Productivity of African Black Oystercatchers Haematopus moquini on , , in the 2001-2002 breeding season

K.M. CALF & L.G. UNDERHILL

Avian DemographyUnit, Universityof , Rondebosch7701, SouthAfrica, e-mail: kathy@adu .uct.ac. za

Calf, K.M. & Underhill, L.G. 2002. Productivity of African Black OystercatchersHaematopus moquini on RobbenIsland, SouthAfrica, in the 2001-2002 breedingseason. Wader StudyBull. 99: 45-49.

Breedingproductivity of African Black Oystercatcherswas monitoredduring the australsummer of 2001- 2002 on RobbenIsland, SouthAfrica. Countswere conductedat two-week intervalsto determinepopulation changesduring the breedingseason. The numberof pairsthat actuallybred was aboutthe sameas the number observedin the first two countsat the startof the breedingseason. A subsequentincrease in the numbersmay be a resultof prospectingbirds arriving later. Sixty-seven percent of breedingpairs produced at leastone fledg- ling (similar to the productivityof oystercatchersbreeding in protectedareas elsewhere). Causes of egg and chick lossprobably include predation by naturalpredators and by cats,dogs and rats (which may be aggra- vated by human disturbance)and a brood reductionstrategy arising because adults are incapableor unwill- ing to provide food for all the chicksof the clutch.

INTRODUCTION unableto catchand handleprey, adultsprovision them until after fledging (Hockey 1984, 1996). Adult workload is thus The African Black OystercatcherHaematopus moquini is the greatestwhen feedingchicks because they needto find suf- only representative of the family Haematopodidae that ficient food to satisfy their own needs and those of their breedsin Africa, populatingthe coastof SouthAfrica and growing offspring (Hockey 1996, Leseberg et al. 2000). Namibia (Martin 1997, Summers& Cooper 1977). With a Foragingtime is limited by accessto the intertidal zone dur- world populationof about5,000 in the early 1980s(Hockey ing low tide. Therefore food availability, coupled with the 1999), it is classified as "near-threatened" both in South effectsof siblingrivalry, may meanthat chicksface starva- Africa and globally (Barnes 2000, BirdLife International tion as well as the risk of predation. 2000). African Black Oystercatchersbreed on the opencoast Populationmodels indicate that oystercatchersneed to and offshoreislands at the peak of the summerholiday sea- fledge an averageof 0.33 young per pair per year to main- son, which runs from November to March (Leseberget al. tain stablepopulations (Hockey 2001). Egg lossand failure 2000). Although eggs and chicks are lost to natural causes of eggsto hatchoccur naturally in mostareas, but the chick- such as storms, windblown sand and depredationby gulls rearing period has been pinpointed as the bottleneck for andpredatory mammals such as genetsand foxes (Leseberg populationstability or growth (Hockey 2000). Becauseoys- et al. 2000), oystercatcherbreeding attempts are alsovulner- tercatchersare long-lived,the effectof elevatedreproductive able to the direct and indirect effects of human disturbance successis evident within 5-6 yearsbut the effect of breed- (Jeffery 1987, Hockey 1999, Adams et al. 1999, Hockey ing failuretakes much longer to detect(perhaps 15-20 years) 2000, Leseberget al. 2000). With ever-increasinghuman use (Hockey 2000). Therefore detailed monitoring of breeding of the SouthAfrican coast,the pressureon breedingAfrican successis essentialto determinethe underlyinghealth of a Black Oystercatchersis likely to intensify. population.This paperreports the resultsof the first year of Robben Island (33ø49'S, 18ø22'E) lies in , intensivemonitoring on Robben Island. 11 km from the of CapeTown, and7 km from the near- est mainlandat .The shorelineis rocky with METHODS variousdegrees of exposure,dependent on the extentof off- shore reefs. Although human disturbanceaffects limited This study was conductedduring the 2001-2002 breeding areasof the shore, the oystercatcherpopulation increased season on Robben Island, which has a coastline of about from 40 individuals in 1977 to 130 in 2001. As a result, the 10 km andis morefully describedby Underhill et al. (200 i). islandis now an importantconservation area for the species Oystercatcherswere counted at two-week intervals from 5 (Barnes 1998, Underhill et al. 2001). November 2001 to 4 May 2002 alongthe entire coastlinein Adult African Black Oystercatchersare territorial and the following categories:adults, pairs, scrapes, nests, chicks moststay on the sameterritory throughout the year.They are and fledglings.Pairs were defined as birds that roostedand long-lived and mate-faithfulwith a naturallylow reproduc- fed closeto eachother, but not necessarilybreeding. Nests tive rate (<2 fledglingsper pair per year (Hockey 1996)). were searchedfor regularlyand, once found, were monitored Somere-nest once a nesthas been destroyed, but birdsnest- at 4-7 day intervals until hatching.Chicks were similarly ing later in the seasonare lesssuccessful than early breed- monitored until fledging. Although young oystercatchers ers (Jeffery 1987). Oystercatcherchicks are sub-precocial; remain with their parentsfor several months,fledging was they leavethe nestsoon after hatchingbut, becausethey are takenas the day from whichchicks are ableto fly well. This

