Diurnal activity budgets of breeding Eurasian Haematopus ostralegus feeding on limpets on rocky shores

NATASHA GHOSH, MIRIAM SPERING •, JOANNE WILSHAW & RAJARATHINAVELU NAGARAJAN2'

Universityof Exeter, School of Psychology, WashingtonSinger Laboratories, Perry Road, Exeter, EX4 4QG, United Kingdom

Ghosh,N., Spering,M., Wilshaw,J. & Nagarajan,R. 2003. Diumal activitybudgets of breedingEurasian - catchersHaematopus ostralegus feeding on limpetson rocky shores. Study Group Bull. 101/102:81-87.

The effectof differentstages of the tidal cycle,weather conditions and the presenceof con- andallo-specifics on the time and activity budgetsof EurasianOystercatchers Haematopus ostralegus was investigated.Data were collectedat a breedingsite on Island in the Bristol Channel,UK, where Oystercatchersfeed on limpetsPatella spp.on rocky shores.Sixteen behaviours were recordedfor 10-minuteperiods throughout the day. Grooming,feeding, antagonistic behaviours and alertness emerged as the four major behaviouralcategor- ies from a PrincipalComponent Analysis that was performedon the frequencyand durationaldata. Oyster- catcherswere more active duringlow tide than high tide. Frequencyand time spenton foragingwere great- est duringhigh falling and low falling tide. Time spenton groomingand frequencywere high duringhigh slackand falling fide. Alertnesswas at its maximumduring high slackand low slacktide. No clear trendwas found in relation to antagonisticbehaviours. Type of habitat occupiedwas also found to have a significant effect on behaviouralpatterns. Amount of time allocatedto differentbehaviours and the frequencyof behav- ioursdisplayed a cyclical patternbased on the fide.

INTRODUCTION studiedas extensively as feeding behaviour.Furthermore, most of the studieson feeding behaviour have been con- must allocatetheir time to exploit their habitat to ducted on Oystercatchersfeeding on bivalves especially satisfybasic requirements for food,movements, social inter- (see review by Blomert et al. 1996, Goss-Custard actionand rest. Time activitybudgets have been studiedex- 1996). The effects of different habitatshave also been stud- tensivelyas the allocationof time and energywhich is im- ied: estuaries(e.g. DeVlas et al. 1996, Kersten 1996) and portantto the survivalof all species.A recurringidea in eco- sandybeaches (e.g. Zwarts et al. 1996a) have beenthe most logical literatureis that the life historystrategies of animals commonlystudied sites. This patternof researchposes the are inextricablylinked with allocationpatterns of time and questionwhether there are differencesbetween behaviours energy(King 1974). Estimationof an 's needsthrough at the varioustypes of observationsite. Indeed, a studycon- the annualcycle requires an understandingof how thesefac- ductedby Kersten(1996) in the Dutch Wadden Sea showed tors affect the pattern of activity and how the habitatsare that behavioursdid differ dependingon the type of territory used(Frederick & Klaas 1982). Knowledgeof daily activ- occupied;different habitats led to differentbehavioural time ity patternsis essentialfor the constructionof time-energy allocations.Hence the presentpaper which is aimed at de- budgetsand for evaluatingforaging and survival strategies scribingthe time budgetof Oystercatchersand the factors of animalsin seasonalenvironments (Risenhoover 1986). that control it. The EurasianOystercatcher Haematopus ostralegus is a with a markedtemporal variation in its activity budget. STUDY AREA Studieson the feeding and breedingbehaviour of this bird haveshown that time-activity budgets are governed by a tidal LundyIsland (51ø10'N, 4ø40'W), with an area of 3.5 km2, is regime (e.g. Kersten & Visser 1996a, Meire 1996); tidal located in the Bristol Channel, 17 km off the north coast of cycleshave the effect of limiting certainbehaviours. It was Devon, England(Fig. 1). There aremany habitattypes along found that the amount of food intake as well as the choice of its shoreline,ranging from shelteredcoves to jagged rocks main prey speciesvaried dependingon whetherit was high that are frequently exposedto vigorous wave action and or low tide (DeVlas et al. 1996). There is also someevidence strongwinds. The two studysites at whichobservations were showinga possibleconnection between social behaviours madewere selectedfor their varyingexposure to wave action and the tidal cycle (Ens et al. 1996). and distributionof rocks. The Landing Bay is east facing, Social interactions such as territorial encounters in rela- borderedby an eroding slate cliff, about 30 m high, and is tion to the tidal cycle and habitat, however, have not been partly sheltered.The North Lighthouseis a steep,exposed,

