RICHARD A[ PHILLIPS %\ DAVID R[ THOMPSON and. KEITH C[ HAMER$ ...... Recent work "e[g[ Bolton\ Houston + Monaghan. 0881# suggests that egg ...
Journal of Applied Ecology 0888\ 25\ 107Ð121
The impact of great skua predation on seabird populations at St Kilda] a bioenergetics model RICHARD A[ PHILLIPS %\ DAVID R[ THOMPSON and KEITH C[ HAMER$ Applied Ornithology Unit\ DEEB\ IBLS\ University of Glasgow\ Glasgow G01 7QQ^ and $Department of Biological Sciences\ University of Durham\ South Road\ Durham DH0 2LE\ UK
Summary 0[ The number of great skuas Catharacta skua Brunnich breeding at St Kilda\ Outer Hebrides\ has increased rapidly in recent years\ making it currently the second largest colony outside Shetland and the fastest growing in the UK[ In comparison with Shetland\ where the diet consists mostly of sandeel Ammodytes marinus Raitt and discarded gadoid _sh\ and very rarely of birds\ great skuas at St Kilda feed far more extensively upon seabirds[ This paper incorporates metabolic\ dietary and demo! graphic data to estimate the total mass of di}erent prey consumed at St Kilda and to assess the potential impact of this predation on other seabird populations[ 1[ On the basis of a bioenergetics model incorporating fundamental life!history par! ameters\ the great skua population at St Kilda was estimated to require 030 × 095 kJ of energy per season\ most of which "77=9)# was necessary for the maintenance and activity of breeding adults[ Energy demand was considerably lower for non!breeders "1=4) of the total#\ and for chicks and ~edglings "8=1)#[ 2[ In addition to seabirds\ great skua diet at St Kilda also included a considerable proportion of _sh and goose barnacles Lepas sp[ However\ because of di}erences in mean meal mass and caloric density\ meals of larger seabird prey were more important in terms of their energetic contribution in the diet than in terms of their relative abundance[ 3[ Combining the bioenergetics and prey consumption models\ it was estimated that a total mass of 01=1 tonnes of _sh\ 0=5 tonnes of goose barnacles and 7=7 tonnes of seabirds was consumed by the great skua population at St Kilda to ful_l its total energy requirement in 0885[ Overall seabird consumption was estimated to be 39\799 seabirds of seven di}erent species[ Although a proportion of birds killed are likely to be visiting non!breeders\ the magnitude of this _gure nonetheless suggests that great skuas may have a considerable impact on the internationally important populations of some seabirds at St Kilda[ Key!words] activity costs\ diet\ energy requirements\ maintenance costs\ pellet analysis[ Journal of Applied Ecology "0888# 25\ 107Ð121
Introduction Considerable attention has been focused in recent years on the dynamics of the relationship between raptors and their avian or mammalian prey in ter! restrial ecosystems "e[g[ Cresswell + Whit_eld 0883^ Korpimaki 0883#[ Much less is known about the inter! actions between their ecological counterparts in mar!
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Correspondence and present address] Dr R[A[ Phillips\ Department of Biological Sciences\ University of Durham\ South Road\ Durham DH0 2LE\ UK[ E!mail] r[a[phil! lipsÝdurham[ac[uk^ Fax] 9080 2631306
ine ecosystems\ the predatory gulls "family Laridae#\ skuas "family Stercorariidae# and giant petrels Mac! ronectes spp[ and the impact they may have on seabird populations "but see Furness 0870^ Watanuki 0875^ Young 0883#[ In the southern hemisphere\ brown\ Tristan and Falkland skuas Catharacta antarctica "Lesson#\ and south polar skuas C[ maccormicki "Saunders#\ feed mostly by predation on seabirds\ with mammals and scavenged prey important only at a few speci_c sites "Jones + Skira 0868^ Schramm 0872^ Green 0875^ Furness 0876^ Norman + Ward 0889^ Ryan +
108 R[A[ Phillips\ D[R[ Thompson + K[C[ Hamer
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Moloney 0880^ Young 0883^ Mund + Miller 0884#[ The only exceptions to this are south polar skuas\ which have been recorded feeding at sea on _sh and krill\ usually in areas of sympatry with brown skuas C[ antarctica lonnbergi "Young 0852^ Trivelpiece + Volkman 0871^ Hemmings 0873^ Pietz 0876#[ The UK holds ¼ 59) of the world population of great skuas Catharacta skua Brunnich "Furness 0876#[ For most of the current century\ comparatively few great skuas bred outside the main strongholds of Ork! ney and Shetland\ although numbers appeared to be increasing in the Outer Hebrides during the 0879s "Furness 0876^ Rennie 0877#[ At the large Shetland colonies\ great skuas appeared to be dependent pri! marily on sandeels Ammodytes marinus Raitt and dis! carded white_sh from trawling activities "Furness + Hislop 0870^ Hamer\ Furness + Caldow 0880#[ Except in localized instances of dietary specialization\ sea! birds tended to be of relatively minor importance\ at least up until the late 0879s when sandeel availability in Shetland waters declined dramatically "Hamer\ Furness + Caldow 0880#[ However\ a recent study of diet and breeding ecology at St Kilda\ Outer Hebrides\ has found that adults at that site feed far more exten! sively upon seabirds yet maintain high levels of chick growth and annual productivity "Phillips et al[ 0886#[ Great skuas were _rst recorded breeding on Hirta\ the main island in the St Kilda archipelago "46>38?N 97>24?W#\ in 0852\ and the size of the breeding popu! lation increased steadily up until at least 0889 "see Phillips et al[\ in press#[ In recent years there has been a rapid expansion to 102 pairs on Hirta and 05 pairs on the other islands in 0885\ which makes Hirta cur! rently the second largest colony outside Shetland and also the fastest growing colony in the UK "Phillips et al[\ in press#[ The birds at St Kilda represent × 0) of the world population of this species[ Seabird populations at St Kilda are of major inter! national importance\ and for this reason the archi! pelago has been designated as a UK National Nature Reserve\ EC Special Protection Area and World Heri! tage Site[ The archipelago holds ¼ 19) of the North Atlantic population of northern gannets Morus bas! sanus "L[#\ ¼ 19) of the world population of the grabae race of Atlantic pu.ns Fratercula arctica "L[#\ the largest colony of northern fulmars Fulmarus gla! cialis "L[# in western Europe\ the largest colony of Leach|s petrel Oceanodroma leucorhoa "Viellot# in the eastern Atlantic and the largest colony of British storm petrels Hydrobates pelagicus "L[# in the UK "Tasker\ Moore + Scho_eld 0877#[ The impact pre! dation by great skuas may be having on these species is unknown[ In this paper we develop a model of great skua energy requirements at St Kilda using fundamental life!history parameters[ By incorporating information on the meal size and caloric value of the main food items\ and the proportion of the diet of breeding adults\ non!breeders and chicks made up by di}erent
prey\ it is possible to estimate their relative con! tribution to the overall population energy demand[ Since the species composition of avian prey is also known "Phillips et al[ 0886#\ we can then estimate approximately how many seabirds are consumed by great skuas at this site during the breeding season\ and hence assess the potential impact this predation may have on prey populations[
Methods BIOENERGETICS MODEL OF POPULATION ENERGY REQUIREMENTS
The St Kilda archipelago consists of four main islands "Hirta\ Dun\ Soay and Boreray# and several large sea stacks[ The majority of the data included in this paper were collected during 0885 on the largest island\ Hirta\ which holds × 89) of the breeding adults[ A bioener! getics model was constructed\ incorporating and developing elements from a number of previous stud! ies "Wiens + Scott 0864^ Furness 0867^ Wiens 0873^ Cairns et al[ 0889#[ A complete census of great skuas was undertaken in 0885 by marking all nests on Hirta and Dun\ and counting apparently occupied territories on the two other islands "see Phillips et al[\ in press#[ The mean number of non!breeders attending the only club site at St Kilda\ on Hirta\ was determined by direct counts at intervals throughout the breeding season[ Energy requirements were estimated separately for breeding adults\ non!breeders and chicks throughout the entire time spent at the colony\ including the pre! incubation and post!~edging periods "see Table 0 for details#[ The major components in the model are illus! trated in Fig[ 0[ All _nal estimates of energy require! ments\ including egg production costs of breeders\ took assimilation e.ciency into account[ This was assumed to be 9=65 for all food types\ a value obtained during controlled feeding trials of captive great skuas fed whole sandeels and whiting Merlangius merlangus "L[# "G[ Hilton\ unpublished data#[ The energy requirements for the maintenance and activity of individual adults were calculated assuming an overall _eld metabolic rate "FMR# throughout the season equivalent to 2=4 times basal metabolic rate "BMR#[ This FMR]BMR ratio was the average esti! mated by Caldow "0877# for great skuas breeding on Foula\ Shetland\ in 0862Ð65 on the basis of time! budgets and incorporating multiplicands of BMR for speci_c activities\ and lies in the middle of the range recorded in most seabird studies to date\ with one or two exceptions "see Discussion#[ Non!breeders do not have to produce a clutch or provision chicks\ but are probably not as e.cient at _nding food as breeding adults and hence were assumed to have approximately equivalent foraging and maintenance costs "following Cairns et al[ 0889#[ Seasonal energy requirements were calculated assuming a constant number of birds was