45 Bulletin 99 December 2002 46 Wader Study Group Bulletin occurs around the age of five weeks, when the chicks are were lost (Table 2). Sixty percentof the eggsand chicks that about two-thirds of adult body mass. disappearedwere from nestsin that part of the southcoast Each chick was ringed on the right leg with a metal ring defined as Area 1 in Table 3 and shownin Fig. 1 and 12% and a blue plastic ring and on the left leg with a numbered were from nests in Area 2 in the north where Kelp Gulls engravedblue and white plasticring. Thirty-five adult oys- Larus dominicanusbred in 2001-2002 (Calf et al. in press). tercatchershad alreadybeen ringed with a metal ring on the The othernests from whicheggs or chicksdisappeared were right leg anda uniquecombination of colourrings on the left scatteredaround the remaining coast.When egg and chick leg. During counts,some breeding adults were identifiedby lossesare related to coast length, the number of lossesin their ring combinations,others by the ring combinationsof Area1 is significantly greater than that expected though this their chicks or their location in a territory. is not the case for Area 2 (Table 3). Becauseof their behaviour,it was easyto misschicks and The total numberof breedingpairs was 63. The nestsof fledglingsduring counts. Therefore any seen within two days 57 pairs were found and the chicksof six pairs were found before or after a count were included in the total. If chicks after hatching(Table 2). The chicksof the secondnest of a or fledglings were not seen,but parentsshowed defensive pair were found after hatchingand thesedata have been in- behaviour,a pair was countedas possiblyhaving chicks or cluded in "broodsfound after hatchingin the table. Of the fledglings.Chicks were differentiated from fledglingson the pairs with chicksfound after hatching,four pairsproduced basisof hatchingdate andyoung were countedas fledglings one fledgling and three producedtwo (Table 2). if they were over five weeksof age. There were five one-eggclutches, 57 two-egg clutches andsix three-eggclutches. Three of the one-eggclutches suc- RESULTS cessfullyproduced fledglings. Of the 57 two-eggclutches, six pairsraised two chicksto fledging,five desertedthe re- Number of birds mainingegg after onehad hatched and, of the 32 pairswhich had two chickshatch, 20 pairs fledged a singlechick. Over the breedingseason, the numberof adult oystercatch- ers increasedfrom 123 individuals,including 61 pairs, in DISCUSSION early Novemberto 193, 96 pairs,in mid-March (Table 1). In late April, there were still 193 birds, 85 pairs. The median Population trends count was 176 birds. The maximum number of fledglings countedwas 51 in early April, decreasingto 26 in early May The earliest counts of African Black Oystercatcherson (Table 1). Robben Island known to us are of 40 in 1977 and 89 in 1980. The median of five irregular countsmade betweenDecem- Nests and nest losses ber 1998 and August 2001 was 135, and the median of 13 two-weekly counts in summer 2001-2002 was 177 birds Altogether,68 nestswere locatedaround the coastof Robben (Table4). At the time of the early counts,the islandwas used Island during the 2001-2002 breeding season(Table 2). as a prison,and a substantialproportion of the prisonoffi- Although the first eggs were found in mid-November,the cialswere recreationalfisherman, who probablycaused sub- peak of the incubation period was not until mid-January, stantial disturbancealong the shoreline,both throughbait when there was a maximum count of 25 nests with eggs collectionand angling (LGU pers.obs.). Because of thenature (Table 1). Chick-rearingpeaked the following monthwith a of runninga prison,there would always be staffoff-duty, and total of 53 pairswith chickson 13 February(Table 1). Of the it is likely that there was a near-continuouspresence of 68 nests,the eggsdisappeared from 14 and 11 pairslost their peopleon the coastthroughout the day. This disturbancemay chicks(Table 2, Fig. 1). All the chicksthat disappeared did have resulted in oystercatchers leaving their eggs to be so within two weeks of hatching.Twelve pairsre-laid after exposedto the elementsand to predators(Jeffery 1987). In their first clutch or chicks were lost and five of thesere-lays 2001-2002, the residenthuman population of the islandwas

