Coastal waders in - - aerial survey results and seasonal occurrence on selected low water plots

Jacob Christian Salvig, Sten Asbirk, Jorgen Peter Kjeldsen, Palle A.F. Rasmussen,Alqueia Quade, John Frikke & Erik Christophersen

Salvig, J.C., Asbirk, S., Kjeldsen,J.P, Rasmussen,P.A.F., Quade, A., Frikke, J. & Christophersen,E. 1997. Coastalwaders in Guinea-Bissau- aerial surveyresults and seasonal occurrenceon selected low water plots. Wader Study Group Bull. 84: 33-38.

Previouswinter surveys in partsof coastalGuinea;Bissau have suggested that aboutI millionwaders winter in the area. DuringJanuary 1994, an aerial surveywas carriedout which confirmedthis. Monthlycounts at selectedsites in the Bijag(•sArchipelago provided an indicationof the size and compositionof wader populationsthat are presentat different times of the year. The wader numbersin coastal Guinea-Bissauare fairly constantduring the summer, althoughthe proportionof migrantwader populationsvaried with species. Duringautumn, there was an influxof birds of birds in Septemberand anotherin November. Duringspring, waders were presentuntil May when most birds left the area. Individualspecies showed considerablevariations in their seasonal patternsof occurrencebut, overall, 15% over- summered. From the observedannual occurrence,it can be concludedthat coastal Guinea-Bissauis importantfor waders all year.

J. Salvig,J.P. Kjeldsen,P.A.F. Rasmussen,J. Frikke & E Christophersen,National EnvironmentalResearch Institute, Department of IA•ldlifeEcology, Gren&vej 12, Kale, Dk-8410 Ronde; S. Asbirk,National Forest and Nature Agency, Haraldsgade53, Dk-2100 Kobenhavn; A. Quade, Gabinetede PlanificacaoCasteira, Apartado 23, Codex 1031-B•sau, Guinea-B•sau.

Correspondenceto J.C. Salvig,COWl, Parallelvej15, Dk-2800 Lyngby,Denmark.

was carried out. The aerial survey allowed relativelyquick INTRODUCTION cov6rage of the entire coastal zone of Guinea-Bissau, althoughusing this method has certain problems,as The coast of has since the 1960s been known describedbelow. This paper reportsthe results of a to be of internationalimportance for winteringPalearctic combinedair and groundsurvey during January - February waders(Altenburg et al. 1982). Ir•terms of numbers,The 1994, coveringthe entirecoastal zone of Guinea-Bissau. Banc d'Arguinin Mauritania is knownto be the most This paper also uses the resultsof monthlycounts in the importantwintering site holdingtwo millionwaders (Smit & BijagosArchipelago to presentdata on the size and Piersma 1990). Coastal Guinea-Bissauis the second- compositionof wader populationsduring the migratory most importantwintering area for Palearcticwaders in seasonsand summer. Finally,the importanceof coastal West Africa (Zwarts 1988, Salvig et aL 1994a). In Guinea- Guinea-Bissauis quantifiedduring the entire annual cycle. Bissau,several previoussurveys of waders have been carried out: Poorter & Zwarts (1984) surveyed81 km2 of intertidalfiats duringthe 1982/83 winter;Zwarts (1988) surveyed278 km2 during the 1986/87 winter and Salvig et a/. (1994a)'surveyed 602 km• duringthe 1992/93 Winter. SURVEY AREA On the basis of these surveys it was estimated that about one millionwaders use the area duringwinter. These Coastal Guinea-Bissauconsists of a biologicalvery rich three surveys concentratedon the eastern part of the delta region(Figure 1), providing1,570 kmß of intertidal Bijag6sArchipelago, since gettingto the western part is fiats within a radius of only c. 220 km (Zwarts 1988). The not an easy task. In addition,providing good countsalong BijagosArchipelago is situatedjust offshoreand consists the entire mainland coast is difficult,and only a few counts of 48 islands and islets. Between the islands and along have been carried out there. Hence, little is known about the coastline,extensive mudfiats are drained by a network these huge areas. In order to continuethe work of former expeditions,and especiallyto performcounts in areas whichhad not yet beenvisited, an aerial surveyof the area