119 Impact of great skua predation
Table 0[ Parameters used in the great skua bioenergetics model Parameter
Value
Source
Total breeding population in 0885 Mean non!breeding population Pre!breeding period Incubation period Chick!rearing period Post!~edging period at colony Adult BMR Adult FMR]BMR ratio Mean clutch size Mean fresh egg mass Mean egg energy density Egg synthesis e.ciency Food assimilation e.ciency Mean brood size at 19 days Mean brood size at ~edging Mean chick ~edging mass Energy requirement of ~edglings
118 pairs ¼02 birds ¼29 days 18 days ¼36 days ¼07 days 427 kJ day−0 2=4 0=80 85 g 5=34 kJ g−0 9=64 9=65 0=95 9=73 0069 g 0914 kJ day−0
Phillips et al[ "in press# This study P[ Catry\ personal communication Furness 0867 This study R[W[ Furness\ personal communication Bryant + Furness "0884# See Methods Hamer\ Furness + Caldow "0880# Furness "0867# Meathrel + Ryder "0876#^ Meathrel\ Ryder + Termaat "0876# Ricklefs "0863\ 0872# G[ Hilton\ unpublished data This study Phillips\ Thompson + Hamer "0886# Phillips\ Thompson + Hamer "0886# See Methods
Non-breeder daily energy requirement for maintenance and activity (3.5 × BMR)
Breeder daily energy requirement for maintenance and activity (3.5 × BMR)
Metabolic energy requirement of chicks* 1.015 (35.15 × Mc )
Energy requirement of fledglings until departure*
Assimilation efficiency
Total population energy requirement
Fig[ 0[ Structure of the bioenergetic model of population energy requirements[ Calculation already incorporates assimilation e.ciency[
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present throughout the breeding period\ although not necessarily the same individuals in the case of non! breeders[ The energy cost of clutch formation "Eclf in kJ# for each breeding pair was estimated according to the equation Eclf Cls × Megg × Calegg × "0:Esyn#\ where Cls is mean clutch size\ Megg is mean fresh egg mass in g\ Calegg is average energy density of larid eggs in kJ and Esyn is egg tissue synthesis e.ciency "for references see Table 0#[ Total metabolic energy requirement "ME in kJ# from hatching to ~edging of each chick was estimated from the equation ME 24=04 × Mc0=904 where Mc is chick mass in g "Drent\ Klaassen + Zwaan 0881#[ Energy demand of each ~edgling during the short period before departure was estimated from a regression equation _tted to daily energy requirement "in kJ# at ~edging "from Fig[ 2 in Drent\ Klaassen + Zwaan 0881# vs[ ~edgling mass "Table 0 in Drent\ Klaassen + Zwaan 0881# for four larid species[ Great
skuas ~edge weighing c[ 0069 g\ only slightly heavier than herring gull Larus argentatus Pontoppidan ~edglings\ the largest species in that study[ Both of these equations take account of assimilation e.ciency[ A sensitivity analysis of the model was performed in two ways] by increasing the value for each input parameter by 0) "following the approach of Furness 0867#\ and also by raising or lowering the value for each input parameter to what were considered to be reasonable extremes and noting the e}ect this had on the output of the model "following the approach of Diamond\ Gaston + Brown 0882#[ Likely extremes for many parameters\ particularly those measured directly for the St Kilda population in 0885\ and others that exhibit little variation in published studies\ were taken to be 2 09) of the starting value[ Slightly nar! rower limits "2 7)# were set for assimilation e.ciency\ as upper and lower values for this variable are between 9=61 and 9=71 even across a wide range of bird species "Gabrielsen et al[ 0880^ Drent\ Klaassen
110 R[A[ Phillips\ D[R[ Thompson + K[C[ Hamer
+ Zwaan 0881^ Mehlum\ Gabrielsen + Nagy 0882#[ Wider potential extremes were considered appropriate for mean duration of the pre!breeding and post!~edg! ing periods\ ~edgling energy requirements and FMR]BMR ratios of breeding adults and non!bree! ders\ as these were not measured directly[ The average number of non!breeders foraging around the colony was also given a high upper\ but not lower boundary\ for reasons outlined in the Discussion[
DIET COMPOSITION
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Diet composition of breeding adults and non!breeders at St Kilda in 0885 from June onwards\ i[e[ shortly before the start of the chick!rearing period\ was known from analysis of regurgitated pellets "see Phil! lips\ Thompson + Hamer 0886^ Phillips et al[ 0886 for details#[ Diet prior to this was assessed from regur! gitates of adults nest!trapped during incubation\ on the assumption that these accurately re~ected the pro! portion of meals in the three main prey categories Ð seabirds\ _sh and goose barnacles Lepas sp[ Ð and using the ratio of di}erent bird species in pellets col! lected in June as an index of species composition "see below#[ A similar approach was adopted to determine the diet composition of the young\ on the basis of the relative proportion of the three main prey types in regurgitates obtained from chicks during routine ringing and measuring\ and using the ratio of bird species in pellets of adults collected throughout chick! rearing as an index of species composition[ Di}erent bird species in the diet were identi_ed from feather characteristics and morphology of legs\ feet\ bill and wings found in pellets\ and di}erent _sh species from the presence of vertebrae and sagittal otoliths "Phillips et al[ 0886#[ Pellets containing bird remains were identi_ed to the appropriate species with the exception of a proportion of pellets of auk "Alci! dae# and storm petrel "Hydrobatidae#[ Di}erent spec! ies were assumed to occur in a similar proportion among these pellets as those positively identi_ed[ A small number of pellets of other food types "squid\ decapod crustacea\ and bird eggs# and single pellets of uncommon bird species were recorded in 0885\ but because of their relatively low incidence these were not considered further[ In the great majority of cases\ pellets contained the remains of only one type of prey and can therefore be assumed to represent individual meals "Furness + Hislop 0870#[ The exceptions to this are pellets result! ing from predation of Leach|s petrel and British storm petrel[ These small species are swallowed whole\ and on the evidence of groups of pellets found together on breeding territories clearly consisting of combinations of wings\ whole legs or body feathers\ it was estimated that at least three pellets result from a meal of a single individual[ A correction factor\ that three pellets of these species represented a single meal\ was therefore applied throughout[ This was not the case for the
larger seabirds\ which are not eaten whole[ Pellets of these species were much more likely to occur singly on territories and did not contain wings or legs[ The extent to which pellet ratios adequately re~ect the proportion of meals of di}erent prey is considered in the Discussion[ In our analyses\ we considered a single meal to be the quantity of food present in a bird|s proventriculus on its return from a foraging trip[ Information on relative prey frequencies during di}erent stages of the season was integrated\ on the basis of the relative duration of each stage\ into an index for the overall proportion of meals of di}erent types consumed during the entire period that breeding adults\ non!breeders and chicks spend at the colony[
MEAL MASS AND CALORIC DENSITY