Table 1. Countsof African Black Oystercatcherson Robben Island,South Africa between 5 November2001 and 4 May 2002.

Date No. of No. of pairs No. of pairs No. of pairs No. of pairs No. of pairs adults of adults with scrapes with eggs with chicks with fledglings

5 November 2001 123 61 0 0 0 0 15 November 2001 127 62 1 1 0 0 29 November 2001 158 68 4 13 0 0 18 December 2001 161 80 14 22 11 0 2 January2002 162 81 8 24 16 0 16 January2002 179 81 0 25 25 1 1 February2002 179 82 0 13 45 5 13 February 2002 191 71 0 8 53 9 4 March 2002 191 92 0 3 27 31 14 March 2002 193 96 0 1 16 43 1 April 2002 172 85 0 0 9 51 20 April 2002 193 85 0 0 1 42 4 May 2002 177 43 0 0 0 26 Median 177

Bulletin 99 December 2002 Calf& Underhill:Productivity ofAfrican Black Oystercatchers onRobben Island 47

N

Quary

Area 2

ng range Harbour

Airstrip

Western Perimeter Road

Boundary Road Settlement

Alpha I

Area I

1000 0 1000 2000metres

Fig.1. Locationof68 African Black Oystercatcher nestsand six pairs with chicks found after hatching onRobben Island, South Africa, duringNovember 2001 to March 2002. Empty (O) and solid (©) circles indicate the location ofnests producing fledglings and nest fail- ureson the first attempt respectively. Empty(A) and solid (,•) triangles indicate thelocation ofsecond nesting attempts thatproduced fledg- lingsand that failed respectively. Squares ([•) indicate locations where post-hatching chicks were found. smallerthan in prisontimes, and few of the RobbenIsland of SouthAfrica has been attributed to thechange in oyster- Museumpersonnel were observed to engagein angling catcherdiets from indigenouslimpets and mussels to the (KMC, LGU pers.obs.). On rockyshores, African Black invasivemussel (Hockey & vanErkom Schurink 1992). The Oystercatchersfeed predominantly on limpets and mussels colonisationof thismussel may have therefore contributed (Hockey& Underhill1983). The Palearctic mussel Mytilus to theincreased numbers of AfricanBlack Oystercatchers on galloprovincialishas recently invaded the southern African RobbenIsland. Regular monitoring is neededto determine coastwhere it has outcompetedthe indigenousmussel if thisupward trend will continueor if theisland is already Aulacomyaater. An increasein theAfrican Black Oyster- at carryingcapacity. catcherpopulation on otheroffshore islands on the westcoast

Bulletin 99 December 2002 48 Wader Study Group Bulletin

Table 2. Breedingparameters of AfricanBlack Oystercatcherson Robben Island, South Africa,during the 2001-2002 breedingseason.

