47

Changesin populationsof breedingwaders on the machair of North Uist, Scotland, 1983- 1998

R.J. Fuller & D.B. Jackson

Fuller,R.J. & Jackson,D.B. 1999. Changesin populationsof breedingwaders on the machairof North Uist, Scotland,1983 - 1998. WaderStudy Group Bull. 90: 47-55.

Surveysbetween 1983 and 1987confirmed that habitats close to the westcoasts of the islandsof North Uist, Benbeculaand South Uist supportedthe densestconcentrations of breedingwaders in Britain. An extensive surveyin 1995 revealedthat large declines in numbersof breedingwaders had occurredsince the 1980s, especiallyon SouthUist andBenbecula. Predation by a recentlyestablished population of hedgehogsappears to be a majorfactor contributing to the declinesof somespecies on SouthUist. In 1998, a surveywas undertakento clarify the statusof breedingwaders on threemachair sites on North Uist which are hedgehog-free.This surveyestablished that Ringed Plover and Dunlin haddeclined at two of the sitessince the 1980s;at onesite, the reductions exceeded 50%. The causesare unclear, though several hypotheses are discussed.In contrast,Oystercatcher had remainedstable at one sitebut had increasedat two sites. There was no evidenceof long-termpopulation change at any sitefor Lapwingor Redshank.Variation in detectability with the stageof breedingcan be a majorsource of errorin countsof breedingwaders; implications for measuringpopulation changes are considered.Future monitoring requirements for waderson the Uists are also discussed.

R..J. Fuller, British Trustfor Ornithology,The Nunnery, Thetford, Norfolk IP24 2PU, UnitedKingdom. D.B. Jackson,Royal Society for the Protectionof Birds,Dunedin House, 25 RavelstonTerrace, Edinburgh EH4 3TP, United Kingdom.

INTRODUCTION waderswere maintainedwithin the studyareas. Landlying on theAtlantic fringe of theislands of SouthUist, Benbecula and North Uist in the Outer Hebrides is In 1993 and 1995, surveysof breedingwaders were characterisedby machair,which is a level plain of vegetated undertakenon the machairof the Uistsand B enbecula using calcareoussand. Partsof thismachair plain are cultivated; methodsthat were comparable with thoseof the earlier otherareas are permanentlywaterlogged. A transitionzone of surveys(Whyte & O'Brien 1995). Thesesurveys indicated roughgrazing land, known asblackland, lies betweenthe that overallnumbers of RingedPlover, Dunlin, Redshank and machairand the acidicpeatlands and moorland that occupythe Snipehad declinedsince 1983 but that therewas no clear greaterpart of theseislands. The machairand blackland cover changein Oystercatcherand Lapwing. The patternof decline approximately200 km2. Thesehabitats support the densest was most consistent on South Uist where the reductions since concentrationsof breedingwaders in Britain and form one of 1983 were estimatedas: RingedPlover -58%, Dunlin -65%, the mostimportant wader breeding grounds in westernEurope. Redshank-43% (Whyte & O'Brien 1995). In the 1980s,the northernend of SouthUist, especiallyto the westof LochBee, In 1983 an extensivesurvey of breedingwaders was carded supportedexceptional concentrations of breedingRinged out on the machair and much of the blackland of the Uists and Plover andDunlin (Etheridge1982, Fuller et al. 1986); these Benbecula(Fuller et al. 1986). The followingpopulation arenow greatlyreduced. On NorthUist the overallpattern estimateswere made: OystercatcherHaematopus ostralegus was alsoone of decliningnumbers of RingedPlover and (2700 pairs),Ringed Plover Charadrius hiaticula (2250 pairs), Dunlin,but no clearchange in the otherspecies. LapwingVanellus vanellus (4350 pairs),Dunlin Calidris alpina(3300 pairs,corrected for detectability),Snipe Predationrates on RingedPlover, Dunlin andRedshank nests Gallinagogallinago (750 pairs)and Redshank Tringa totanus in SouthUist weremuch higher in the 1990sthan in themid (2650 pairs). The numbersof Snipewere probably greatly 1980s(Jackson 1998). The mainnest losses of RingedPlovers underestimated(Fuller et al. 1983). Subsequentsurveys were are dueto predationby CommonGulls Larus canus (Jackson undertakenin sampleareas of machairon North Uist and 1998). For Dunlin and Redshank,however, much of the SouthUist in eachyear between 1983 and 1987 (Fuller & increasedpredation on SouthUist wasby an expanding Percival1988). Throughoutthis period,high densitiesof populationof hedgehogsErinaceus europaeus. Hedgehogs 48