By weighing chicks before and after a feed\ Furness + Hislop "0870# found that a mean of 80 g "range 49Ð 014 g\ n 07# of discarded gadoid _sh or bird meat was fed to great skua broods at each meal[ We there! fore assumed a mean meal mass for great skua adults of 099 g "following Furness + Hislop 0870# for meals of _sh or larger seabird prey[ This may be a more or less universal value for Catharacta skuas "see Dis! cussion#[ In order to simplify the analyses\ we assumed the indigestible portion of the meal\ which is later regurgitated as the pellet\ was of negligible mass[ In contrast to the larger seabird prey species\ Leach|s petrel and British storm petrel are small enough to be swallowed whole[ Consequently\ meals of this type were assumed to be equivalent to adult mass of the two species at St Kilda "Cramp + Simmons 0866#[ Meals of goose barnacle were assumed to weigh 39 g\ for reasons outlined below[ Over 89) of otoliths recorded in great skua pellets at St Kilda in 0885 were Norway pout\ Trispoterus esmarki "Nilsson#\ or poor cod T[ minutus "L[#\ whiting and haddock Melanogrammus aegle_nus "L[# "Thomp! son\ Hamer + Phillips 0887#[ Based on the length of intact otoliths and regression equations "Harkonen 0875#\ the whiting and haddock appeared to be large _sh "over 099 g# and were probably not taken whole[ The Norway pout and poor cod in the diet will have weighed considerably less[ However\ most pellets of these last species contained otoliths from several _sh\ and so the average meal size of 099 g suggested by Furness + Hislop "0870# seems appropriate[ Fisheries samples of whiting caught in June and August had a mean caloric value of 4=1 kJ gÐ0 fresh mass\ which appeared to be fairly typical of gadoid _sh during the summer "Hislop\ Harris + Smith 0880#[ The edible portion of an individual seabird consists primarily of the skin\ pectoral and leg muscles\ viscera "kidney\ liver\ gut\ heart#\ fat deposits and blood[ No single study has addressed the question of what pro! portion of fresh body mass these components comprise[ Pectorals\ leg muscles and viscera together constituted a mean of 22=8) "range 18=4Ð27=8)# of
111 Impact of great skua predation
fresh carcass mass of fulmars\ kittiwakes Rissa tri! dactyla "L[#\ common guillemots Uria aalge "Pon! toppidan#\ and pu.ns "G[ Hilton\ unpublished data#[ Blood volume of birds is between 5 and 02) of body mass "Dein 0875#[ Approximately 7) of total body mass of breeding guillemots and\ assuming samples of adults collected at St Kilda shortly after egg!laying were representative\ ¼ 5)\ ¼ 03) and ¼ 04) of body mass of pu.ns\ fulmars and Manx shearwaters Puf_nus puf_nus "Brunnich#\ respectively\ was lipid "Osborn + Harris 0873^ Furness\ Thompson + Har! rison 0883#[ Most\ if not all of this fat will be in accessible tissues\ although some will already have been accounted for in muscles and visceral components[ For the purposes of analysis\ it was assumed that\ overall\ ¼ 54) of fresh body mass from carcasses of the larger seabirds "kittiwakes\ pu.ns\ fulmars\ guillemots and Manx shearwaters# would be consumed[ These prey therefore provide sev! eral meals per carcass if we assume that great skuas carry 099 g of food\ on average\ back to the colony[ Several great skuas were frequently observed feeding from single corpses below the cli}s around the colony\ so this assumption seems to be reasonable[ If less than 54) of each carcass is consumed\ the _nal _gure for total seabird consumption will be underestimated to a greater or lesser extent "see sensitivity analysis#[ There are published values for mean mass of adults of the relevant prey species at various UK colonies\ and in some cases St Kilda itself "Cramp + Simmons 0866\ 0872^ Galbraith 0872#[ Caloric values of whole birds show some degree of interspeci_c variation\ ranging from 6=4 to 09=8 kJ gÐ0 fresh mass in adults and fully grown chicks "Brisbin 0857^ Dunn 0864^ and references therein^ Adams et al[ 0880#[ This appears to depend largely on relative fat content\ which is high in seabirds[ The edible body components extracted from carcasses of the larger prey species by great skuas will be the most energy!dense tissues\ and so the
maximum value of 09=8 kJ gÐ0 was used in analyses[ Leach|s petrels and British storm petrels are swal! lowed whole\ and consequently a greater proportion of the prey ingested will be indigestible or of low caloric value[ Mean energy content of meals of these two species was therefore adjusted downwards\ by assuming that only 54) of each petrel consists of digestible material of high calori_c value\ as above[ Although 099 g is probably an appropriate mass for meals of bird meat and _sh\ meals of goose barnacles weigh considerably less[ The total mass of seven large undigested goose barnacles regurgitated by a nest! trapped adult on Hirta in 0885 was 24 g[ Counts of large fragments and complete shell plates found in pellets suggest this is towards the upper end of the range in terms of the number consumed per meal[ Even allowing for a degree of water absorption since ingestion in the sample collected\ a single meal of goose barnacle is therefore unlikely to weigh more than 39 g on average\ and this value was used in fur! ther analysis[ As a published value was not available\ mean caloric density of goose barnacles from the regurgitate was determined by drying to constant mass\ followed by homogenization in an electric mill and direct combustion of weighed samples in a bal! listic bomb calorimeter "CB!269\ Gallenkamp\ London\ UK# calibrated with benzoic acid[
PREY CONSUMPTION MODEL
Using the above values for mean meal mass and caloric value per gram fresh mass\ it is possible to estimate the energy content of meals of a particular type[ The relative importance of di}erent prey in terms of the overall dietary energy requirement was then calculated by combining information on relative energy content with data on the proportion of meals of each type in the diet of breeding adults\ non!bree! ders and chicks[ Since energy requirements in kJ have
Table 1[ Energy requirements of great skuas at St Kilda in 0885 Individual:pair basis
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Entire colony
Daily
Per season
Daily
Per season
Breeding adults "118 pairs# Maintenance and activity Egg production costs
1379 kJ bird−0 Ð
296×092 kJ bird−0 1969 kJ pair−0
0024×092 kJ Ð
030×095 kJ 364×092 kJ
Non!breeders "02 individuals# Maintenance and activity
1379 kJ bird−0
296×092 kJ bird−0
21=1×092 kJ
2=88×095 kJ
Ð
34=6×092 kJ chick−0
Ð
00=0×095 kJ
0914 kJ ~edgling−0
07=4×092 kJ ~edgling−0
086×092 kJ
2=44×09 5 kJ
Ð
Ð
Ð
059×095 kJ
Chicks Hatching to ~edging "0=95 chicks per pair# Fledging to departure "9=73 ~edglings per pair# Total colony energy requirement
112 R[A[ Phillips\ D[R[ Thompson + K[C[ Hamer
been estimated using the bioenergetics model "see Table 1#\ the total amount of energy provided by each type of prey can be quanti_ed\ and from this _gure it is straightforward on the basis of caloric densities to back!calculate the total mass of each food type necessary to satisfy that requirement[ By inco! rporating data on mean mass of the prey species\ the numbers of birds eaten can then be calculated on the assumption that all the edible components "i[e[ 54)# of carcasses of the larger seabird prey species are con! sumed[
petrel were collected at St Kilda and the mean was therefore considered to be within 2 09) of that of birds consumed in 0885[ Wider likely extremes "2 49)# were applied for the proportions of di}erent prey eaten by great skuas\ because of possible prob! lems in the use of pellets as indicators of diet "see Discussion#[
Results BIOENERGETICS MODEL
Energy requirements
SENSITIVITY ANALYSIS
The relative importance of variation in input values for each element in the prey consumption model was considered in a sensitivity analysis\ as above\ by increasing the value for each input parameter by 0)\ and also by raising or lowering the value for each input parameter to what were considered reasonable extremes[ Likely extremes for mean caloric density were taken to be 2 14)\ as this encompasses most variation in published _gures for bird meat\ crustacea\ and gadid _sh species caught during the summer "Bris! bin 0857^ Hunt 0861^ Adams et al[ 0880^ Hislop\ Harris + Smith 0880#[ Probable lower and upper lim! its to mean meal mass for _sh and the large seabirds were considered to be 69 g and 029 g\ i[e[ 2 29) "Young 0852^ Furness + Hislop 0870#[ Mean mass of meals of goose barnacles was given a higher upper boundary "¦ 49)# as more assumptions were involved in deriving the original estimate[ The data on mass of adult Leach|s petrel and British storm
Energy requirements of each section of the great skua population are indicated in Table 1[ As a proportion of the seasonal total for the colony\ by far the largest component "77=9)# was for the maintenance and activity of breeding adults[ Energy demand was much lower for chicks and ~edglings "8=1) of seasonal totals#\ and for the small number of non!breeders "1=4)#[ Egg production costs were low\ at 9=2) of the seasonal total for the colony\ or less than 9=3) of adult activity and maintenance requirements[
Sensitivity analysis Results from the sensitivity analysis are indicated in Table 2[ Potential variation in input values for the number of non!breeders foraging around the colony\ and their FMR]BMR ratio\ had a comparatively minor e}ect on the overall estimate for energy demand of the entire great skua population[ Similarly\ vari!