33 of canals and creeks as the tide recedes. The tidal low water: at high water huge numbersof waders roosted amplitude in this area varies from about 3 m on neap tides in the mangroves,where they were impossibleto see from to about4.5 m on springtides (Zwarts 1988). This means the air. The entire coastalzone was surveyedduring eight that large intertidalfiats are exposedat low water, which flightslasting 3 - 3« hourseach. In orderto plotthe are highly productivein terms of prey items for waders and distributionof the birds,the area surveyedwas dividedinto other waterbirds. Most of the upper intertidalzone in 31 sub-areas (Figure 1). Due to economicconstraints, only Guinea-Bissau is fringedwith mangroves. one riversidewas counted. The planeflew a surveyroute which enabled a full count of all wader species present in the survey area. Waders were recordedfrom the aircraft at a height of 50 - 100 metres, and at a speed of 150 kilometresper hour followingthe methodsdescribed by METHODS Pihl & Frikke (1992).

Since aerial counts do not give accurate results, ground Aerial surveys countswere made to calibratethe aerial surveys. Unfortunately,simultaneous air and groundcounts were Duringthe 1994 winter, coastal Guinea-Bissauwas not possibledue to practicalconstraintsø Consequently, surveyedfrom the air usinga small twin-engined,high- groundcounts were performedin selectedareas of the winged aircraft (PartinaviaObserver). Two observers,one archipelagoafter the aerial surveyswere completed. The each side of the aircraft, performedthe observations, aerial surveyswere performedduring the period21 - 27 which were recordedon dictaphones. Both observerswere January1994, whereasthe groundcounts, in six parts of experiencedin carryingout aerial surveysof Danish the BijagosArchipelago, were performedduring the period coastalwaters, and the observerin front (besidethe pilot) 31 January- 12 February1994. was familiar with the area. All countswere performedat

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Figure 1 Map of Guinea-Bissaushowing the count areas used.

34 Monthly oounts The speciescomposition of boththe aerial and the ground counts in 1994 are shown in Table 1; 59.4% of the waders Duringthe surveyin 1992/93, we discoveredthe tidal fiats countedfrom the aircraft in the BijagosArchipelago were west of Bubaqueand east of Soga to be an area where identified. Kentish Plover Charadrius alexandrinus, huge numbersof wadersoccur during low water. We CommonSandpiper Actiris hypoieucos and Dunlin Cal•lris selectedthis tidal area for monthlycounts: groundcounts alpinawere not detectedfrom the air, while Ringed Plover of waders at ten areas of 14 hectares each were performed Charadriushiaticula, Turnstone Arenaria interpres, once a month duringlow water as close to the highest Redshank Tringatotanus and Grey Plover Pluvialis springtide as possiblefrom Februaryto December1994. squatarolawere only poorlyrecorded, assuming that the The count areas are shown in Figure 1. All countswere groundcounts were random samples. made by two observers,travelling by dinghybetween the tidal fiats, and using binoculars,telescopes and Table 2 presentsthe estimatednumber of wintering handcounters. No December counts were performed in waders in the BijagosArchipelago during the three 1994 due to transport restrictionsin Guinea-Bissau. surveys. In all three surveys,Curlew SandpiperCalidris ferrugineaoccurred in by far the highest numbers. Kentish Plover, Bar-tailed Godwit Umosa lapponicaand Little Stint Calidris minuta had all decreased in numbers since the first survey;only the Curlew Sandpiperhad increasedin RESULTS numbers. Whimbrel Numeniusphaeopus, Turnstone, Grey Plover, Ringed Plover, Common Sandpiperand Dunlin showed rather consistentnumbers duringthe three Winteringnumbers surveys,whereas SanderlingCalidris alba, Knot Calidris canutus,Redshank, Oystercatcher Haematopus ostralegus Duringthe aerial survey,a total of 427,354 waders were and Curlew Numeniusarquata all showedhighly countedon the tidal fiats of the BijagosArchipelago, and fluctuatingnumbers. 131,478 on the mainland coast. The air and ground counts carried out within a few days of each other in The spatial distributionat low water showedthat waders restrictedareas of the Bijagos Archipelago gave a were presentall over the tidal fiat zones in coastal Guinea- detectionrate for the aerial survey of 57%. The ground Bissau;80% of the birdswere recordedin the archipelago surveyscovering the tidal fiats west of Bubaqueand south- and 20% along the mainlandcoast. Half the birds in the east of Soga (sub-areaA15, Figure 1) counted105,000 archipelagowere recordedin four subareas(A7, A8, A12 waders, whereas 59,000 were counted in the same area from the air. and A15), whereashalf the birdsalong the mainlandcoast were recordedin two subareas (K6 and K11).