populationson threelarge areas of machairon NorthUist and B anadjacent island which he had previously surveyed each year Harris from1983 to 1987.The aims of the1998 survey were (a) to C•k•Soundof clarifythe current status of breedingwaders in theseareas . wherehedgehogs have yet to becomeestablished and (b) to estimatethe size of thewader populations before colonisation by hedgehogs.This paper summarises the results of theseand earlierwader surveys on threeareas of NorthUist. North Uist STUDY AREAS Thelocations of eachstudy area are shown on Figure 1. Detailsof eacharea are given below. Eachof theseareas was freeof hedgehogsin 1998;we canbe surethis is the case becausesearches for hedgehog scats were negative, local Benbecul croftershave not encountered hedgehogs and no road kills werefound in theseareas. Furthermore, studies of wader breedingsuccess on Baleshareand Soilas in 1997 did not find anyevidence of egglosses due to hedgehogs (D.B. Jackson unpublished).There is also concemthat American mink Mustelavison, which have established feral populations on Lewisand Hams, might spread to theUists. At thetime of I Machairsand dunesand writing,we areunaware of anyconfirmed records of minkon NorthUist or Berneray. ;• 150mLand over

10 Baleshare(Grid referenceNF790610) 1 An islandat the southwestcomer of North Uist connectedto km themain island by a causeway.The island contains large samplesof all thetypical machair and blackland habitats (Fuller1981). Waders were surveyed across the entire island (totalarea 8.8 km2). Hedgehogs have colonised adjacent parts of southwestNorth Uist but thereis no evidencethat a Sound of populationhas become established onBaleshare. Grazing Eriskay pressurefrom sheep is currentlyhigher on parts of Baleshare

Figure 1 The approximatedistribution of machairand sand dunes on North thanon the other study areas. Cattle are also grazed on parts of the island. Uist,Benbecula and South Uist. Locations of three areas surveyed in 1998 are shown:(A) Baleshare,(B) Soilas,(C) Berneray. Soilas(Grid referenceNF815764) Thissite is formedby thepeninsula of cultivatedand uncultivatedmachair and dunes that projects into the Sound of were introducedinto southernSouth Uist in 1974 and 1975 Hamsfrom Sollas and Middlequarter (study area 4.5 km2). (Angus1993) and have subsequently spread throughout South Thereare only two permanently marshy areas, both on the Uist,Benbecula and into the southern part of NorthUist. At southernboundary. At thetime of thesurvey there were presentall themajor wader breeding areas on North Uist and substantialnumbers of cattlegrazing parts of the machair. adjacentislands remain free of hedgehogs.It is especially important,therefore, to documentcurrent population sizes in Berneray(Grid referenceNF905820) theseareas so that any future impacts of hedgehogson wader The site coversthe entire areaof cultivatedand uncultivated populationscan be measured. machairand dunesthat forms the westernhalf of the island (studyarea 5.5 km2). This is the driest of thethree study areas In 1997,breeding success of waderson hedgehog-free areas withno permanently marshy areas, though there are onNorth Uist wasfar higherthan on SouthUist butwas substantialareas of duneslack which are flooded in winter. At similarto thatduring the mid 1980sfor SouthUist (Jackson thetime of the survey cattle, but no sheep, were grazing the 1998).In viewof thishigh breeding success and the high site uncultivated machair and dunes. fidelityof thesebirds (Jackson 1994), it is surprisingthat the 1993and 1995 surveys recorded a decrease inpopulations of SURVEY METHODS RingedPlover, Dunlin andRedshank on someof theseNorth Thesurvey methods were exactly the same as those employed Uistsites. In 1998one of us(RJF) resurveyed breeding wader in theperiod 1983-87 except that much of Baleshareand 49

Table 1 Timing of surveysundertaken at the threesites 1983-1998. 'Early June'indicates that visits were madeduring the first two weeksof June

Baleshare Sollas6 Berneray

1983• earlyJune earlyJune earlyJune 19842 5-6 June 1-2 June 8 June 19852 7-8 June 6 June 10 June 1986: 9-11 June 5 June 4 June 19872 3-7 June 2 June 10 June 19933 31May-3 June earlyJune earlyJune 19954 19-21 June 9 & 16 June 23 June 1998• 3-8 June 4-5 June 9-11 June