Table 2[ Sensitivity tests for the bioenergetic model[ Shown in the table are the percentage changes in output "population energy requirement# resulting from a 0) increase in the value of an input parameter\ and the percentage change in output at probable maximum and minimum extremes in input parameters
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Parameter
) change in output for 0) increase
Likely extremes ")#
) change in output at extremes
Total breeding population Non!breeding population Pre!breeding period Incubation period Chick!rearing period Post!~edging period at colony Adult BMR FMR]BMR ratio of breeders FMR]BMR ratio of non!breeders Clutch size Fresh egg mass Egg energy content Egg synthesis e.ciency Food assimilation e.ciency Brood size at 19 days Brood size at ~edging Chick ~edging mass Energy requirement of ~edglings
ables associated with energy requirements of chicks and ~edglings for growth and maintenance\ and those of breeding adults for clutch formation\ tended to have only a small in~uence[ More important deter! minants of the overall population energy demand were the durations of the pre!breeding and post!~edging periods\ primarily because insu.cient information was available for St Kilda and wide potential extremes were deliberately set[ Not surprisingly\ as the largest element in the energy budget for the colony was the activity and maintenance energy requirements of breeding adults\ possible variation in their BMR\ FMR]BMR ratio and\ to a lesser extent\ assimilation e.ciency\ have a major in~uence on the _nal _gure[
"Table 3#[ Most importantly\ although goose bar! nacles made up 05) of meals of breeding adults and 13) of those of non!breeders\ they accounted for around 1) and 3)\ respectively\ of the energy intake of these two groups[ Meals of Leach|s petrel and Brit! ish storm petrel were also much less important ener! getically than their relative number would suggest[ In contrast\ meals of larger seabirds "kittiwakes\ pu.ns\ fulmars\ guillemots and Manx shearwaters# were con! siderably more important in terms of their energetic contribution in the diet than in terms of their relative abundance[
Mass of prey consumed PREY CONSUMPTION MODEL
Relative ener`y content of different meals On the assumption that meals of these two types were of the same mean mass "099 g#\ the estimated energy content "in kJ# of a single meal of bird meat from one of the larger seabirds was just over double that from _sh\ because of di}erences in caloric value per gram of fresh tissue "Table 3#[ Measured caloric value of goose barnacles was low[ The mean value 2 SD for goose barnacle tissue itself was 16=91 2 0=70 kJgÐ0 dry mass "n 6#\ with an overall energy density equivalent to 0=8 kJ gÐ0 fresh mass for the whole sample including shell plates[ Because meals of goose barnacles were small "39 g# in addition to being poor energetically\ their total energy content was under 04) of a _sh meal and under 6) of a meal of the larger bird species[ Estimated energy content of Leach|s petrel and British storm petrel meals were also low\ simply because of the small size of adults of these species[ Variation in meal energy content had a very strong e}ect on the relative contribution that di}erent prey made to the overall energy demand of the colony
The total energy requirement of breeding adults\ non! breeders\ chicks and ~edglings "from Table 1# was apportioned according to the relative contribution of each prey type from Table 3[ These _gures were then converted to a value for the mass of prey consumed according to di}erences in prey caloric density[ In total\ an estimated 01=1 tonnes of _sh\ 0=5 tonnes of goose barnacles and 7=7 tonnes of seabirds were consumed in 0885 by great skuas at St Kilda "Table 4#[ Maintenance and activity requirements of breeding adults accounted for over 74) of the total mass of each prey type consumed by the colony[
Number of seabirds consumed On the basis of published values for adult mass "Cramp + Simmons 0866\ 0872^ Galbraith 0872#\ and assuming that 54) of each carcass of the larger spec! ies was consumed\ the total numbers of seabirds eaten at St Kilda can be calculated "Table 5#[ The most important species in terms of overall numbers were Leach|s petrel\ British storm petrel and guillemot[ In total\ an estimated 39 799 seabirds were eaten by the
Table 3[ Mean meal mass\ energy content and frequency of di}erent prey types at St Kilda in 0885 and their relative contribution to total energy requirements[ For estimation of mean meal mass see Methods Breeding adults Caloric value "kJ g−0#
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Fish Goose barnacle Leach|s petrel British storm petrel Kittiwake Pu.n Guillemot Fulmar Manx shearwater
Percentage Percentage Percentage Percentage Percentage Percentage of all of energy of all of energy of all of energy meals consumed meals consumed meals consumed 34=3 05=2 4=4 1=7
28=3 1=0 1=8 9=7
41=5 9 5=7 2=3
27=6 9 2=9 9=8
40=2 13=2 6=3 2=9
46=1 2=8 4=9 0=1
1=1 2=7 07=7 3=7 9=4
3=0 5=8 23=1 7=7 9=7
1=7 3=6 12=1 5=9 9=5
3=2 6=1 24=7 8=1 9=8
9=7 6=0 9 4=5 9=4
0=7 05=5 9 02=0 0=1
114 R[A[ Phillips\ D[R[ Thompson + K[C[ Hamer
Table 4[ Total _sh\ goose barnacle and seabird consumption by great skuas at St Kilda in 0885 Total mass consumed "kg# by
Fish Goose barnacle Leach|s petrel British storm petrel Kittiwake Pu.n Guillemot Fulmar Manx shearwater
Breeding adults
Chicks and ~edglings
Non! breeders
Colony total
09 559 0424 479 059 414 789 3304 0024 009
0989 9 51=8 06=6 46=3 86=0 370 013 00=7
328 72=2 17=3 5=4 5=4 59=8 9 36=7 3=3
01 089 0519 569 074 489 0949 3784 0294 014
Table 5[ Total numbers of seabirds consumed by great skuas at St Kilda in 0885 Total numbers consumed by
great skua population during the 013!day breeding season[ Sensitivity analysis
Þ 0888 British Ecological Society\ Journal of Applied Ecology\ 25\ 107Ð121
Changes in caloric density by 2 14)\ or in mean mass of meals of _sh or the larger bird species by 2 29)\ generally resulted in a 09Ð19) change in the estimated mass of _sh and goose barnacles\ and estimated total numbers of the larger birds consumed during the 0885 season "Appendix#[ Changes to the value for goose barnacle caloric density had very little in~uence\ whereas a potential increase "by 49)# or decrease "by 14)# in the mean meal mass had a sub! stantial e}ect on total consumption of this item\ but trivial consequences for the estimated quantities of other prey consumed[ Altering the mass of the two small petrels by 2 09) had very little e}ect on the output of the model[ The total number of seabirds consumed "and particularly of the _ve larger species# was more sensitive to a change in the percentage uti! lization of each carcass[ An increase to 014) of the original value resulted in a 09) decline in overall numbers eaten\ whereas a decrease to 64) of the