1Present address: Psychologisches Institut, University of Heidelberg,HauptstraBe 47-51, D-69117 Heidelberg, 2Present address: Department of Zoology,Division of WildlifeBiology, AVC College,Mannampandal-609305, Tamilnadu, India, e-mail: @rediffmail.com *Correspondingauthor

81 Bulletin 101/102 August/December 2003 82 Wader Study Group Bulletin

Pt•inGulJey Paths

N

BrazenWar• St JamesS{one

St Marks Stone

Dead Cow Point

Land•r•B•y

Fig. 1. Lundy Island in the BristolChannel, SW Englandshowing the two studysites (star symbol):the LandingBay and the North Light- house,

Bulletin 101/102 August/December 2003 Ghosh et aL Diurnal acbwty budgets of breeding Euras•an Oystercatchers 83

granitecliff siteon the northwest side of the island.In spring, Inactivity is restingand sleepingon one or two legswith the the prevailingwinds are from the northwestand make the head tucked betweenthe wings. area relatively rough and exposed. Each year, 15-20 pairs of Oystercatchersbreed along Conditions Lundy'sshoreline where they feedmainly on commonlim- petsPatella vulgata and black-footedlimpets P. depressa. Roughnessof sea was measuredusing the Beaufort Wind Theseoccur at both siteswhere they areexposed at low water Scale (Kerman 1993) and rainfall (mm) information was (R. Nagarajanunpublished data). Compared with the numer- obtainedfrom the Lundy IslandWeather Records. The prox- ousstudies of EurasianOystercatchers feeding on blue mus- imity andnumber of otherOystercatchers and sea close selsMytilus edulisand common cockles Cerastoderma edule, to the focal animal were also noted. therehave only beena few studiesof Oystercatchersfeeding on limpets(e.g. Harris 1964, Feare 1971, Coleman et al. 1999). Tidal cycle

METHODS Observationswere madeduring complete tidal cycles.Dur- ing the study,tidal amplituderanged from 2.29 m to 5.10 m. Data collection The tidal cycle was divided into six segmentsof approxi- matelytwo hourseach: high slack,high falling, low falling, The diurnal time activity budgetsof Oystercatcherswere low slack, low rising and high rising. The segmentshigh studiedduring April 1998. Observationswere madefrom a slack and low slack were the two-hour periods centred on distanceof 30-70 m, thuskeeping observer disturbance to high water and low water respectively. a minimum.Other human disturbance was infrequent and did not appearto affect the birds' behaviour.During all obser- Data analysis vations, 1-8 Oystercatcherswere present. Diurnal time activitybudgets were compiledby sampling PrincipalComponent Analysis (PCA) was usedto groupthe focal animals,as describedin Altman (1974), for 10-minute behaviours.This methodwas used to factorisea groupof 16 observationalperiods throughout the tidal cycle.Focal birds different behaviours and Varimax rotation was used for fac- were selectedat random. Observationswere occasionally tor extraction.Factors with an eigenvalue> 1 were extracted curtailedby the departureof the birds;but thoselasting >7 and the variableswere orderedand groupedby the size of minuteswere usedin the analysis,giving a samplesize of 77 loadingsto facilitateinterpretation. The 2-3 behaviourswith periods.The numberof intra- and inter-specifickleptopara- highestscores were consideredas significantloading in each sites such as Herring Gulls Larus argentatus and Lesser factor. A comparisonof behaviourswas made using PCA Black-backedGull L. fuscuswithin a 3 m radiusfrom each becauseit is a multivariatetechnique that helpsto elucidate focal bird were also counted.All kleptoparasiticencounters underlyingfactors without any a priori assumptions.Step- were noted. Observationswere made using a 15-60x tele- wisemultiple regression was used to predictdifferent groups scopeand 8x40 binoculars.All observationswere madeby of behaviours.The explanatoryvariables used were: time of two observers who dictated a continuous record of the behav- day, habitat, roughnessof sea, rainfall, presenceof other iour of focal birds to one recorder. The recorder measured the Oystercatchers,presence of otherbirds and the six stagesof frequencyand duration of eachbehaviour using a stopwatch. the tidal cycle. For the categorised variables of tide and conspecifics,low slacktide and pairs of Oystercatcherswere Behavioural descriptions usedas reference categories as they were the modalcategor- ieswithin their respective variable groups. Where significant The frequencyand duration of eachbehaviour was recorded resultswere obtained,indicating that further analysiswas as well as all periodsof inactivityusing a focal animalsam- appropriate,stepwise regression analyses were then con- pling method.Sixteen different behaviourswere observed. ductedto determinethe mostefficient way to describethe These were assignedto the following major categories: data. As inter- and intra-tidal cyclesare highly correlated, grooming,foraging, antagonistic behaviours and alertness. eachvariable was analysedseparately with all the othervari- These categoriesmatch the behaviourclassifications made ables.Analysis using intra-tidal variation resultedin more in previousstudies (e.g. Ens & Goss-Custard1986, Heg et precisemodels than those with inter-tidalvariations and were al. 1993). The categorydefinitions are: thusused throughout the analysis.