Notes: JSurveys by NCC/Wader Study group teams of surveyors( Fuller et al. 1986) 2SurveysbyWader Study Group teams. RJF involved in allthese surveys 3Balesharesurveyed byRJF. Soilas and Berneray surveyed byRoyal Air Force Ornithological Society (RAFOS) (Dodds etal. 1995) 4Surveyby RSPB contract surveyors ( Whyte & O'Brien1995) 5SurveybyRJF with Vickie Heaney (Soilas) and Ruth Fuller (Baleshare) 6Thesite boundary usedin 1984-87 and 1998 differed tothat used inthe 1993 and 1995 surveys. The latter surveys included enclosedfields between the machair plain and the A865 road. The 1993 and 1995 counts have been adjusted by reference tothe pairsummary maps to makethem comparable with other years

Berneraywas surveyedby a singleobserver (RJF) in 1998 thanfactors that influence their absolute accuracy i.e. their rather than by a team of two or three observers.This is not relationshipto real populationsize. consideredto reducecomparability with earliersurveys. Sollaswas covered by a pair of observersin 1998 aswas the Variationin detectabilitywith thestage of breedingcan be a Eachkamishdune system at the southernend of Baleshare. majorsource of errorin countsof breedingwaders. Detailsof themethodology are given by Reed& Fuller(1983) Oystercatcher,Ringed Plover and Lapwing are highly visible andFuller et al. (1986)but a summaryof thekey pointsis throughoutthe breeding season and errors generated by low given here for convenience.The estimationof numbers detectabilityare unlikely to be a problem. However,for revolvescomplete coverage of the chosenstudy area by Dunlinand Redshank, detectability is highlydependent on the walkingtransects placed at 100 or 150m intervalsdepending stageof thebreeding cycle. Thesespecies are very on habitat,terrain and wader densities. A singlevisit is made inconspicuousduring incubation but becomefar more duringthe first two weeksof June. Multiplevisits were detectablewhen they have young, advertising their presence to considered undesirable because of the disturbance caused to observerswith alarmcalls and mobbing behaviour. The thebirds and associated predation risks. Exactlythe same surveyperiod, early June,has been chosen to coincidewith the transectlines were walkedin 1998 asin the surveysconducted time whenmost pairs have chicks. m 1983-87. Locationsof birdsare plotted as accurately as possibleon 1:10,000Ordnance Survey maps using Themain disadvantage of earlyJune surveys is thatyear to species-specificcodes (note, however, that the activitycodes yearvariation in thetiming and success of breedingwill describedin Appendix1 of Reed& Fuller(1983) were greatly influencethe efficiency of thecount. This is a particular. simplifiedfor usein subsequentsurveys). Field recordsof problemfor Lapwing,Dunlin and Redshank. Conducting wadersare interpreted as 'pairs' using species-specific criteria. surveysat approximatelythe sametime of yearcannot guaranteedirect comparability of countsfrom year to year. The timingof countsat thethree sites in eachof theyears for Ideally,it wouldbe best to surveyat thesame stage of which data are available is shown in Table 1. Counts have breedingeach year, but this assumes a high level of nesting beenundertaken during the first two weeksof Juneat all sites synchronywithin the population, which is unrealistic. in all yearswith theexception of 1995when the surveyswere Furthermore,the elevated predation rates experienced in recent somewhatlater, especially at Baleshareand Berneray. years,especially on SouthUist, may have served to reduce synchronyeven further as a resultof higherlevels of FACTORS AFFECTING THE INTERPRETATION re-nesting. OF COUNTS Estimatesofnumbers ofbreeding waders for large' areas, In thecase of Dunlinand Redshank, in a lateseason many especiallywhere the birds are at relativelyhigh density, are birdsmay still be incubatingin earlyJune and detectability necessarilyinexact and must be interpretedcautiously. Here will be relativelylow, Furthermore,in someyears, Redshank we describefactors that may affectthe reliability of the counts cansuffer extremely high local breeding losses (Jackson asindices of populationlevel and population trends, rather 1988),which may result in