original value resulted in a 05) increase in seabird consumption[ Altering the proportion of a particular prey in the diet of non!breeders or young to the proposed upper and lower extremes of 2 49) generally had a very minor in~uence on the output of the model[ This was not the case for breeding adults[ An increase to the proposed limit of 049) of the observed proportion of _sh in the diet of breeders resulted in a considerable rise in the estimate of total _sh consumption\ and a substantial decline in goose barnacle and seabird consumption[ A 49) decrease\ not surprisingly\ had the opposite e}ect[ Raising or lowering the proportion of goose barnacle in the diet of breeding adults resulted in very large changes in estimated con! sumption of barnacles\ but not of any other prey type[ Adjusting the observed proportion of each bird spec! ies in the diet of breeders to the 2 49) extremes had dramatic consequences for that particular species\ but this was balanced by smaller changes in the opposite direction for the other species[ Consequently\ at most there was a 04) change in the overall _gure when the proportion of Leach|s petrel changed\ but generally only a 09) or smaller change occurred when the
115 Impact of great skua predation
the proportion of one of the other six species was altered[
Discussion
Þ 0888 British Ecological Society\ Journal of Applied Ecology\ 25\ 107Ð121
As in all bioenergetics studies of this nature\ there are a number of potential sources of error in the esti! mation of values for the di}erent parameters used in the model[ The results of the sensitivity analysis highlighted the importance of several variables\ in par! ticular those associated with total number and daily energy expenditure of breeders[ One of the most cru! cial parameters was clearly the FMR]BMR ratio of breeding adults\ because the overall energy require! ment of this group was very high compared to that of non!breeders or young "chicks and ~edglings#[ The use of a seasonal average of 2=4 times BMR\ however\ seems a reasonable approach\ because this is in the mid!range of published values for seabirds\ which tend to average between three and four times BMR across the season as a whole "Gabrielsen\ Mehlum + Nagy 0876^ Birt!Friesen et al[ 0878^ Gabrielsen et al[ 0880^ Montevecchi\ Birt!Friesen + Cairns 0881^ Mehlum\ Gabrielsen + Nagy 0882^ Furness + Bryant 0885#[ A few species\ including Cape gannets Morus capensis "Lichtenstein# and northern gannets\ which use sus! tained ~apping ~ight and plunge!dive during foraging\ and common guillemots\ which are wing!propelled pursuit divers\ have an FMR]BMR ratio in excess of 3 ] 0 "Birt!Friesen et al[ 0878^ Cairns et al[ 0889^ Adams et al[ 0880#[ However\ the foraging modes of these species are likely to be energetically expensive in com! parison with great skuas[ The equations employed in this study to estimate total energy demand of growing chicks and daily energy requirements of ~edglings were based on detailed work on four closely related semi!precocial species which exhibited very similar patterns of total metabolizable energy intake during development\ and as such are likely to be appropriate for great skuas[ Recent work "e[g[ Bolton\ Houston + Monaghan 0881# suggests that egg production may be limited by the availability of speci_c nutrients at the time of clutch formation rather than energetic constraints[ If this is the case\ adult foraging e}ort may be com! paratively high over a short period prior to egg!laying\ but this would not be incorporated in the component for egg production using the partitioning method adopted here[ However\ adult activity costs during the season as a whole are taken account of elsewhere in the model\ and\ assuming any increase in activity occurred for a reasonably brief period during egg for! mation\ the e}ect on overall energy requirements is likely to be slight[ One other possible error is the estimate of the num! ber of non!breeders present at the colony\ hence the comparatively high upper boundary proposed for this parameter in the sensitivity analysis[ Counts of birds at the club site at St Kilda remained fairly uniform
during the course of the season[ However\ turnover rates of non!breeding skuas are usually high\ with individuals at Foula generally in attendance for 05Ð 10 days at the colony\ although ringing recoveries sug! gest that non!breeding great skuas remain in northern latitudes throughout the summer "Klomp + Furness 0881#[ Whether these birds forage in close proximity to breeding colonies when not actively in attendance at a club is unknown[ If they do feed near the colony\ then the predicted energy requirement for non!breed! ing skuas at St Kilda will be underestimated in this study to an uncertain extent[ Sensitivity tests on related bioenergetics models generally found that variation in the exponents and multiplicands of metabolic equations had the greatest e}ect on output "Furness 0867^ Diamond\ Gaston + Brown 0882#[ This model did not require the use of these very general equations "those used in this study to calculate chick and ~edgling energy demands were derived from closely related species# and so avoided many of the potential complications[ Therefore\ while bearing the results of the sensitivity analysis and the above considerations in mind\ we can be fairly con! _dent that the _nal _gures for population energy demand are reasonably accurate[
LIMITATIONS OF PREY CONSUMPTION MODEL
Determination of the total mass of di}erent prey necessary to ful_l the population energy demands of great skuas relied heavily on adequate estimation of mean meal mass and on the use of pellets to assess diet composition[ The sensitivity analysis indicated that large increases or decreases in the proportion of di}erent prey in the diet of non!breeders or chicks had comparatively little e}ect on the estimation of total food consumption\ whereas any change in the prey ratio of breeding adults was much more in~uential[ This underlines the importance of accurate charac! terization of diet\ particularly of breeding birds[ Many previous studies of seabirds have incor! porated the analysis of prey remains in pellets as a means of assessing diet "e[g[ Du}y + Jackson 0875^ Hamer\ Furness + Caldow 0880^ Mund + Miller 0884#[ Pellet analysis does have several inherent limi! tations[ Soft!bodied prey\ e[g[ _sh o}al\ are likely to leave few\ if any\ remains[ However\ this was unlikely to be a problem in this study because great skuas obtain very little discarded o}al from _shing vessels\ as fulmars compete much more e}ectively for this resource "Hudson + Furness 0878#[ Foraging ranges of great skuas feeding on goose barnacles or discards might be greater and\ in theory\ pellets sampled at the colony could be biased towards seabird remains if the latter are consumed closer inshore[ However\ great skuas regurgitate freshly obtained prey to their mate or chicks after returning from almost all absences from the territory "Furness 0876^ Hamer\ Furness + Caldow 0880#[ In addition\ attendance of great skuas
116 R[A[ Phillips\ D[R[ Thompson + K[C[ Hamer
