctic Skuas, their hosts (black-legged kittiwake Rissa tridactyla, common guillemot Uria aalge, Atlantic puffin Fratercula arctica, Arctic tern Sterna paradisaea) and an apex predator (great skua Stercorarius skua) over 24 years (1992-2015) in Scotland. We used digital mapping and statistical models to determine relative effects of bottom-up (host productivity) and top-down (great skua density) pressures on Arctic Skuas across 33 colonies, and assess variation between three colony types classified by host abundance. Arctic Skuas declined by 81% and their hosts by 42%-92%, whereas at most colonies great Skuas increased. Annual productivity declined in Arctic Skuas and their hosts, and reduced Arctic skua breeding success was a driver of the species' population decline. Arctic skua productivity was positively associated with annual breeding success of hosts and negatively with great skua density. Intercolony variation suggested Arctic skua trends and productivity were most sensitive to top-down pressures at smaller colonies of host species where great Skuas had increased most, whereas bottom-up pressures dominated at large colonies of host species. Scotland's Arctic skua population is declining rapidly, with bottom-up and top-down pressures simultaneously reducing breeding success to unsustainably low levels. Marine food web alterations, strongly influenced by fisheries management and climate change, are driving the decline, and this study demonstrates severe vulnerability of seabirds to rapid change in human-modified ecosystems. Potential but untested conservation solutions for Arctic Skuas include marine protected areas, supplementary feeding within colonies and management of great Skuas.
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interactions with great Skuas stercorarius skua as a factor in the long term decline of an arctic skua stercorarius parasiticus population
Ibis, 2011Co-Authors: Neil M Dawson, Colin D Macleod, Malcolm Smith, Norman RatcliffeAbstract: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.
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Interactions with Great Skuas Stercorarius skua as a factor in the long‐term decline of an Arctic Skua Stercorarius parasiticus population
Ibis, 2010Co-Authors: Neil M Dawson, Colin D Macleod, Malcolm Smith, Norman RatcliffeAbstract: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.
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Predation by great Skuas at a large Shetland seabird colony
Journal of Applied Ecology, 2004Co-Authors: Stephen C Votier, Norman Ratcliffe, Richard A Phillips, Stuart Bearhop, Robert W FurnessAbstract:Summary 1. Skuas are top predators in marine ecosystems and may have detrimental effects on seabird communities they prey upon. However, predation rates are poorly understood and poorly quantified. Using a bio-energetics model we estimate seabird predation by great Skuas, Stercorarius skua , at a large UK colony (Hermaness, Shetland). We investigate the influence of dietary specialization and fishery management on predation and explore the effect of experimental removal of specialist bird predators. 2. Great Skuas at Hermaness required 491·5 × 10 6 kJ and 546·6 × 10 6 kJ of energy in each of two breeding seasons. Breeding Skuas fell into one of two groups: a small proportion (5%) of specialist bird predators or the vast majority (95%) that fed opportunistically on birds or specialized on fishery discards. During 1999, great Skuas consumed ≈ 80 000 kg of fish, which increased to over 90 000 kg in 2001. About 13 000 seabirds were consumed by great Skuas each year, with 26‐29% being consumed by specialist bird predators. 3. Although it is difficult to assess, great Skuas appear to be having a negative impact on seabird populations. Altering model inputs to test differing scenarios revealed that reductions in fishery discards would result in increased seabird predation rates. However, proposed changes in fishery management over the period of the study did not reduce discarding rates, which instead increased. 4. Synthesis and applications . The use of a bioenergetics model reveals that great skua predation may negatively affect seabird populations. Availability of fishery discards is an important factor influencing seabird predation rates, but predicting the effect of changes in fishery management may be difficult in the short term. Specialist bird predators consume large quantities of seabird prey, but this is less significant at the population level. Although experimental removal of specialist bird predators may reduce predation at a minimal loss of Skuas, it is unclear whether conspecifics may replace them and retain high rates of predation.
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REPRODUCTIVE CONSEQUENCES FOR GREAT Skuas SPECIALIZING AS SEABIRD PREDATORS
The Condor, 2004Co-Authors: Stephen C Votier, Norman Ratcliffe, Stuart Bearhop, Robert W FurnessAbstract:Abstract Most of the Great Skuas (Stercorarius skua) breeding at Hermaness, Shetland, exhibit dietary specialization: a small proportion feed almost exclusively upon seabird prey, a small proportion feed as generalists, and most feed on fishery discards. We investigated the foraging dynamics, reproductive performance, and survival of Great Skuas that specialized in depredating other seabirds compared with those feeding predominantly on fish. Around half of the specialist bird predators defended combined breeding and feeding territories that included a section of seabird colony; the remainder of the predatory Skuas foraged away from breeding territories. Specialist bird predators retained their feeding habit and, if present, feeding territory, across years. Time budgets revealed that specialist bird predators spent less time foraging than Skuas feeding predominantly on fish. Results of radio-telemetry indicated that bird-specialist Skuas have smaller home ranges than other birds. In a comparison of reprodu...
Stuart Bearhop - One of the best experts on this subject based on the ideXlab platform.