Total nests Nests in which Total chicks Chicks hatched Nests Total Fledglings at least one hatched per nest producing fledlings per nest chick hatched at least one fledgling produced

Initial clutches

1-egg 5 3 3 0.60 3 3 0.60 2-egg 48 30 51 1.06 30 36 0.75 3-egg 4 2 5 1.25 2 2 0.50 Totals 57 35 59 1.04 35 41 0.72

Second clutches

1-egg 0 0 0 0 0 0 0 2-egg 9 6 6 0.67 6 7 0.78 3-egg 2 2 3 1.50 1 2 1.00 Totals 11 8 11 1.00 8 9 0.81

Grand total 68 43 70 1.03 43 50 0.74

Broods found 7 ? ? ? 7 9 1.29 after hatching

The 2001-2002 breeding season successof 70% foundfor oystercatchersbreeding in areasof theWestern Cape that are protected from humandisturbance, The largestcounts of oystercatchers,both of 193, weremade and far larger than the 25% recordedin unprotectedareas on 14 March and 20 April 2002, including96 and 85 pairs, (Hockey2000). At 0.87 fledglingsper pair in 2001-2002, the respectively,with theremaining birds occurring as singles or Robben Island oystercatcherscomfortably exceededthe small flocks. Acrossthe breedingseason from 5 November thresholdof 0.33 youngper pair estimated by Hockey(2001) 2001 to 20 April 2002, the mediannumber of pairspresent asthe fledgingsuccess necessary to maintaina stablepopu- was 81. Sixty-three pairs were known to have bred. The lation. numberof pairsthat bred is almostthe sameas the number It seemslikely that,though the oystercatchersof Robben of pairsobserved in the first two countsmade in November Island breed successfully,they operatea brood reduction (61 and 62 pairs) (Table 1). We thereforeconsider that the strategy,preferentially feeding the largestchick or first to breedingbirds were presentin pairs on the island in their hatch(O'Connor 1984) becausemost pairs only rearedone territoriesat the startof the breedingseason, before the first chick to fledging despite93% of nestscontaining two or nestswere found. The increasein the number of pairs dur- threeeggs. Fifty-seven (84%) of all the clutchesfound con- ing the breedingseason may be a.resultof birds arriving on sistedof two eggs but only 10 of theseraised both chicks the island to prospecta breedingterritory for the following (Table 2). Oystercatchersmay be unableto providetwo or year. The occurrenceof nonbreedingadult oystercatchers in more chicks with sufficient food as a result of poor food pairswas observed on MarcusIsland, South Africa, in Janu- availabilityor the adultsmay be unwilling to expendtheir ary 1976 by Summers& Cooper (1977). own energyin rearingtwo chicks,thereby sacrificing their Seventy-sixpercent of the monitoredbreeding pairs of longevityand lifetime reproductivesuccess (Ens et al. 1992, AfricanBlack Oystercatchers on RobbenIsland produced at Kersten 1997). These factors are currently being investi- leastone fledgling (Table 1). This is similar to the fledging gated.

Table 3. Observed and expected lossesof African Black Oyster- Table 4. African Black Oystercatchercounts on Robben Island, catcher eggs and chickson Robben Island, South Africa. Expected South Africa (data from Underhill et al. 2001 and this study). numberswere calculatedassuming that losseswere proportionalto the lengthof coastline.Area 1 is the sectionof coast between Alpha Observers Date Count 1 and the intersection of Western Perimeter Road and Boundary Road (Fig. 1). Area 2 is the sectionof coast betweenthe quarryand G. Arkell, C. Marais 18 Aug. 1977 40 the old shootingrange. The differencesbetween the observedand G.D. Underhill, LGU, H.G. Robertson 8 Dec. 1980 89 expectedvalues are statistically significant (Z22 = 12.0,P < 0.001). P.A. Whittington 26 Dec. 1998 149 Avian DemographyUnit 30 Nov. 2000 135 Area I Area 2 Remainder Earthwatch Team 3 Mar. 2001 143 of island LGU, KMC 18 May 2001 110 LGU, KMC 12 Aug. 2001 126 Coast length (km) 1.7 1.3 7.7 KMC Median of 13 counts between Observed number of 5 Nov. 2001 and 4 May 2002 177 lossesof eggsor chicks 15 3 7 Expectednumber of lossesof eggsor chicks 4 3 18