at St Kilda is very high\ indicating that foraging trips are relatively short "Phillips et al[ 0886#[ It is therefore unlikely that many foraging trips are of su.cient dur! ation to allow for digestion of prey and the production of pellets away from the colony[ Pellets persist for di}ering time periods after pro! duction depending on their composition\ potentially biasing analyses towards more resilient pellet types\ particularly those of feathers "Furness + Hislop 0870#[ Restricting the comparison to the chick!rearing period\ when the diet of adults was assessed solely by pellets\ and that of chicks by regurgitates\ an estimated 46) of meals of breeding great skuas\ and 29) of those of non!breeding skuas were of birds[ If birds were being grossly overestimated\ this should be detectable in a comparison with their relative inci! dence in chick regurgitates\ which can reasonably be considered an unbiased representation of diet[ Bird meat did occur in a lower proportion of chick regur! gitates "36)\ n 08# compared with breeders| "although not nonbreeders?# diet during this period\ but the di}erence was only moderate "09)#[ In addition\ Watanuki "0878# found a high positive cor! relation "r 9=71# between the percentage of seabirds in pellets or food remains and their occurrence in food loads delivered to slaty!backed gull Larus schistisagus Stejneger chicks[ Annett + Pierotti "0878# also con! cluded\ in a comparison of pellets\ stomach contents\ and chick and mate!feedings of western gull Larus occidentalis Audubon\ that pellets re~ected dietary composition with reasonable accuracy[ As far as estimating meal mass is concerned\ the assumption that meals of _sh and seabirds "other than the two smallest species# had a similar mean mass is probably valid[ Because of wing!loading consider! ations\ it is likely that a maximum payload exists just below that which would jeopardize ~ight perform! ance[ The maximum mass of food loads delivered by adult Catharacta skuas to their mate or brood is surprisingly consistent\ at 008Ð039 g "Young 0852^ Furness + Hislop 0870^ Reinhardt 0886#[ Given that there is bound to be some variance in payload mass somewhere below this level\ an assumed mean meal mass of 099 g seems appropriate[ Bearing this and the above considerations in mind\ it is therefore reason! able to conclude that the _nal estimates of total mass of prey and numbers of seabirds consumed are at least broadly applicable\ and certainly in the right orders of magnitude[
RELATIVE ENERGETIC CONTRIBUTION OF DIFFERENT PREY
Þ 0888 British Ecological Society\ Journal of Applied Ecology\ 25\ 107Ð121
Goose barnacles\ although constituting 05) and 13) of meals of breeding adults and non!breeders\ respec! tively\ contributed comparatively little "under 3)# to total energy consumption[ Goose barnacle tissue has a caloric value of 16=9 kJ gÐ0 dry mass\ which is higher than\ for example\ whole pelagic amphipods and krill\
or shrimp "Massias + Becker 0889^ Mehlum\ Gab! rielsen + Nagy 0882#[ However\ because of the large shell plates\ the overall caloric density is very poor "0=8 kJ gÐ0 fresh mass#\ just under 64) of the value for non!gravid\ {shelled| benthic invertebrates such as star_sh\ crabs and urchins measured by Hunt "0861#[ Crustacea appear to be a suboptimal food choice in general[ An adult great skua feeding solely on goose barnacles would require a total of 051 kg per season for activity and maintenance alone\ which at 39 g a meal would require 22 meals a day\ representing a very large number of foraging trips and:or long per! iods spent away from the colony leaving chicks unguarded and open to predation[ This compares with a total of 48 kg of _sh or 17 kg of birds\ which would require around _ve meals and two meals "of the larger species# a day\ respectively[
IMPACT ON SEABIRD POPULATIONS
One possibility to consider before discussing the potential impact of predation on other species is that great skuas might obtain a proportion of seabird prey by scavenging rather than by active hunting[ Studies of several species\ including guillemots and kittiwakes at sites where presumably predation pressure is low\ conclude that survival of breeding adults is very high during the breeding season "Birkhead 0863^ Aebischer + Coulson 0889#\ although of course\ at very large colonies\ this might still mean a fair number of car! casses could be available[ In addition\ non!breeder mortality may be greater\ and therefore more corpses might be available from this source[ A critical determinant of whether predation might have an impact on prey populations is whether hunt! ing e}ort is directed mostly at breeding adults\ visiting non!breeders\ chicks or ~edglings[ A few pellets of kittiwake\ fulmar and pu.n\ and many pellets con! taining guillemot remains found at St Kilda in 0885\ were from chicks or ~edglings "Phillips\ Thompson + Hamer 0886^ Phillips et al[ 0886#[ Unfortunately it is di.cult to determine the exact proportion because\ excepting those that include an entire skull\ pellets from adult prey rarely have a distinguishing charac! teristic[ Positive identi_cation will therefore often be slightly biased towards chicks[ These are swallowed whole and tend to leave more obvious remains\ par! ticularly legs in the case of guillemot ~edglings[ A large percentage of great skua predation might be directed at visiting non!breeders[ In a study on Dun\ St Kilda\ during the 0869s\ of 1599 pu.ns killed by great black!backed gulls Larus marinus\ 32) were non!breeders "Harris 0879#[ It is extremely di.cult to assess relative number of immatures compared to breeding birds attending colonies\ and usually imposs! ible to determine the breeding status of prey[ Transient non!breeding Leach|s petrels and British storm petrels visit colonies in huge numbers "Fowler\ Okill + Mar! shall 0871#[ In addition\ detailed demographic and
117 Impact of great skua predation
Þ 0888 British Ecological Society\ Journal of Applied Ecology\ 25\ 107Ð121
ringing studies suggested that numbers of non!breed! ing Manx shearwaters and fulmars were equivalent to 64) and 199Ð499)\ respectively\ of breeding popu! lation sizes "Brooke 0889^ Dunnet 0880#[ Similarly\ immature auks also visit colonies in large numbers[ Up to 49) of pu.ns trapped on the Isle of May "Scotland# during the latter part of the season were probably too young to breed\ and an analysis of com! mon guillemot life!table data indicated that non! breeder numbers were equivalent to ¼ 29) of the breeding population "Harris 0873^ Cairns et al[ 0889#[ This potential availability of very large numbers of immatures or non!breeders should be borne in mind\ because if a large proportion of skua attacks are directed at this group\ the impact on prey species| population dynamics will be considerably lower than would otherwise be expected[ Surveys at St Kilda in 0876 by Tasker\ Moore + Scho_eld "0877# estimated that there were 51 799 apparently occupied fulmar nest sites\ 6799 apparently occupied kittiwake nest sites\ 129 499 pu.n burrows and 11 699 individual guillemots[ Assuming\ for illustrative purposes only\ that great skua predation was directed solely at breeding adults\ and excluding guillemots because we know that a very substantial proportion of birds consumed are chicks or ~edglings\ the seasonal total for each species eaten by great skuas in Table 5 would be equivalent to 1=9) of breeding fulmars\ 04=4) of breeding kittiwakes and 9=8) of breeding pu.ns[ In fact\ the impact on the population is likely to be less than this\ as an unknown proportion of birds eaten will be non!breeders\ chicks or ~edglings\ even for these species[ It is currently impossible to assess the impact of predation on Leach|s petrel\ British storm petrel and Manx shearwater by great skuas[ Even the approximate sizes of their breeding populations are unknown due to the heterogeneous nature of the nest! ing habitat "which includes grass slopes\ stone walls\ crevices in boulder scree\ etc[# and the inaccessibility of two islands\ Soay and Boreray[ Nonetheless\ it is clear that very large numbers of these petrels\ whether breed! ing adults or non!breeders\ are being killed[ Other studies have concluded that substantial mor! tality occurs as a consequence of predation by skuas or gulls[ During the 0869s\ great black!backed gulls on Dun were estimated to consume ¼ 0=4) of the breeding population of pu.ns per year "Harris 0879#[ A recent study at a Newfoundland colony also found a very high rate of predation by great black!backed gulls\ with 1=8 pu.ns found dead for every 099 breed! ing pairs of pu.ns present "Russell + Montevecchi 0885#[ Annual predation rate of Leach|s petrels by a colony of 2499 pairs of slaty!backed gulls on Daikoku Island\ Japan\ was apparently even higher\ with up to 38 999 petrels killed in a single month and an estimated 02=1) of all adult petrels attending the colony killed annually "Watanuki 0875#[ This last study dem! onstrates that a large colony of gulls can have a very severe impact on a seabird population[
The great skua colony on Hirta has grown extremely rapidly in the last few years\ the population having almost doubled since 0882 "Phillips et al[\ in press#[ Extensive surveying of St Kildan seabird col! onies in 0876 by Tasker\ Moore + Scho_eld "0877# did not provide any evidence that fulmar\ kittiwake\ pu.n or guillemot populations had declined since great skua colonization in 0852\ although that survey was undertaken before the recent expansion in skua numbers[ More recent monitoring of subcolonies vis! ible from land indicates no change in the number of apparently occupied fulmar nest sites\ an increase in guillemot numbers\ but a substantial decline in occu! pied kittiwake nest sites at St Kilda in 0885 compared with 0882 "Thompson + Walsh 0886#[ It is possibly no coincidence that of these species\ the kittiwake is the one on which we might predict that great skua predation would have the greatest impact[ In terms of available space\ the great skua population at St Kilda could continue increasing for a number of years "Phil! lips et al[\ in press#[ Given that St Kilda is considered to hold amongst the largest European populations of pu.ns\ fulmars\ Leach|s petrel and British storm petrel\ the exact numbers of which are unknown for the last two species\ close monitoring could be deemed a judicious approach in this situation[
Acknowledgements Fieldwork in 0885 on Hirta was supported by a grant from Scottish Natural Heritage[ For help with _eld! work or logistics at St Kilda we thank John Love\ Stuart Murray\ Gail Churchill and Phil Sharkey[ We are extremely grateful to Geo} Hilton for allowing us access to unpublished data on assimilation e.ciency and body composition of seabirds[ Robert Furness\ Kate Thompson and an anonymous referee made many helpful comments on the manuscript[
References Adams\ N[J[\ Abrams\ R[W[\ Siegfried\ W[R[\ Nagy\ K[A[ + Kaplan\ I[R[ "0880# Energy expenditure and food con! sumption by breeding Cape gannets Morus capensis[ Mar! ine Ecology Progress Series\ 69\ 0Ð8[ Aebischer\ N[J[ + Coulson\ J[C[ "0889# Survival of the kit! tiwake in relation to year\ breeding experience and position in the colony[ Journal of Animal Ecology\ 48\ 0952Ð0960[ Annett\ C[ + Pierotti\ R[ "0878# Chick hatching as a trigger for dietary switching in the western gull[ Colonial Water! birds\ 01\ 3Ð00[ Birkhead\ T[R[ "0863# Movement and mortality rates of Brit! ish guillemots[ Bird Study\ 10\ 130Ð143[ Birt!Friesen\ V[L[\ Montevecchi\ W[A[\ Cairns\ D[K[ + Macko\ S[A[ "0878# Activity!speci_c metabolic rates of free!living northern gannets and other seabirds[ Ecology\ 69\ 246Ð256[ Bolton\ M[\ Houston\ D[ + Monaghan\ P[ "0881# Nutritional constraints on egg formation in the lesser black!backed gull] an experimental study[ Journal of Animal Ecology\ 50\ 410Ð421[ Brisbin\ I[L[ "0857# A determination of the caloric density
118 R[A[ Phillips\ D[R[ Thompson + K[C[ Hamer
Þ 0888 British Ecological Society\ Journal of Applied Ecology\ 25\ 107Ð121
and major body components of large birds[ Ecology\ 38\ 681Ð683[ Brooke\ M[ de L[ "0889# The Manx Shearwater[ T[ + A[D[ Poyser\ London[ Bryant\ D[M[ + Furness\ R[W[ "0884# Basal metabolic rates of North Atlantic seabirds[ Ibis\ 026\ 108Ð115[ Cairns\ D[K[\ Montevecchi\ W[A[\ Birt!Friesen\ V[L[ + Macko\ S[A[ "0889# Energy expenditures\ activity budgets\ and prey harvest of breeding common murres[ Studies in Avian Biology\ 03\ 73Ð81[ Caldow\ R[W[G[ "0877# Studies on the morphology\ feeding behaviour and breeding biology of skuas with reference to kleptoparasitism[ PhD Thesis\ University of Glasgow[ Cramp\ S[ + Simmons\ K[E[L[ "0866# The Birds of the West! ern Palearctic\ Vol[ 0[ Oxford University Press\ Oxford[ Cramp\ S[ + Simmons\ K[E[L[ "0872# The Birds of the West! ern Palearctic\ Vol[ 2[ Oxford University Press\ Oxford[ Cresswell\ W[ + Whit_eld\ P[ "0883# The e}ects of raptor predation on wintering wader populations at the Tyn! inghame estuary\ southeast Scotland[ Ibis\ 025\ 112Ð121[ Dein\ F[J[ "0875# Hematology[ Clinical Avian Medicine and Surgery "eds G[J[Harrison\ +\ L[R[Harrison#[ Saunders\ USA[ Diamond\ A[W[\ Gaston\ A[J[ + Brown\ R[G[B[ "0882# Stud! ies of high latitude seabirds[ 2[ A model of the energy demands of the seabirds of eastern and Arctic Canada[ Can! adian Wildlife Service Occasional Papers no[ 66\ Ottowa[ Drent\ R[H[\ Klaassen\ M[ + Zwaan\ B[ "0881# Predictive growth budgets in terns and gulls[ Ardea\ 79\ 4Ð06[ Du}y\ D[C[ + Jackson\ S[ "0875# Diet studies of seabirds] a review of methods[ Colonial Waterbirds\ 8\ 0Ð06[ Dunn\ E[H[ "0864# Growth\ body components and energy content of nestling double!crested cormorants[ Condor\ 66\ 320Ð327[ Dunnet\ G[M[ "0880# Population studies of the fulmar on Eynhallow\ Orkney Islands[ Ibis\ 022 "Suppl[ 0#\ 13Ð16[ Fowler\ J[A[\ Okill\ J[D[ + Marshall\ B[ "0871# A retrap analysis of storm petrels tape!lured in Shetland[ Ringing and Migration\ 3\ 0Ð6[ Furness\ R[W[ "0867# Energy requirements of seabird com! munities] a bioenergetics model[ Journal of Animal Ecol! ogy\ 36\ 28Ð42[ Furness\ R[W[ "0870# The impact of predation by great skuas Catharacta skua on other seabird populations at a Shet! land colony[ Ibis\ 012\ 423Ð428[ Furness\ R[W[ "0876# The Skuas[ T[ and A[D[ Poyser\ Calton\ Sta}ordshire[ Furness\ R[W[ + Bryant\ D[M[ "0885# E}ect of wind on _eld metabolic rates of breeding northern fulmars[ Ecology\ 66\ 0070Ð0077[ Furness\ R[W[ + Hislop\ J[R[G[ "0870# Diets and feeding ecology of great skuas Catharacta skua during the breeding season[ Journal of Zoology\ London\ 084\ 0Ð12[ Furness\ R[W[\ Thompson\ D[R[ + Harrison\ N[ "0883# Bio! metrics and seasonal changes in body composition of com! mon guillemots Uria aalge from north!west Scotland[ Sea! bird\ 05\ 11Ð1[ Gabrielsen\ G[W[\ Mehlum\ F[ + Nagy\ K[A[ "0876# Daily energy expenditure and energy utilization of free!ranging black!legged kittiwakes[ Condor\ 78\ 015Ð021[ Gabrielsen\ G[W[\ Taylor\ J[R[E[\ Konarzewski\ M[ + Mehlum\ F[ "0880# Field and laboratory metabolism and thermoregulation in dovekies "Alle alle#[ Auk\ 097\ 60Ð67[ Galbraith\ H[ "0872# The diet and feeding ecology of breeding kittiwakes Rissa tridactyla[ Bird Study\ 29\ 098Ð019[ Green\ K[ "0875# Observations on the food of the south polar skua\ Catharacta maccormicki near Davis\ Antarctica[ Polar Biology\ 5\ 074Ð075[ Hamer\ K[C[\ Furness\ R[W[ + Caldow\ R[W[G[ "0880# The e}ects of changes in food availability on the breeding