Grooming coverspreening and relaxation,which is defined RESULTS as short,l-2s periodsof restbetween preening episodes and may includevarious shaking movements. Behavioural groups Foraging coverssearching, handling and swallowing. Antagonisticbehaviour has three components: piping, chas- A PrincipalComponent Analysis with Varimaxrotation con- ing andpseudosleeping. In this study,pseudosleeping is firmed, for both frequencyand durationof behaviour,that definedas a form of apparentresting where alertnessis the a priori groupingsof behaviourwere appropriateand that maintained.It occursduring territorial encounters(Ens they were mutually orthogonal.Four factorscorresponding et al. 1996). to grooming,foraging, antagonistic behaviour and alertness Alertnesshas two components:major and minor vigilance. basedon the original 16 observedbehaviours emerged for Major vigilanceis a stateof high alertnesswhere the tar- boththe frequencyand duration of behaviours(Tables 1 and get is identifiableand is characterisedby a suddenhead 2). The four factors cumulatively explained 65.5% of the or bodymovement. Minor vigilancedoes not involve any variancein the frequencydata. The durationdata yielded six body movement,just a slightoutward head movement. factors- feeding,antagonistic behaviour, grooming, contact,

Bulletin 101/102 August/December 2003 84 Wader Study Group Bulletin

Table 1. PrincipalComponent Analysis with varimax rotation on frequencyof observedbehaviours in Oystercatchers(n=77). Numbers in bold denote behaviours used in comprisingthe factors. observed Factor I Factor 2 Factor 3 Factor 4 behaviours grooming feeding antagonistic alertness behaviours majorvigilance 0.179 -0.055 0.007 0.614 minorvigilance -0.215 -0.102 0.167 0.680 balancing -0.235 -0.525 -0.113 -0.271 preening 0.967 -0.003 0.006 0.098 relaxing 0.941 -0.004 -0.016 0.133 searching -0.133 0.915 -0.030 -0.256 swallowing -0.136 0.912 -0.035 -0.251 chasing -0.081 -0.060 0.873 -0.070 pseudosleeping -0.134 -0.014 0.877 0.169 piping 0.382 0.027 0.665 0.007 calling 0.151 0.009 -0.027 0.572 comfort -0.127 -0.378 0.137 -0.293