reduceddetectability at thetime of 50 the surveyor possiblyin prematuredeparture of birds. There RESULTS is, however,little evidencethat this factor has introducedlarge All availablecounts since 1983 are summarisedin Figure 2. A annual variation into the Redshank counts for the three sites comparisonof the 1998 countswith thoseundertaken in the studiedhere. In the caseof Lapwing,by the secondweek of period 1984-87 is givenin Table 2. Resultsfor eachspecies June, sometimesearlier, the birds are startingto gatherin are discussedindividually below. flocks. Theseflocks are excludedfrom countsbecause they may be mixturesof adultsand juveniles, which havenot Oystercatcher necessarilybred on the studyarea concerned.Inevitably, B alesharehas consistentlyheld an extremelylarge population breedingnumbers will be underestimatedonce flocking has of breedingOystercatchers. There is no evidenceof a startedand this problem becomes increasingly severe the later long-term trendon Baleshare(Figure 2). The 1998 estimate the count. Annual variation in weather conditions and wasvery closeto the highestestimate for the period1984-87. predationpressure, will affectsuccess of first clutchesof In contrast,Oystercatchers appear to have increasedat both Lapwingsand consequentlyinfluence the numberof birdsthat Sollasand Berneray. For both sites,the 1998 estimatewas are still on territoryin early June. In yearswith a high failure higherthan those for eachof theyears 1984-87 andaverage rate early in the season,many birdswill lay repeatclutches numberswere approximately40% higher than in the mid and thereforestill have eggsor small youngin early June. In 1980s (Table 2). Oystercatcherestimates for 1995 are low yearsof relatively high early success,a muchlarger proportion relative to otheryears in the 1990s (Figure 2). It seems of the populationwill have youngclose to fledgingin early unlikely that this is a consequenceof the relativelylate survey June. datesin 1995because Oystercatcher is not a speciesfor which surveytiming is a criticalfactor. Many variationsin estimatednumbers between years are likely to reflectdifferences in countingefficiency and two Ringed Plover particularpoints need to be bornein mind. First,the survey There is clear evidence of a marked decline at both Baleshare methodcannot reliably detectsmall population changes. and Berneray(Figure 2). The declineoccurred earlier at Observationsof the behaviourof individually markedbirds, Berneraythan at Baleshare. In 1993 estimatednumbers on andexperience of variationin the timing of breeding,justify Berneraywere very similar to thosein 1995 and 1998 and in us giving a cautiousindication of the scaleof the problem. As eachof theseyears were lessthan half the averagefor the a roughguide, we suggestthat variationin detectability period 1984-87. At Baleshare,however, the 1993 estimate betweenyears, due solely to the abovefactors, may introduce was very similar to the 1987 estimatebut thereafternumbers spuriousfluctuations of as •nuchas +/- 20% for Lapwing, droppedconsiderably and in 1998 were on average34% lower Dunlin and Redshankbut perhapsless for Oystercatcherand than in the mid 1980s (Table 2). The situationat Sollasis less RingedPlover. Second,there is a strongpossibility that some clear. The 1993 and 1995 surveysfollowed a very similar low countsmay be artefactsarising from low detectabilityor patternto that at Bernerayi.e. a very markeddecrease since late surveys.When interpretingpopulation changes we the mid 1980s. The 1998estimate was, however, considerably emphasisecases where there appears to be a systematic higherand was only slightlylower thanthe 1987 estimate(142 directionaltrend in numbersor wherethere are large changes and 159pairs respectively). The RingedPlover population at between 1998 and the counts of the mid 1980s. We have Sollasappears now to be very similar to that in the mid 1980s arbitrarilydefined 'a largechange' as 40%; thisminiraises the butnumbers are now far loweron Bernerayand the situation is risk of drawing attentionto changesin numbersthat may intermediateat Baleshare.The relativelylate countdates in actuallybe spurious. 1995are unlikely to haveaffected estimates of thisspecies.