Bullerin 99 December 2002 Calf & Underhill:Productivity of African Black Oystercatcherson Robben Island 49

During the summerof 2001-2002, virtually all visitorsto REFERENCES the RobbenIsland Museum undertooka bus tour along the south coast from the settlementpast Alpha 1 to Boundary Adams, N.J., Kerley, G.I.H. & Watson, J.J. 1999. Disturbanceof incubat- Road, an area that held a high densityof breedingoyster- ing AfricanBlack Oystercatchers: is heating of exposedeggs a problem? Ostrich 70: 225-228. catchers(Fig. 1). There, the perimeterroad is very close to Barnes, K.N. (Ed.). 1998. The Important Bird Areas of southernAfrica. the shoreline (less than 10 m). Becauseit is near the settle- BirdLife SouthAfrica, Johannesburg. ment, it is also an area where residentswalk their dogs.This Barnes,K.N. (Ed.). 2000. TheEskom Red Data Bookof birdsof SouthAfrica, is the sectionof coastwhere oystercatchersexperienced the Lesothoand Swaziland.BirdLife SouthAfrica, Johannesburg. most severeloss of eggs and chicks, with 60% of all losses BirdLife International. 2000.Threatened birds of theworld. Lynx Edicions taking place in Area 1 (Table 3, Fig. 1). It is likely that this and BirdLife International,Barcelona and Cambridge. arisesfrom disturbanceresulting in adultsbeing away from Calf, K.M., Cooper, J. & Underhill, L.G. in press.First breedingrecords of Kelp Gull Larus dominicanusat RobbenIsland, Western Cape, South their nestsor chicksfor longperiods making them more vul- Africa.S. Afr. J. Mar. Sci. nerableto predation.The impact of cats,dogs and rats from Ens, B.J., Kersten, M., Brenninkmeijer, A. & ltulscher, J.B. 1992.Terri- the settlementhas not yet beenevaluated but it is likely that tory quality,parental effort andreproductive success of Oystercatchers they take oystercatchereggs or chicks.In Area 2, betweenthe (Haematopusostralegus). J. Anim. Ecol. 61: 703-715. quarrywhere Kelp Gullsbred and the old shootingrange, the Hockey, P.A.R. 1984.Growth and energetics of the AfricanBlack Oyster- numberof losseswas not particularlyhigh (Table 3, Fig. 1). catcherHaematopus moquini. Ardea 72:111-117. Therefore the Kelp Gulls did not apparentlyhave a large Hockey, P.A.R. 1996.Haematopus ostralegus in perspective:comparisons with otheroystercatchers. pp. 251-285 in: J.D. Custard(ed.). The Oyster- impact on the breedingsuccess of the oystercatchers. catcher.From indivMualsto populations.Oxford UniversityPress, Ox- ford. RECOMMENDATIONS Hockey, P.A.R. 1999. Conservationperspectives: African Black Oyster- catcher.Under Currents 4(1): 4-5. Robben Island is protectedas a National Historical Monu- Hockey, P.A.R. 2000. OystercatcherConservation Programme Newsletter. ment and hasWorld HeritageSite statusbased on its histor- OystercatcherTidings 1: 1. Hockey P.A.R. 2001. Updateon breedingperformance. Oystercatcher Tid- ical and cultural heritage.It alreadyqualifies as a Wetland ings 2: 2-3. of InternationalImportance in termsof the RamsarConven- Hockey, P.A.R. & Underhill, L.G. 1983.Diet of the AfricanBlack Oyster- tion becauseit supportsmore than 1% of the globalpopula- catcherHaematopus moquini on rocky shore:spatial, temporal and sex- tion of the African Black Oystercatcher,and