Given a 57% detection rate, an estimated 750,000 waders were presenton the intertidalfiats of the Bijagos Archipelago. This is very much the same as estimated Seasonal variations in wader numbers in coastal duringthe surveysin 1986187(699,120 individuals,Zwarts Guinea-Bissau 1988) and 1992/93 (710,120 ind., Salvig et al. 1994a). On the mainland coast, an estimated 230,000 waders were The monthlycounts of total numbersof waders at ten presentin the 1994 winter. feedingareas in 1994 are presentedin Figure2. Two influxeswere recordedin autumn:one in Septemberand On the basis of one survey along one of the rivers entering one in November. Numbers peaked in November the Rio Grande de Buba, it was estimated that 25,000 suggestinghigh numbersduring all winter months waders were present. This compares favourablywith the (November- January). Numberswere fairly constant resultsof the groundsurveys carried out by Zwarts (1988), duringspring until May. Overall, the speciescomposition who covered 42.7% of the tidal fiats of the Rio Grande de in summer and winter in the coastal zone of Guinea- Buba and estimated that 30,940 waders were present. Bissau,appears to be roughlythe same. Curlew Sandpiperis by far the most numerouswader speciesin It is not possibleto extrapolatethe counts in the Rio both summer and winter, followedby Bar-tailedGodwit. Grande de Buba to other rivers in Guinea-Bissau, since the Little Stint is the exception,with a low ratio of summering Buba river consists of stone fiats, while the Geba river is to winteringnumbers. Comparingthe total numberof soft mud, and the rivers in the south of Guinea-Bissau waders in summer and winter, approximately15% of the havea mixtureof sandand mud(Zwarts pers. comm.). winter populationis still presentin the northernsummer. Table 3 presentsthe proportionof the most abundant In total, we estimate that about 980,000 waders were wader populationsremaining in the BijagosArchipelago winteringin the archipelagoand alongthe mainlandcoast. duringthe arctic summer months. Taking into accountthe estimated25,000 waders along the Rio Grande de Buba (this study)and the extratens of thousands along the other rivers,we estimate that between 1 and 1.1 million waders wintered in coastal Guinea- Bissau in 1994.

35 Thousands Seasonal pattern 16 From the observed annual occurrence, it is obvious that coastal Guinea-Bissauis importantfor waders all year round. Individualspecies showed considerable variations in the pattern of seasonal distribution. Most waders arrivedin September,only RingedPlover arrivedin November. Several speciesshowed very similar seasonal distributionpatterns: Curlew Sandpiper, Sanderling, Grey Plover, Bar-tailed Godwit and Redshank showed an autumn increasefollowed by fluctuationsthroughout winter 4 and springuntil May. This patternsuggests that most of the birdsthat arrive in autumn remain in the area during 2 winter, althoughprobably moving around within the coastal zone of Guinea-Bissau. The seasonal patternsof O Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Whimbrel, Turnstone,Knot and Little Stint showedpeak numbersin autumnwith the bulk of the populationsleaving the area in February. This would suggestthat these birds exploitedmore than one habitatduring winter and spring Figure2 Seasonalchanges in numbersof totalwaders recorded at beforetheir returnmigration to the breedinggrounds. This ten feedingplots west of Bubaque,Guinea-Bissau phenomenonhas been recognisedby other authorsat otherwintering sites alongthe East AtlanticFlyway, e.g. Ntiamoa-Baidu(1991) and Salvig (1995). A shift in the DISCUSSION use of the coastalzone by Knotfrom autumnto springhas also been recordedin Ghana (Ntiamoa-Baidu 1993). Winter numbers