Eigenvalue 2.558 2.134 1.950 1.215 % of varianceexplained 21.3 17.8 16.3 10.1 cumulative% explained 21.3 39.1 55.4 65.5 exploringand alertness - andthese factors explained 84% of highslack and high falling tide, while alermess frequency and total variance. Factors that arose in the durational data were durationwere highest during low risingand low slacktide. exploring(walking and scanning) and contacting (flying and Antagonisticbehaviours did nothave an apparenttrend. calling).However, these were not analysedfurther due lack of samplesize of thesebehaviours during the observation Factors influencing the behaviour periods. Groomingbehaviour was influenced positively by roughness Time activity budgets of thesea and negatively by rainfall.Also, there was a differ- encebetween the habitats indicating that grooming frequency Time activitybudgets of Oystercatchersboth frequency and washigher in the LandingBay thanat the NorthLighthouse. durationof the activitiesare shownin Figures2 and 3. Both The modelsfor foragingbehaviour, both frequency and du- the time spenton foragingand the frequencyof foragingbe- ration,yielded the samevariables. Foraging behaviour was haviourwere greatest during high falling and low fallingtide. positivelyinfluenced by rainfalland increased in relationto Groomingand alertness (vigilance) behaviours also showed an roughnessof the sea.It wasalso prevalent during high fall- apparentcyclical pattern corresponding to time within the tidal ing andlow fallingtide. Frequency and duration models ac- cycle.Grooming duration and frequency were greatest during countedfor 37.2% and45.8% of the total varianceexplained

Table 2. PrincipalComponent Analysis with varimax rotation on durationof observedbehaviours in Oystercatchers(n=77). Numbers•n bold denote behavioursused in comprisingthe factors. observed Factor I Factor 2 Factor 3 Factor 4 Factor 5 Factor 6 behaviours feeding antagonistic grooming contacting exploring ,alertness behaviours majorvigilance -0.009 0.015 -0.032 0.066 0.268 0.848 minorvigilance -0.082 0.047 -0.020 0.023 -0.133 0.889 preening -0.023 -0.051 0.937 -0.016 0.145 -0.029 relaxing -0.062 -0.019 0.942 0.007 0.111 -0.022 walking -0.001 0.251 0.220 -0.101 0.768 0.074 flying -0.001 0.026 0.015 0.943 0.019 0.084 searching 0.958 -0.052 -0.116 -0.057 -0.083 -0.109 swallowing 0.934 -0.032 -0.087 -0.057 -0.116 -0.124 chasing -0.034 0.907 -0.053 -0.004 0.047 -0.052 pseudosleeping -0.045 0.787 -0.105 -0.043 0.017 0.158 piping 0.051 0.815 0.097 0.048 0.209 -0.034 calling -0.016 -0.030 -0.024 0.935 -0.021 0.003 scanning -0.046 0.030 0.063 0.067 0.897 0.023 inactivity -0.760 -0.098 -0.237 -0.203 -0.409 -0.300

Eigenvalue 2.779 2.403 2.108 1.942 1.505 1.019 % of varianceexplained 19.8 17.2 15.1 13.9 10.7 7.3 cumulative% explained 19.8 37.0 52.1 65.9 76.7 84.0

Buffetin 101/102 August/December 2003 Ghosh et al. Diurnal activity budgets of breeding Euras•an Oystercatchers 85

[] other behaviours [] grooming f•lfeeding [] antagonistic behaviour

_ []alertness

high high high low low low dsing slack falling falling slack rising state of tide

Fig. 2. Mean frequency of alertness, antagonistic behaviour, feeding, grooming, and other behaviours occurring over the six segments of each tidal cycle. respectively.Models of antagonisticbehaviour showed that DISCUSSION bothfrequency and duration decreased across the day as well as duringthe low risingand low falling fide. The time spent The Principal ComponentAnalysis yielded factorsthat fit on antagonisticbehaviours increased depending on therough- into the behaviouralcategories derived from earlier studies nessof the seawhereas frequency of antagonisticbehaviour confirmingthe appropriatenessof thesecategories. For most decreased.The alertnessfrequency model explained22.5% behaviours,both frequency and duration varied significantly of the total variance.Frequency of alertnessdiffered signifi- with the tidal cycle as shown by the regressionanalysis. cantlybetween the two studysites: Oystercatchers were more Some behaviourswere also influenced by weather condi- alert in the LandingBay than at the North Lighthouse.In the tions. presenceof otherOystercatchers, alertness decreased. Alert- nessfrequency decreased during high slack and low falling tide but increasedduring low rising tide (Table 3).