of the nominate relatedvariation. S. Aft'. J. Zool. 19:1-11. race of the Swirl Tern Sterna bergii bergii (Barnes 1998, Hockey,P.A.R. & van Erkom Schurink,C. 1992.The invasivebiology of Underhill et al. 2001). We thereforerecommend that South the musselMytilus provincialis on the southernAfrican coast.Trans. Roy. Africa nominate Robben Island as a Ramsar wetland. Soc. S. Afr. 48: 123-139. Jeffery, R.G. 1987.Influence of humandisturbance on the nestingsuccess Because of its status as a National Historical Monument, of African Black Oystercatchers.S. Aft. J. WiM1.Res. 17:71-72. the island is a tourist attraction and the number of visitors has Kersten, M. 1997. Living leisurelyshould last longer.PhD thesis,Univer- increasedin recentyears. We thereforerecommend that, dur- sity of Groningen,The Netherlands. ing the oystercatcherbreeding-season (November-April), Leseberg,A., Hockey, P.A.R. & Loewenthal, D. 2000. Human disturbance touristsbe directedaway from the sensitiveshoreline areas and the chick-rearingability of African Black Oystercatchers(Haema- where African Black Oystercatchersbreed. Tourists should topusmoquini): a geographicalperspective. Biol. Conserv.96: 379-385. Martin, A.P. 1997.African Black OystercatcherHaematopus moquini. pp. not, for example,be permittedto ventureon foot beyondthe 374-375 in Harrison, J.A., Allan, D.G., Underhill, L.G., Herremans, M., Alpha 1. Moreover residentsshould be discouragedfrom Tree, A.J., Parker,V. & Brown, C.J. (eds). The atlas of southernAJkican walking along the section of road between Alpha 1 and birdsVol 1. Non-passerines.BirdLife SouthAfrica, Johannesburg. BoundaryRoad, and shouldkeep their dogson a lead. The O'Connor, R.J. 1984.The Growth and Development of Birds.Wiley, Chich- impactof domesticand feral catson the oystercatcherpopu- ester. lation should be also be monitored and if necessarycon- Smith, C. 1997. RobbenIsland. Struik, Cape Town. Summers,R.W. & Cooper,J. 1977.The population,ecology and conserva- trolled. tion of the Black OystercatcherHaematopus moquini. Ostrich 43: 28-40. In 2000, Kelp Gulls bred on RobbenIsland for the first Underhill, L.G., Whittington, P.A. & Calf, K.M. 2001. Shorelinebirds of time, and 15 pairsbred in 2001-2002 (Calf et al. in press). RobbenIsland, Western Cape, South Africa. WaderStudy Group Bull. 96: The Kelp Gull breedingseason coincides with that of the oys- 37-39. tercatchers,but in 2001-2002 the area where they bred was not one with pooroystercatcher breeding success (Table 3). However, Kelp Gulls are known predatorsof African Black Oystercatchers(Summers & Cooper 1977). We therefore recommendthat the Kelp Gull breedingpopulation and its impacton the oystercatchersbe monitored.If the Kelp Gulls becomea problemfor theoystercatchers, they should be con- trolled.

ACKNOWLEDG EM ENTS

This researchwas supportedby the NationalResearch Foun- dation, the Earthwatch Institute, the Darwin Initiative and the Centrefor IsotopeResearch at the Universityof Groningen. RobbenIsland Museum and Marine and CoastalManage- mentprovided logistical support. All the willing field work- ers involved in this project are thankedfor their assistance.

Bufferin 99 December 2002