In thispaper we summariseall availablecounts for Baleshare, Lapwing Sollasand Berneray for the period 1983 to 1998. However, Thereis little evidenceof a systematictrend in Lapwing we focusespecially on a comparisonof the countsin 1998 populations,though numbers may now be lower at Berneray with thosemade in 1984-87. The reasonis that during the thanin the mid 1980s. The relativelylow estimatesof 1984-87 periodconsiderable consistency of surveypersonnel Lapwingsin 1995were probably a consequenceof thelate andfield methodswas achieved. RJF participatedin all of the visit dates. This factoris probablyresponsible for muchof the surveysduring 1984-87 as well as undertakingthe 1993 variationbetween years evident in Figure2. Balesharecount. This approachseems prudent because there is some evidence that different observers count waders Dunlin breedingat high densityin differentways (Fuller 1983). We Thereis strongevidence of a largedecline in Dunlin on donot present estimates of Snipenumbers because the method Berneray sincethe mid 1980s. The 1998 estimatewas 59% is far lesssuitable for estimatingdensities and trends in Snipe lower thanthe averagefor 1984-87 (Table 2). Each of the thanfor the otherspecies (Fuller et al. 1983). countsmade on Bernerayin the 1990shas been far lower than any of the countsmade in the 1980s(Figure 2). This is in strikingcontrast to Baleshareand Sollaswhere numbers have 51

300 250

250 - RINGED 2OO / / PLOVER 200 1 50

150-

1 O0

5O

8:3 84 85 86 87 9:3 95 98 83 84 85 86 87 93 95 98

Year Year

250 DUNLIN 2OO

150

100 •

83 84 85 86 87 93 95 98 83 84 85 86 87 98

Year Year

180

16o REDSHANK

140

120 KEY

•100 -... Baleshare ß Soilas ß Berneray 80 -

60 :! , 83 84 85 86 87 93 95 98

Year Figure2 Breedingpopulation estimates (pairs) of Oystercatcher, Ringed Plover, Lapwing, Dunlin and Redshank atBaleshare, Soilas and Berneray 1983-1998. 52

Table 2 Estimatedpopulations of breedingwaders (pairs) in themid 1980sand in 1998with % change betweenthese two periods

Estimates Estimate % change2 1984-19871 1998

Oystercatcher Baleshare 239 (193-270) 264 + 11% (-2%, +37%) Soilas 98 (85-114) 140 +44% (23%, +65%) Berneray 85 (77-94) 120 +41% (+28%, +56%)

Ringed Plover Baleshare 120 (97-166) 79 -34% (-19%,-52%) Sollas 177 (159-205) 142 -19% (-11%, -31%) Berneray 176 (141-209) 76 -56% (-46%, -64%)

Lapwing Baleshare 267 (242-328) 266 0 (-19%, +10%) Soilas 172 (153-232) 131 -23% (-14%,-44%) Berneray 179 (151-197) 119 -33% (-21%, -40%)

Dunlin Baleshare 101 (97-118) 84 -17% (-13%,-29%) Sollas 63 (46-84) 74 +19% (-12%, 61%) Berneray 168 (142-199) 68 -59% (-52ø,4,-66%)

Redshank Baleshare 96 (90-158) 107 +12% (-32%, +19%) Sollas 28 (26-37) 36 +29% (-3%, +38%) Berneray 84 (69-91) 73 -13% (-20%, +6%)

Notes: •Themedian estimate forthe four years isshown with the two most extreme estimates inparentheses. 2%change isthe median change between 1998 and each of the four years 1984-87 with the two most extreme valuesin parentheses. beenrelatively stable (Figure 2). Numbersmay be gradually morethan 50% sincethe mid 1980s. In contrastto Berneray, decliningat Balesharebut at Sollassubstantially more birds thereis no clearevidence of a populationdecline of Ringed were estimated in 1998 than in 1993 and 1995. Plover or Dunlin at Sollas. The situation at Baleshare for these two speciesseems to be intermediatebetween the large Redshank decreaseat Bernerayand apparent stability at Soilas. Breedingpopulations of Redshankwere relatively stable at all threesites (Figure 2). At Balesharethere was an exceptionally Thisstudy indicates that predation by hedgehogsis notthe largeestimate of 158 pairsin 1984 but otherwisenumbers onlyfactor driving recent population changes in breeding haveremained within the range 86 to 108 pairs. There has waders on the Uists. It also demonstrates that on North Uist alsobeen extremely little variationin estimatednumbers at thereare considerable differences between study areas in Sollas. At Berneray,with the exceptionof two low estimates populationtrends. At leastfour types of hypothesiscan be in 1983 and 1995, numbershave ranged between 69 and91 advancedto explainthe declinesin RingedPlover and Dunlin. pairs. The extremelylow estimatefor Bernerayin 1995 was We stressthat this study has not in any way elucidatedthe probablya consequenceof the very late surveydate (23 June) causesof thepopulation changes; these can only be by whichtime it is likely thatmany birds would have departed determinedthrough more detailedand intensivework. The from theimmediate vicinity of the breedingareas. hypothesesare: (a) reducedsurvival outside the breeding season,(b) increasinginter-specific competition, (c) changein DISCUSSION OF POPULATION CHANGES qualityof breedinghabitat, (d) increasingpredation pressure. Populationsizes of Lapwing andRedshank have been largely Thesehypotheses are not mutually exclusive. For example, stableat eachof thethree sites examined between the early therecan be interaction between habitat change and predafion 1980sand the late 1990s. Thereis evidenceof a gradual pressure(Jackson 1988). increasein Oystercatchernumbers at Sollasand Berneray. RingedPlover and Dunlin populations,however, have The firsthypothesis is improbablebecause if therewere a definitelydeclined at somesites. The declinesare most patternof increasingmortality, one would expect a moreeven markedat Berneraywhere both species have decreased by patternof declinein breedingpopulations across the different 53