100

ß 80 .:_ [] inactivity 60 [] other behaviours [] grooming I•feeding 40 [] antagonistic behaviour [] alertness : 20

high high high low low low rising slack falling falling slack dsing state of tide

Fig. 3. Percentageof time spent in alertness,antagonistic behaviour, feeding, grooming,other behavioursand inactivityoccurring over the s•x segments of each tidal cycle.

Bulletin 101/102 August/December 2003 86 Wader Study Group Bullebn

Table 3. Results of stepwise multipleregression analyses. Values are unstandardisedco-efficients with standard errors in parentheses

Independent grooming feeding feeding antagonistic antagonistic alertness variables frequency frequency duration duration frequency frequency time of day .... 0.035'* -0.124'* - (0.008) (o.o31) - habitat 2.132** .... 0.268** (0.749) .... (0.085) other birds ...... solitary ...... other - - - 0.017 - -0.290'* oystercatchers - - - (0.009) - (0.102) rainfall -0.120'* 0.085' 0.015' - - - (0.039) (0.035) (0.006) - - - roughness 0.635* 0.471'* 0.081'* 0.005 -0.019 - of sea (0.283) (0.135) (0,024) (0.003) (0.010) - high rising ...... high slack ...... 0,374** ..... (0.111) high falling - 1.036' 0.219'* - - - - (0.467) (0.082) - - - low rising .... 0.027* -0.099* 0,186 - - - (0.010) (0.042) (0.122) low falling - 1.444** 0.347** -0.031 ** -0.134* -0.267* - (0.393) (0,069) (0.010) (0.041) (0.112)

R2adj 11.9% 37.2% 45.8% 18.6% 10.6% 22.5%

F 11.29'* 12.24** 17.06'* 5.33** 5.51'* 6.52**

* denotesp < 0.05, ** denotesp < 0.01.

Grooming trast, exactly the oppositeresults were observedin the cur- rent study.On Lundy duringrough weather, the Oystercatch- Groomingfrequency was predictedby habitat,rainfall and ers took larger limpetsfrom wet rocks, and ran amongthe roughnessof sea.However, there was no significantmodel rocksas soonas wavessplashed on them in orderto prey on for the durationof grooming,suggesting that this behaviour largerlimpets (R. Nagarajanpersonal observation). Hockey may be triggeredin a ballisticmanner, for exampleby spray (1981) and Frank (1982) reportedthat other oystercatcher from waves. speciesfeed near the water's edgewhere the limpetsare ac- tive and their shellsdo not adheretightly to the substratum. Foraging Therefore,the rain couldmake the limpetsmore vulnerable to Oystercatchersby wettingthe rocks. Both frequencyand duration measures of foragingshowed that it increasedwith greaterrainfall and roughnessof sea, Antagonistic behaviours and duringhigh falling and low falling fide. Theseresults are similar to the findingsof previousstudies of Oystercatchers The duration of antagonisticbehaviour was significantly feedingon otherprey types(Kersten & Visser 1996a,Meire relatedto time of day, the presenceof otherOystercatchers, 1996).As mightbe expected,foraging is greatestduring fall- roughnessof sea, low rising and low falling tide. The in- ing tidesas prey is exposedand still activethus making it creasein the presenceof other Oystercatchersis to be ex- morevulnerable than in otherphases of thetidal cycle.Time pected,as the focal animalswere engagedin protectingtheir spentsearching for prey alsoincreased in high rainfall and territoryfrom otherbirds. The frequencyof territorialbehav- roughnessof sea.This could be due to weatherconditions iours also increasedwith a higher roughnessof sea. This having an adverseaffect on the accessibilityof the limpets, mightbe dueto the roughsea disturbing Oystercatchers from sothat longer periods of searchingbehaviour are required to their roostingand low-level foragingsites, and causing them yield the samebenefits; or it could be that the repeatedre- to congregate.In thisway they may makeintrusions into one exposureof limpetsunder rough conditions made foraging another'sterritories, leading to antagonisticencounters. On more profitable so that the birds devotedmore time to it. the islandof Schiermonnikoogduring extreme high fidesand Rainfall causesworms to bury deeper in the sandin the stormyweather, breeding Oystercatchers are sometimesdis- Waddensea,making them lessor even completelyinacces- turbed by the sea from their roostingsites. This can cause sibleto the predators(Dankers et al. 1981).Possibly this is neighbouringpairs or individualsto roost closertogether, why, duringheavy rain on theisland of Schiermonnikoogin increasingthe frequencyof aggressiveencounters. It canalso the ,Oystercatchers were observedto stopfor- meanthat someOystercatchers are oustedfrom their feed- aging altogether(D. Heg personalcommunication). In con- ing territoriesaltogether, making intrusions into other(neigh-