The aerial survey in coastal Guinea-Bissauconfirms that this coastal winteringsite holds about 1 millionwaders. Status of summering waders However,the use of aerial surveysfor the assessmentof bird numberspresents a numberof methodological The overall percentageof waders summeringin the problems,since aerial surveysdo not reflectaccurate coastalzone of Guinea-Bissaucorresponds reasonably numbers. Birds can be overlooked, and flock size can be well with reportsfrom otherAfrican wetlands (cf. Table 3). over- and underestimatedby observersbecause of the However,the median percentageof waders presenti n the high speed of the aircraft and the short observationtimes boreal summer at Langebaan Lagoon in South Africa involved. Waders are generallydifficult to surveyfrom an duringthe period1975- 1986 was 11%, but varied aircraft;only the largerspecies, e.g. Oystercatcherand between2.5% and 31% (Underhill1987). These Bar-tailedGodwit, are easy to see becausethese large fluctuationsare thoughtto be due to yearly differencesin species are more conspicuousfrom the air and more the productionof young(Prokosch 1988). As the monthly readilydetected, even when they occurin small flocks countsat Bubaqueare continuing,the extentof these (Salvig et al. 1994b). In the coastalzone of Guinea- fluctuationswill be investigatedin the comingyears. The Bissau,the surveyshad to be carriedout at low water, oversummeringbirds are presumedto be immaturebirds. whenthe waderswere dispersedover the intertidalfiats The ratio of number of summeringto number of wintering and, consequently,rather hard to survey. waders in coastal Guinea-Bissauvaried from speciesto species, but the percentagesare within the ranges of what The discrepanciesin numbersfor the BijagosArchipelago has been recordedelsewhere e.g. on Banc D'Arguinin (Table 2) for most of the wader speciesrecorded between Mauritaniaand in the coastalwetlands of Ghana (Van Dijk the three surveysmay resultfrom i) differencesin the et al. 1990, Ntiamoa-Baidu1991). coverageof the area duringthe three surveys;ii) considerablecount errors; iii) a shift in the distributionof The numbersof waders presentduring summer varied the birds;and iv) an actual changein populationnumbers. from speciesto species(cf. Table 3). On the Banc Given the similarityof the overall numbersof waders D'Arguin in Mauritania, fluctuationsin the numbers of countedduring each survey, it looks, as if the deviationsin migrantwaders recorded in June have also been reported numbersfor CurlewSandpiper, Little Stint and Bar-tailed (Van Dijk et al. 1990): in somespecies, e.g. Knotand Bar- Godwit (and perhapsalso KentishPlover) reflectreal tailed Godwit, substantial numbers remained in some changes in numbers betweenthe surveys. The deviations yearswhile in otheryears all the birdsdeparted. Small in numbersfor these speciesseem to confirmthe theory numbersof Little Stint remainingin summer have also that numbers of waders breedingin tundra areas seem to been recordedin other African countries,e.g. on Banc vary more, due to high annual breedingsuccess, than D'Arguinin Mauritaniaand in Egypt (Van Dijk et al. 1990, those of waders breedingin temperate zone (Salvig et al. 1994a). Goodman& Meininger1989). Van Dijk eta/. (1990) suggestedthat this was because, unlike most other species,Little Stint starts breeding in its first summer.