studyareas. There is alsono evidencethat retum rates of on both Baleshareand Berneray since at leastthe 1970sand individuallymarked adult Dunlin, RingedPlover and probablyfor manyyears previously. There are relativelyfew Redshankhave changed since the 1980s. It is possiblethat as breedingCommon Gulls on the Soilaspeninsula, the site theoverall population level droppeddue to increasing showingleast evidence of declinesin breedingwaders (though mortality,birds might increasingly concentrate in thebest DBJ found that therewas severepredation pressure from qualitybreeding areas which would lead to greaterdeclines in CommonGulls at Sollasin 1997). Unfortunatelythere have someareas than others. However, this would requirehigh beenno long-term systematiccounts of thesegull coloniesbut natalor breedingdispersal which Dunlin andRedshank do not thereis no evidenceof an increasein nestinggulls thatmight exhibitaccording to workundertaken in SouthUist, though be driving increasedpredation on waders. In fact, on RingedPlover does show higher levels of dispersal(Jackson B aleshareit appearsthat gullshave decreasedsince the early 1994). Thereis no evidenceto supportthe competition 1980s. In 1982 RJF estimatedthat 400-500 'pairs'of hypothesis.The onlyspecies of waderthat appears to be CommonGulls, 200 'pairs' of Black-headedGulls Larus increasingis Oystercatcher,which utihses different food ridibundusand 10+ 'pairs' of HerringGulls Larus argentatus resourcesto RingedPlover and Dunlin. were nestingin the dunesystem (Eachkamish) at the southem end of Baleshare. The numbers of Common Gulls now The thirdhypothesis concerns habitat. BreedingRinged breedingon Baleshareare very much lower than these Ploversare stronglyassociated with cultivatedmachair on the estimates.In 1998, DBJ estimated154 alarmingadult U•sts (Fuller et al. 1986, Jackson1994). There has beenno CommonGulls, which may representsome 100 'pairs',and a large-scalereduction in the areaof cultivatedmachair since similar numberin 1997. Thoughno systematiccounts were the 1980sthat could account for the declinesof this specieson made, it is worth noting that in 1998, RJF observedHooded B aleshareor Bemeray. Grazingpressure from sheephas Crowsfar more frequentlyon the Bemeraymachair than at the mcreasedon Balesharesince the early 1980s. There are other two sites. possibleimplications for breedingsuccess of Dunlin,which may be moreexposed to predationin heavilygrazed areas Couldpredation pressure on breedingwaders actually have wherethere is reducedvegetation cover. The qualityof habitat increased,even if gull numbershave beendecreasing? This is for feedingchicks may alsobe reduced,as the vegetation possiblefor severalreasons. First, this could happen where a becomesmore uniform in structureunder heavy grazing. habitatchange, such as might be broughtabout by increased However, this factor doesnot seemespecially relevant on the grazingpressure, alters vegetation structure and reducescover otherstudy areas, where we do not perceivelarge changes in for nests and chicks. Second, the number of fences has grazingpressure. On Berneraythe population decline of increasedon the machairand dunessince the early 1980s. Dunlin andRinged Plover appears especially marked within Thesemay providelook-outs for HoodedCrows, making it the dune slack areas. This doesnot necessarilymean that a easierfor them to locateeggs and chicksof waders. Third, it is habitat-specificfactor (such as dryingout of duneslacks or possiblethat changesin gamekeepingactivity may have higherpredation rates in duneslacks compared with other resultedin increasedpredation pressure from Hooded Crows in habitats)is responsiblefor the decline. Birds may have someareas. Fourth,the importance of wadersin the diet of graduallyredistributed into the mostpreferred habitats as gullsmay dependon the availabilityof otherfood resources. overallpopulation level hasfallen (Fretwell& Lucas1970). If CommonGulls are, for any reason,fmding it hard to collect RingedPlovers nesting in cultivatedhabitats on the Uistsshow