Bulletin 101/102 August/December 2003 Ghosh et al.: D•urnal acbwty budgets of breeding Euras•an Oystercatchers 87 bouring)territories more likely to occur(D. Heg, personal D. Taylor and Peter Goods who organisedthe necessary communication).The effectof the time of day on antagonis- equipmentfor this research. tic behaviourwas a decreasein the afternoon;antagonistic behavioursalso decreasedduring low rising and low falling REFERENCES tide, probablybecause the birds were more dispersed. Altman, J. 1974. Observationalstudy of behaviour:sampling methods. Behav. 49: 227-265. Alertness Blomert, A-M., Ens, B.J., Goss-Custard, J.D., Hulscher, J.B. & Zwarts, L. (Eds.) 1996. Oystercatchersand their estuarinefood sup- Alertnessdecreased during high slackand low fallingtide but plies. Journalof the NetherlandsOrnithologists' Union, Amsterdam. increasedduring low rising tide. It might be expectedthat Coleman, R.A., Goss-Custard,J.D, Durell, S.E.A Le V dit & Hawkins, vigilancebehaviour will increaseduring the high slack period, $.J. 1999. Limpet Patella spp. consumption by Oystercatchers when otherbehaviours, such as feeding,would not be as fre- Haematopusostralegus: a preference for solitary prey items. Mar. quent due to the inaccessibilityof prey. However, this was Ecol. Prog. Ser. 183: 253-261. Dankers, N., Kuhl, H. & Wolff, W.J. (Eds.) 1981. Invertebratesof the not the case. Wadden Sea. Balkema, Rotterdam. Previousstudies have shown that the behaviour of Oyster- DeVlas, S.J., Bunskoeke, E.J., Ens, B.J. & Hulscher, J.B. 1996. Tidal catchersis affectedby the presenceof gullsleading to a loss changesin the choiceof Nereis diversicolorand Macoma balthica as of foraging time (Harris & Wanless 1997). However, our main prey species in the diet of the Oystercatcher Haematopus resultsdo not showthat the presenceof otherspecies had any ostralegus.Ardea 84A: 105-116. Ens, B.J. & Goss-Custard,J.D. 1986. Piping as a displayof dominance significantimpact on alertness.Similarly, the presenceof by winteringOystercatchers Haematopus ostralegus. Ibis 128: 382- otherOystercatchers resulted in a decreasein the amountof 391. vigilancebehaviour. Ens, B.J., Briggs, K.B., Safriel, U.N. & Smit, C.J. 1996. Life history Vigilancebehaviour differed between our two studysites decisionsduring the breeding season.Pp. 186-218 in The Oyster- in that alertnessoccurred more often at the LandingBay than catcher: From individuals to populations (Goss-Custard,J.D., ed.). at the Lighthouse.This might be due to more humanactivi- Oxford UniversityPress, Oxford. Feare, C.J. 1971. Predationof limpetsand dogwhelksby Oystercatchers. ties nearthe LandingBay. We alsonoted synchronised vigi- Bird Study 18: 121-129. lancebehaviour by pairs.This is consistentwith similar ob- Frank, P.W. 1982. Effectsof winter feedingon limpetsby Black Oyster- servationsby of Ens et a/.(1996). catchers,Haematopus bachmani. Ecology 63: 1352-1362. Frederick, R.B. & Klaas, E.E. 1982. Resource use and behaviour of migratingsnow geese. J. Wildl. Manage. 46: 601-614. CONCLUSIONS Goss-Custard, J.D. (Ed.) 1996. The Oystercatchersfrom individualsto populations.Oxford University Press,Oxford The largestvariations in behaviourwere the increasein for- Harris, M.P. 1964. The food of someLarus Gulls. Ibis 107: 43-53. agingduring high falling andlow falling tide andoverall, the Harris, M.P. & Wanless, $. 1997. The effect of removinglarge numbers level of activity was greatestduring high falling tide. This of gulls Larus spp.on an islandpopulation of OystercatchersHaema- resultis what would be expectedon a priori grounds,and is topus ostralegus:implications for management.Biol. Conserv. 