ecology of great skuas Catharacta skua in Shetland[ Jour! nal of Zoology\ London\ 112\ 064Ð077[ Harkonen\ T[ "0875# Guide to the Otoliths of the Bony Fishes of the Northeast Atlantic[ Danbiv ApS\ Hellerup\ Denmark[ Harris\ M[P[ "0879# Breeding performance of pu.ns Fra! tercula arctica in relation to nest density\ laying date and year[ Ibis\ 011\ 082Ð198[ Harris\ M[P[ "0873# The Puf_n[ T[ + A[D[ Poyser\ Calton\ Sta}ordshire[ Hemmings\ A[D[ "0873# Aspects of the breeding biology of McCormick|s skua Catharacta maccormicki at Signy Island\ South Orkney Islands[ British Antarctic Survey Bulletin\ 54\ 54Ð68[ Hislop\ J[R[G[\ Harris\ M[P[ + Smith\ J[G[M[ "0880# Vari! ation in the calori_c value and total energy content of the lesser sandeel "Ammodytes marinus# and other _sh preyed on by seabirds[ Journal of Zoology\ London\ 113\ 490Ð406[ Hudson\ A[V[ + Furness\ R[W[ "0878# The behaviour of seabirds foraging at _shing boats around Shetland[ Ibis\ 020\ 114Ð126[ Hunt\ G[L[ "0861# In~uence of food distribution and human disturbance on the reproductive success of herring gulls[ Ecology\ 42\ 0940Ð0950[ Jones\ E[ + Skira\ I[J[ "0868# Breeding distribution of the great skua at Macquarie Island in relation to numbers of rabbits[ Emu\ 68\ 08Ð12[ Klomp\ N[I[ + Furness\ R[W[ "0881# Non!breeders as a bu}er against environmental stress] declines in numbers of great skuas on Foula\ Shetland\ and prediction of future recruitment[ Journal of Applied Ecology\ 18\ 230Ð237[ Korpimaki\ E[ "0883# Rapid or delayed tracking of multi! annual vole cycles by avian predators[ Journal of Animal Ecology\ 52\ 508Ð517[ Massias\ A[ + Becker\ P[H[ "0889# Nutritive value of food and growth in common tern Sterna hirundo chicks[ Ornis Scandinavica\ 10\ 076Ð083[ Meathrel\ C[E[ + Ryder\ J[P[ "0876# Intraclutch variation in the size\ mass and composition of ring!billed gull eggs[ Condor\ 78\ 253Ð257[ Meathrel\ C[E[\ Ryder\ J[P[ + Termaat\ B[M[ "0876# Size and composition of herring gull eggs] relationship to pos! ition in the laying sequence and the body condition of females[ Colonial Waterbirds\ 09\ 44Ð52[ Mehlum\ F[\ Gabrielsen\ G[W[ + Nagy\ K[A[ "0882# Energy expenditure by black guillemots "Cepphus grylle# during chick!rearing[ Colonial Waterbirds\ 05\ 34Ð41[ Montevecchi\ W[A[\ Birt!Friesen\ V[L[ + Cairns\ D[K[ "0881# Reproductive energetics and prey harvest of Lea! ch|s storm!petrels in the northwest Atlantic[ Ecology\ 62\ 712Ð721[ Mund\ M[J[ + Miller\ G[D[ "0884# Diet of the south polar skua Catharacta maccormicki at Cape Bird\ Ross Island\ Antarctica[ Polar Biology\ 04\ 342Ð344[ Norman\ F[I[ + Ward\ S[J[ "0889# Foods of the south polar skua at Hop Island\ Rauer Group\ East Antarctica[ Polar Biology\ 09\ 378Ð382[ Osborn\ D[ + Harris\ M[P[ "0873# Organ weights and body composition in three seabird species[ Ornis Scandinavica\ 04\ 84Ð86[ Phillips\ R[A[\ Bearhop\ S[\ Hamer\ K[C[ + Thompson\ D[R[ "in press# Rapid population growth of great skuas at St Kilda] implications for management and conservation[ Bird Study\ in press[ Phillips\ R[A[\ Catry\ P[\ Thompson\ D[R[\ Hamer\ K[C[ + Furness\ R[W[ "0886# Inter!colony variation in diet and reproductive performance of great skuas Catharacta skua[ Marine Ecology Progress Series\ 041\ 174Ð182[ Phillips\ R[A[\ Thompson\ D[R[ + Hamer\ K[C[ "0886# The population and feeding ecology of great skuas Catharacta
129 Impact of great skua predation
skua at Hirta\ St Kilda[ Unpublished Report to SNH\ Inverness[ Pietz\ P[J[ "0876# Feeding and nesting ecology of sympatric south polar and brown skuas[ Auk\ 093\ 506Ð516[ Reinhardt\ K[ "0886# Food and feeding of Antarctic skua chicks Catharacta antarctica lonnbergi and C[ Maccor! micki[ Journal fur Ornithologie\ 027\ 088Ð102[ Rennie\ F[W[ "0877# The status and distribution of the great skua in the Western Isles[ Scottish Birds\ 04\ 79Ð71[ Ricklefs\ R[E[ "0863# Energetics of reproduction in birds[ Avian Energetics "ed[ R[A[ Paynter#\ pp[ 041Ð180[ Nuttal Ornithological Club\ Cambridge\ Mass[ Ricklefs\ R[E[ "0872# Some considerations on the repro! ductive energetics of pelagic seabirds[ Studies in Avian Biology\ 7\ 73Ð83[ Russell\ J[ + Montevecchi\ W[A[ "0885# Predation on adult pu.ns Fratercula arctica by great black!backed gulls Larus marinus at a Newfoundland colony[ Ibis\ 027\ 680Ð 683[ Ryan\ P[G[ + Moloney\ C[L[ "0880# Prey selection and tem! poral variation in the diet of subantarctic skuas at Inac! cessible Island\ Tristan da Cunha[ Ostrich\ 51\ 41Ð47[ Schramm\ M[ "0872# Predation by subantarctic skuas Catharacta antarctica on burrowing petrels at Marion Island[ South African Journal of Antarctic Research\ 02\ 30Ð33[ Tasker\ M[L[\ Moore\ P[R[ + Scho_eld\ R[A[ "0877# The seabirds of St Kilda\ 0876[ Scottish Birds\ 04\ 10Ð18[
Thompson\ D[R[\ Hamer\ K[C[ + Phillips\ R[A[ "0887# Fish prey in the diet of great skuas at St Kilda[ Scottish Birds\ 08\ 059Ð054[ Thompson\ K[R[ + Walsh\ P[M[ "0886# Seabird monitoring on Hirta and Dun\ St Kilda[ 0876Ð85[ Joint Nature Con! servation Committee Report No[ 165\ Peterborough[ Trivelpiece\ W[ + Volkman\ N[J[ "0871# Feeding strategies of sympatric south polar Catharacta maccormicki and brown skuas C[ lonnbergi[ Ibis\ 013\ 49Ð43[ Watanuki\ Y[ "0875# Moonlight avoidance behaviour in Lea! ch|s storm!petrels as a defence against slaty!backed gulls[ Auk\ 092\ 03Ð11[ Watanuki\ Y[ "0878# Sex and individual variations in the diet of slaty!backed gulls breeding on Teuri Island\ Hokkaido[ Japanese Journal of Ornithology\ 27\ 0Ð02[ Wiens\ J[A[ "0873# Modelling the energy requirements of seabird populations[ Seabird Energetics[ "eds G[C[Whittow + H[Rahn#\ pp[ 144Ð173[ Plenum Press\ New York[ Wiens\ J[A[ + Scott\ J[M[ "0864# Model estimation of energy ~ow in Oregon coastal seabird populations[ Condor\ 66\ 328Ð341[ Young\ E[C[ "0852# Feeding habits of the south polar skua Catharacta maccormicki[ Ibis\ 094\ 290Ð207[ Young\ E[C[ "0883# Skua and Penguin] Predator and Prey[ Cambridge University Press\ Cambridge[ Received 3 August 0886^ revision received 19 January 0888
Appendix Sensitivity tests for the prey consumption model[ Shown in the table are the percentage changes in output estimates for kilos of _sh\ kilos of goose barnacles\ and numbers of birds consumed resulting from a 0) increase in the value of an input parameter\ and probable maximum and minimum extremes in input parameters "see Methods#[ Note that goose barnacles and guillemots are absent from the diet of chicks and non!breeders\ respectively[ Values in parentheses are means for 0 Leach|s petrel\ 1 British storm petrel\ 2 kittiwake\ 3 pu.n\ 4 guillemot\ 5 fulmar\ 6 Manx shearwater[ A zero value indicates a negligible change "2³9=994 for a 0) change in an input parameter\ and 2³9=94) for likely extremes#[
Parameter Fish caloric density
Goose barnacle caloric density
Bird caloric density
Fish meal mass
Goose barnacle meal mass
Large bird meal mass
Leach|s petrel mass
British storm petrel mass
Þ 0888 British Ecological Society\ Journal of Applied Ecology\ 25\ 107Ð121
Percentage carcass utilization
) change in ) change _sh input parameter mass "kg#