food (for exampleinvertebrates on the machair and the the capacityto movebetween breeding seasons to better shoreline)they may tum increasinglyto wadereggs and habitat(Jackson 1994). In conclusion,with the exceptionof chicks. It is possiblethat predation impacts on wadersare intensifiedsheep grazing on B aleshare,there have beenno causedby a smallnumber of individualgulls andthat the obviouschanges in habitatsince the 1980sthat could account numberof gullsexhibiting this behaviouris not correlatedwith for the scaleof populationchanges documented here. We theoverall population size of gulls. Initial observationsby cannot,however, rule outthe possibility that subtle changes DBJ on CommonGull feedingecology in 1997suggest that haveoccurred in vegetationstructure or soil moisture. predationon wadersis a specialistactivity involving some 25% of individualgulls. Finally,it shouldbe notedthat small The final hypothesisconcems predation. The mostimportant wadersmay benefitfrom high nestingdensities of larger predatorsof wadersin the threestudy areas are likely to be species,especially Lapwings and terns, which can give air gullsand crows. Bernerayis just 8 km from mink-inhabited , cover'for smallwaders by vigorouslyattacking pr. edators Hams but we have no evidence that mink have colonised the (Dyrcz et al. 1981). However,this doesnot seemrelevant in island. Little is known aboutthe dynamicsof CommonGull explainingthe wader declinesin the Uists becausethere is no or HoodedCrow Corvuscorone populations in the Uists,nor evidenceof a declinein Lapwingsor terns. abouttheir feedingecology there. Spatialvariation in predafionpressure on wadersmay existon themachair Based on these recent observations we make three main dependingon theproximity of gull colonies,availability of suggestions.First, some further work on possiblechanges in alternativefood for gullsand crows, and variation in crow habitatwould be useful. This couldinvolve examining control. Large coloniesof CommonGulls have beenpresent 54 fine-grain habitatattributes (vegetation structure, floristics, derivedfrom the residualfunding from surveysconducted in soil moisture) at sites that have retained waders and those that the 1980s). The 1983-87 surveyswere conductedunder the have lost them;comparisons could also be madewith detailed auspicesof the WaderStudy Group and the formerNature habitat data collected from the three sites in the 1980s (R.J. ConservancyCouncil. The 1993survey was undertaken by Fuller unpublished).Second, the role of avianpredators, the RoyalAir ForceOmithological Society and the 1995 especiallyof CommonGull and HoodedCrow, in the surveyby the Royal Societyfor the Protectionof Birds. Many populationdynamics of machairbreeding waders is worth peoplehave contributed to the countsof breedingwaders on close examination. On South Uist, there is evidence that the Uistsover the years but specialthanks must go to Harry predationpressure from CommonGulls on RingedPlovers has Green, Mike Pienkowski and Steve Percival who contributed increased since the 1980s (Jackson 1998). The reasonsfor this so much to the surveysof Uist wadersin the 1980s. Vickie are unknown. Third, periodicmonitoring of the wader Heaneyof theRSPB andRuth Fuller helped with the 1998 populationsis necessaryusing methods comparable to surveys.Help with preparingthe manuscript was given by Su previoussurveys. Monitoring is especiallyimportant in Goughand Nicki Read. We thankIan Bainbridge,David assessingthe effectsof any futurecontrol measures that might Gibbons and Ken Smith for their comments. be introducedfor hedgehogsand in assessingthe timing of any changesin wadernumbers on North Uist in relationto the REFERENCES expandinghedgehog population. Large sampleareas need to Angus,S. (ed.) 1993.Hedgehogs in theWestern Isles in 1991. be covered, such as those examined from 1984 to 1987, Hebridean Naturalist 11: 73-76. perhapsalso including islands in the Soundof Harris because theseare unlikelyto be colonisedby hedgehogsthough they Dodds,G., Yaxley,R. & O'Brien,M. 1995. Numbersof may be more vulnerableto mink colonisation.We suggestthat breedingwaders on lowlandareas of the OuterHebrides, countsshould be undertakenat threeyearly intervals. 