36 Conservation aspects REFERENCES It is clear that the coastal zone of Guinea-Bissau is an internationalimportant wetland on the East AtlanticFlyway Altenburg,W., M. Engelmoer,R. Mes & T. Piersma1982. Wintering duringboth winter and summer and duringmigration. At waders at the Banc D'Arguin,Mauritania. - Comm. 6 Wadden least 14 wader species not only fulfillthe 1% criterionfor Sea WorkingGroup. Leiden,The Netherlands. internationalimportance, but considerablyexceed it. Furthermore,the value of the area is furtherenhanced by Goodman,S.M. & P.L. Meininger(eds.) 1989. The Birdsof Egypt. the numbersof otherwaterbirds, e.g. terns (Salvig et al. in OxfordUniversity Press, Oxford. press). Ntimoa-Baidu,Y. 1991. Seasonalchanges in the Importanceof CoastalWetlands in Ghanafor Wading Birds. Biol.Cons. 57: In orderto safeguardthe wader populationsoccurring in 139-158. coastal Guinea-Bissauand to secureeffective protection Ntimoa-Baidu, Y. 1993. Trends in the use of Ghanaian coastal for this important area, conservationstrategies have to wetlandsby migratingKnots Calidris canutus. Ardea81: 71- take into account the increasingpressure on the area. In 79. the past few decades, mangrovesin the coastal zone of Pihl,S. & J. Frikke1992. Countingbirds from aeroplane. In: Guinea-Bissauhave been clearedfor rice growing,and Komdeur,J., J. Bertelsen& G. Cracknell(eds.): Manual for tourismand fisheriesare underdevelopment. In April aeroplaneand shipsurveys of waterfowland seabirds.IWRB 1996, the BijagosArchipelago was acceptedby UNESCO Spec. Publ. 19, Slimbridge,UK, 37 pp. as a Man and BiosphereReserve. Poorter,E. & L. Zwarts1984. R•ultats d'unepremiere mission ornitho-•cologiquede I'UICN/WWF en Guinea-Biseau.Ned. In order to strengthenthe conservationstatus of the area Sticht.Intern. Vogelbesch., Zeist. and to safeguardthe wader populations,the Wadden Sea States and Guinea-Bissauhave establisheda co-operation Prokosch,P. 1988. Das Schleswig-HolsteinischeWattenmser ais Frahjahrs- Aufenthaltsgebietarkfischer Watvogel- program to support the conservationof the tidal areas of Populationenam Beispielvon Kiebitzregenpfeifer(Pluvialis Guinea-Bissau(and of the Wadden Sea), with the long- squatarola),Knutt (Calidds canutus) and Phuhisnchnepfe term aim of establishingcomprehensive protection of (Limosalapponica). Corax 12: 273-442. these areas. SaMg, J.C., S. Asbirk,J.P. I½.jeldsen& P.A.F. Rasmussen1994a. Winteringwaders in the BijagosArchipelago, Guinea-Bissau As part of this co-operationprogram, a team of Guinean 1992-1993. Ardea 82: 137-142. technicians has been educated and trained in order to continuethe monitoringactivities of waterbirds in Guinea- SaMg, J.C., K. Laursen& J. Frikke 1994b. Bird Life in the Danish Wadden Sea. Bissau. A database for counts of waterbirds have been set up for the area. Total winter countswill be carried out OpheliaSuppl. 6: 327-332. every 3 or 5 years in the coastal zone of Guinea-Bissau, Saivig,J.C., S. Asbirk,J.P. I•.jeldsen,P.A.F. Rasmussen,A. Quade, the next one being in 1999. These countswill be co- J. Frikke& E. Christophersen.In press.Wintering terns in the ordinatedwith similarcounts in Mauritania(Banc D'Arguin) coastal zone of Guinea-Bissau in 1992-92 and 1994. - Marine and otherWest-African countries by Wetlands Ornithology. International.The monthlycounts performed on the tidal flatswest of Bubaqueand east of Soga are continuing. SaMg, J.C. 1995. Migratorymovements and mortalityof Danish AvocetsRecurvirostra avosetta. Ringing and MigrationNB NB NB.