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Movements, winter distribution and activity patterns of Falkland and brown Skuas: Insights from loggers and isotopes
Marine Ecology Progress Series, 2007Co-Authors: Richard A Phillips, Paulo Catry, Stuart Bearhop, Janet R. D. Silk, Rona A. R. Mcgill, Vsevolod Afanasyev, Ian J. StrangeAbstract:In the first published study of the wintering ranges and activity patterns of Skuas from any colony, we combined tracking (geolocator) and stable isotope analysis in a comparison of migra- tion behaviour of brown Skuas Catharacta lonnbergi and Falkland Skuas C. antarctica from South Georgia and the Falkland Islands, respectively. Brown Skuas, particularly failed breeders, departed and returned to the colony much earlier than Falkland Skuas, and 2 of 3 brown Skuas performed a pre-laying exodus. During winter, brown Skuas were distributed widely over deep, oceanic water within the Argentine Basin (37 to 52° S) between the Antarctic Polar Front and the northern sub-trop- ical Front. Falkland Skuas, by comparison, wintered mainly in subantarctic waters around the central Patagonian shelf-break (40 to 52° S). Much greater overlap existed among core areas within than between species, and sex did not influence distribution. The partial inter-specific spatial segregation was also reflected in a divergence in activity patterns, with brown Skuas in flight for a greater propor- tion, and more time on average, during both daylight and darkness. Both species of skua spent far more time on the water than do foraging albatrosses, and there was limited overlap between their nonbreeding distributions and those of large procellariids from the same archipelagos. Stable isotope signatures of brown skua feathers indicated that distributions of tracked birds were typical of most or all of the breeding population, and were consistent from year to year. None was characteristic of species that winter on adjacent continental shelves or off south-west Europe. Isotope values also sug- gested a mixed diet for brown Skuas of zooplankton, low trophic-level squid and fish, with little or no reliance on seabird predation or fisheries.
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seabird predation by great Skuas stercorarius skua intra specific competition for food
Journal of Avian Biology, 2007Co-Authors: Stephen C Votier, Jonathan E. Crane, Jose Manuel Arcos, Stuart Bearhop, Robert W FurnessAbstract: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.
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Seabird predation by great Skuas Stercorarius skua– intra‐specific competition for food?
Journal of Avian Biology, 2007Co-Authors: Stephen C Votier, Jonathan E. Crane, Jose Manuel Arcos, Stuart Bearhop, Robert W FurnessAbstract: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.
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Nocturnal foraging by great Skuas Stercorarius skua: implications for conservation of storm-petrel populations
Journal of Ornithology, 2006Co-Authors: Stephen C Votier, Jonathan E. Crane, Ana León, Claire A. Mcsorley, Eduardo Mínguez, Ian P. Mitchell, Matthew Parsons, Richard A Phillips, Stuart Bearhop, Robert W FurnessAbstract:At St Kilda, Outer Hebrides, a large colony of great Skuas Stercorarius skua feed extensively on one of the largest colonies of Leach’s storm-petrels Oceanodroma leucorhoa in Europe, but little is known about the dynamics of this predator–prey system. Recently published population estimates of storm-petrels make it possible to estimate the impact of skua predation for the first time. Although Skuas in the southern hemisphere catch petrels attending breeding colonies at night, it is not known whether congeners in the northern hemisphere also forage during the hours of darkness. We found (using radio-transmitters) that Skuas regularly forage at night and (using light intensifying equipment) observed them catching storm-petrels at night. However, Skuas also foraged during daylight hours, and it is unknown whether they might also catch storm-petrels at sea. Data on diet composition reveals that the proportion of storm-petrels in skua diet declined between 1996 and 1997, but remained constant thereafter. Although a large proportion of the storm-petrel prey is likely to consist of non-breeders, numbers consumed suggest that breeders and an unknown quantity of transients may also been eaten. The numbers of storm-petrels eaten are not sustainable and may result in substantial long-term population declines. Under current conditions, maintenance of large populations of both Leach’s storm-petrels and great Skuas at St Kilda appears to be mutually exclusive.
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Predation by great Skuas at a large Shetland seabird colony
Journal of Applied Ecology, 2004Co-Authors: Stephen C Votier, Norman Ratcliffe, Richard A Phillips, Stuart Bearhop, Robert W FurnessAbstract:Summary 1. Skuas are top predators in marine ecosystems and may have detrimental effects on seabird communities they prey upon. However, predation rates are poorly understood and poorly quantified. Using a bio-energetics model we estimate seabird predation by great Skuas, Stercorarius skua , at a large UK colony (Hermaness, Shetland). We investigate the influence of dietary specialization and fishery management on predation and explore the effect of experimental removal of specialist bird predators. 2. Great Skuas at Hermaness required 491·5 × 10 6 kJ and 546·6 × 10 6 kJ of energy in each of two breeding seasons. Breeding Skuas fell into one of two groups: a small proportion (5%) of specialist bird predators or the vast majority (95%) that fed opportunistically on birds or specialized on fishery discards. During 1999, great Skuas consumed ≈ 80 000 kg of fish, which increased to over 90 000 kg in 2001. About 13 000 seabirds were consumed by great Skuas each year, with 26‐29% being consumed by specialist bird predators. 3. Although it is difficult to assess, great Skuas appear to be having a negative impact on seabird populations. Altering model inputs to test differing scenarios revealed that reductions in fishery discards would result in increased seabird predation rates. However, proposed changes in fishery management over the period of the study did not reduce discarding rates, which instead increased. 4. Synthesis and applications . The use of a bioenergetics model reveals that great skua predation may negatively affect seabird populations. Availability of fishery discards is an important factor influencing seabird predation rates, but predicting the effect of changes in fishery management may be difficult in the short term. Specialist bird predators consume large quantities of seabird prey, but this is less significant at the population level. Although experimental removal of specialist bird predators may reduce predation at a minimal loss of Skuas, it is unclear whether conspecifics may replace them and retain high rates of predation.