1993. Reportby RoyalSociety for theProtection of Birds, Edinburgh. For most wader species,rather little is known aboutthe nature of populationfluctuations. It couldbe that stochasticvariation Dyrcz,A., Witkowski,J. & Okulewicz,J. 1981. Nestingof in breedingsuccess and adult survival combinedwith 'timid'waders in thevicinity of 'bold'ones as an antipredator density-dependentforces result in quite wide fluctuations adaptation.Ibis 123: 542-545. within what are long-term stablepopulations. Life history traitswould be expectedto modulatethe size of fluctuations. Etheridge,B. 1982. Distributionof Dunlin Calidrisalpina Long lived species,such as Oystercatcher,would be expected nestson an areaof SouthUist machair.Bird Study29: to showgreater stability than a comparativelyshort-lived 239-243. specieslike RingedPlover. Without this knowledgeit is not possibleto assesswhether observed declines of somespecies Fretwell, S.D. & Lucas, H.L. 1970. On territorial behaviour on North Uist lie withinthe expectednorms of fluctuation. andother factors influencing habitat distribution in birds.I. However, on South Uist and Benbecula, where the declines Theoreticaldevelopment. Acta Biotheoretica 19:16-36. havebeen larger and more consistentbetween sites, and where thereis clearerunderstanding of the factorsresponsible, there Fuller,R.J. 1981. The breedinghabitats of waderson North is firm cause for concern about the future of the wader Uist machair. Scottish Birds 11: 142-152. populations.We stressthat the findingsof thisstudy do not in any way diminishconcern about the impactsof hedgehogson Fuller,R.J. 1983. Somedifferences in the ways thatobservers the breedingwaders of the Uists. Hedgehogshave had a estimatenumbers of wadersbreeding at highdensity. Wader majorimpact on populationsof wadersthat nest in the damper StudyGroup Bull. 40: 8-11. habitatsof SouthUist i.e. Dunlin, Snipeand Redshank. Similarpopulation declines are to be expectedon mainland Fuller, R.J., Green, G.H. & Pienkowski, M.W. 1983. Field North Uist ashedgehogs move furthernorth. observationson methods used to countwaders breeding at Postscript.'Since this article was written, it has been highdensity in theOuter Hebrides, Scotland. Wader Study confirmedthat mink have colonisedNorth Uist. It has also Group Bull. 39: 27-29. becomeclear thatferal ferretsare nowstrongly established on the island. Fuller, R.J., Reed, T.M., Buxton, N.E., Webb, A., Williams, T.D. & Pienkowski,M.W. 1986. Populationsof breeding ACKNOWLEDGEMENTS wadersCharadrii and their habitats on thecrofting lands of the We are gratefulto the North Uist Estate,the Baysof Harris OuterHebrides, Scoff and. BiologicalConservation 37: Estateand the people of Baleshare,Sollas and Berneray for 333-361. permissionto surveythe waderson their land in 1998. The ScottishOrnithologists' Club and the Wader Study Group gave financialassistance with the 1998 surveys(WSG funds 55

Fuller, R.J. & Percival, S.M. 1988. Surveysof breeding Jackson, D.B. 1998. The decline and conservation of Uist's waders in the SouthernIsles of the Outer Hebrides, breedingwaders. Outer Hebrides Western Isles Bird Report 1983-1987. Nature ConservancyCouncil Chief Scientist 1997: 105-112. Directoratecommissioned research report no. 904 Reed, T.M. & Fuller, R.J. 1983. Methods used to assess Jackson,D.B. 1988. Habitat selectionand breedingecology populationsof breedingwaders on machairin the Outer of threespecies of wadersin the WesternIsles of Scotland. Hebrides.Wader Study Group Bull. 39: 14-16. UnpublishedPhD Thesis. Universityof Durham. Whyte,C. & O'Brien,M. 1995 Changesin thenumber of Jackson,D.B. 1994. Breedingdispersal and site-fidelity in wadersbreeding on machair habitatson the Uists and threemonogamous wader species in theWestern Isles, UK. Benbecula,1983-1995. Reportby RoyalSociety for the Ibts 136: 463-473. Protectionof Birds,Edinburgh.