Smit, C.J. & T. Piersma 1989. Numbers,mid-winter distribution and migrationsof waderpopulations using the EastAtlantic flyway. In: H. Boyd& J.-Y. Pirot(eds) Flywaysand reservenetworks ACKNOWLEDGEMENTS for waterbirds: 24-63. IWRB Spec. Publ.9, Slimbridge. Summers, R.W., L.G. Underhill, D.J. Pearson & D.A. Scott 1987. The expeditionwas a DanishMinistry for the Environment Wader migrationsystems in southernand easternAfrica and project,and part of the trilateralWadden Sea Cooperation westernAsia. Wader StudyGroup Bull. 49, Suppl./IWRB (The Netherlands,Germany and Denmark). We express Spec. Publ.7: 15-34. our gratitudeto Torgny Quick and to the Swedish Underhill,L.G. 1987. Waders (Charadrii)and otherwaterbirds at Embassyin Bissaufor supportingthis project. We are LangebaanLagoon, South Africa, 1975-1986. Ostrich58: 145- indebtedto the authoritiesin Guinea-Bissaufor, permission 155. to carry'out the aerial survey. The Foundationof Poul Tholstrupmade it possibleto completethe article. Thanks Van Dijk,A.J., F.E. De Roder,E.C.L. Marteijn& H; Spiekman1990. Summeringwaders on the Bane D'Arguin,Mauritania: a cansus are also due to Leo Zwarts and Bruno Ens, who in June 1988. commentedon the first draft of this manuscript,and to an anonymousreferee for valuable comments on the final Zwarts, L. 1988. Numbers and distributionof coastal waders in versionof the manuscript. Guinea-Bissau. Ardea 76: 42-55.

37 Table 1. The percentagesof wadersspecified during counts in the BijagosArchipelago from aircraft and ground.The aerialcounts represent birdsspecified in the entirearchipelago, whereas the groundcounts represent birds specified on ten feedingareas in the archipelago.

Species aircraft(%) (jrcund(%) SandealingCalid#s alba 0.9 1.8 DunlinCalid•fs alpina 0 0.1 RingedPlover Charadrius hiaticula 0.2 4.4 Kentish Plover Charadrius alexandrinus 0 0.5 Grey PloverPluvialis squatarola 2.4 4.7 CudewSandpiper Calidris ferruginea 58.5 54.4 Little Stint Calidris minuta 10.0 5.4 Knot Calidris canutus 12.1 5.8 TumstoneArenaria interpres 0.1 1.0 CommonSandpiper Actitis hypoleucos 0 0.3 Redshank Trln.qatotanus 1.5 7.1 OystercatcherHaematopus ostrale.qus 1.2 1.1 GreenshankTringa nebularia 0.1 0.3 Bar-tailedGodwit Limosa lapponica 10.7 10.0 Curlew Numeniusarquata 1.1 0.5 Whimbrel Numenius phaeopus 1.2 2.6 100.0 100.0 N= 253•412 N= 25•595

Table 2. Estimatednumber of winteringwaders in the BijagosArchipelago during three surveys.

Species 1986/87' 1992/93** 1994 Sanderling 8,500 24,300 13,500 Dunlin 400 710 800 Ringed Plover 30,400 26,300 33,000 Kentish Plover 9,100 5,000 3,750 Grey Plover 36,500 39,100 35,250 CurlewSandpiper 196,600 326,500 408,000 Little Stint 102,000 59,700 40,500 Knot 90,000 31,300 43,500 Tumstone 10,800 7,900 7,500 CommonSandpiper 2,100 2,900 2,200 Redshank 70,400 38,400 53,250 Oystercatcher 1,600 7,100 8,250 Greenshank 2,920 1,400 2,250 Bar-tailed Godwit 115,800 108,700 75,000 Cudew 3,900 9,300 3,750 Whimbrel 18,100 221000 191500 Total 699,120 710•610 750•000

*: Zwarts(1988), **: Salviget al. (1994).

Table 3. The ratioof numbersummering to numberwintering of waderscounted at ten feedingareas in the BijagosArchipelago in 1994 (this study)presented as percentages.The correspondingpercentages are shownas wellfor the BancD'Arguin in Mauritania.

Species BancD'Arguin This study % % Sanderling 24 16 RingedPlover 13 35 Grey Plover 45 19 CurlewSandpiper 9 22 Little Stint 1 6 Knot 7 3 Tumstone 15 19 Redshank 36 11 Bar-tailed Godwit 6 18 Whimbrel 47 33 Total 13 15 *: Van Dijket el. (1990).

38