82: 167-171. consistentwith resultsfor Oystercatchersforaging on other Heg, D., Ens, B.J., Burke, T., Jenkins,L. & Kruijt, J.P. 1993.Why does prey types.However, in severalcases variations in the less the typically monogamousOystercatcher Haemotopus ostralegus en- dominantbehaviours were not as predicted.One of the most gage in extrapaircopulations? Behav. 126: 247-289. strikingfeatures of this studyis the considerableproportion Hockey, P.A.R. 1981. Feeding techniquesof the African Black Oyster- of time spentinactive: an overallaverage of 85% of time per catcherHaematopus moquini. Pp. 99-115 in Proceedingsof the sym- 1O-minute observation period. Can the Oystercatcher'sallo- posiumon birds of the sea and shore,1979 (Cooper,J., ed), African Seabird Group, Cape Town. cationof somuch time to inactivitybe regardedas efficient? Kerman, B.R. 1993. A multifractal equivalentof the Beaufort scalefor However, inactivity could also be consideredas "surplus sea state.Geophys Research Letters 20: 297-300. time" that may be reallocatedto the mostprofitable of other Kersten, M. 1996. Time and energy budgetsof OystercatchersHaema- activities(Kersten 1996). Moreover, the physiologicalneed topusostralegus occupying territories of different quality.Ardea 84A: for restfor the properand efficient functioningof the meta- 291-310. bolic systemalso needs to be considered.This compensation Kersten, M. & Visser, W. 1996a. Food intakeby OystercatchersHaema- topusostralegus by day and night measuredwith an electronic nest of energyexpenditure indicates that the time spentinactive, balance. Ardea 84A: 57-72. for exampledigestion after feeding, is not an inefficientallo- Kersten, M. & Visser, W. 1996b. The rate of food processingin the cationof time but ratheran activity that mustbe engagedin oystercatcher:food intake and energy expenditureconstrained by a for the overall fitnessof the bird, sinceOystercatchers can digestive bottleneck. Functional Ecol. 10: 440-448. ingest food at a much greater rate than it can be digested King, J.R. 1974. Seasonalallocation of time and energy resourcesin birds. Pp. 127-204 in Granivorousbirds in ecosystems(Pinowski, J. (Kersten& Visser 1996b, Zwarts et al. 1996b). Despitethe & Kendeith, S.C. Eds). CambridgeUniversity Press,Cambridge. restrictionof observationsto daytimehours and the moder- Meire, P.M. 1996. Feeding behaviour of OystercatchersHaematopus ate number of observationsmade, the present study does ostralegusduring a period of tidal manipulations.Ardea 84A: 509- show clear trends in the time allocation of behaviours. 524. Risenhoover, K.L. 1986. Winter activity pattern of moosein Interior Alaska. J. Wildl. Manage. 50: 727-734. ACKNOWLEDGEMENTS Zwarts, L., Wanink, J.H. & Ens, B.J. 1996a. Predicting seasonaland annualfluctuations in the local exploitationof differentprey by Oyster- We thank S.E.G. Lea for his valuable comments on the catchersHaematopus ostralegus: a ten-yearstudy in the WaddenSea. manuscriptand stimulatingdiscussions regarding the study, Ardea 84A: 401-440. to HumphreyP. Sitters,Bruno Ens andDik Heg for valuable Zwarts, L., Cayford, J.T., Hulscher, J.B., Kersten, M., Meire, P.M. & commentson earlierdrafts of the manuscript,to Arie van der Triplet, P. 1996b. Prey selectionand intake rate. Pp. 30-55 in The Oystercatcher:From individualsto populations(Goss-Custard, J.D., Lugt and Britta Osthausfor stencillingthe map and Rachel ed.). Oxford University Press,Oxford. Kirby for computerassistance. Our appreciationalso goes to

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