Oxpeckers in north-eastern Namibia: recent population trends andthe possible negative impacts of drought and ®re

Antony Robertson*, Alice M. Jarvis

Directorate of Environmental A�airs, Ministry of Environment and Tourism, Private Bag 13306, Windhoek, Namibia

Received 15 December 1998; received in revised form 28 April 1999; accepted 10 May 1999

Abstract

Both redbilled oxpeckers (Buphagus erythrorhynchus) and yellowbilled oxpeckers (Buphagus africanus) have experienced recent

population decreases in southern Africa largely as a result of cattle dipping against ticks, their primary source of food. In Namibia,oxpeckers are con®ned largely to the Caprivi region in the north-east and the yellowbilled oxpecker is classi®ed as a Namibian RedData species. Counts in 1983±1984 estimated populations of 2285±3780 redbilled oxpeckers and 2062±2613 yellowbilled oxpeckers

on cattle in East Caprivi. In this study, three counts of oxpeckers were made in 1997±1998. Bird numbers had not increased inproportion with cattle numbers, the primary host which had increased three-fold in the last 15 years. Current redbilled oxpeckernumbers (3627±4902) were similar to those in the 1980s. Too few yellowbilled oxpeckers were observed to derive robust population

estimates, but our counts suggest a decline to about 330 birds. There have been no o�cial cattle dipping programmes in Caprivi,and private tick control is virtually non-existent. We suggest that the current dry conditions, in combination with the increasinglywidespread and more frequent practice of veld burning are having a substantial, di�erential e�ect on some of the tick speciesfavoured by the two species of oxpeckers. We hypothesise that these two factors act as a control mechanism on ticks favoured by

yellowbilled oxpeckers, so precipitating the dramatic decline of this species. If current trends continue, yellowbilled oxpecker willsoon disappear from Caprivi. # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Oxpeckers; Namibia; Ticks; Caprivi; Population decline

1. Introduction

Oxpeckers (Buphaginae) feed primarily on ticks andshare a symbiotic relationship with a wide range ofungulate host species (Stutterheim, 1981; Stutterheimand Panagis, 1985a; Hustler, 1987; Hart et al., 1990;Mooring and Mundy, 1996a; Koenig, 1997). Globally,they are con®ned to Africa south of the Sahara (Stut-terheim, 1982). In southern Africa, the redbilledoxpecker (Buphagus erythrorhynchus) is more wide-spread and numerous than the yellowbilled oxpecker(Buphagus africanus) (Mundy, 1997a,b). However, bothspecies are currently recovering from general populationdeclines experienced in part as a result of the earlier useof arsenic-based cattle dips in many areas of the sub-region (Stutterheim and Brooke, 1981; Stutterheim,1982). Both yellowbilled and redbilled oxpeckers occur

in Namibia with distributions con®ned largely to thehigher rainfall, north-eastern part of the country(Caprivi region and Okavango River). The yellowbilledoxpecker is also found along the Kunene River near theborder with Angola (Mundy, 1997b). While many pre-vious studies have identi®ed the relative importance ofdi�erent ungulate species as hosts for the two species ofoxpecker (Stutterheim and Panagis, 1985a; Hustler,1987; Hart et al., 1990), more recent work by Mooringand Mundy (1996a) suggests that host selection byoxpeckers is related to a variety of factors a�ectingforaging e�ciency, such as host body mass, herd size,hair length, behavioural response and the prevalenceand composition of ticks on hosts, rather than to hostspecies per se. Their study was also the ®rst to assess thee�ects of seasonal changes in tick abundance onoxpecker host preference and numbers. Studies of fora-ging preferences have shown some di�erences in the tickspecies favoured by the two oxpeckers (Bezuidenhoutand Stutterheim, 1980; Stutterheim and Brooke, 1981;Stutterheim, 1982), with symbiont overlap, at least in

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* Corresponding author. Tel.: +264-61-249015; fax: +264-61-

240339.

E-mail address: tr@dea.met.gov.na (A. Robertson).

east Africa, largely occurring as a result of a sharedpreference for the widespread ixodid tick Rhipicephalusappendiculatus. With the exception of white rhinoceros(Ceratotherium simum) and black rhinoceros (Dicerosbicornis), all of the preferred mammalian host speciesidenti®ed in other studies are present within theoxpeckers' ranges in Namibia. However, cattle are byfar the most numerous host (with ca. 140 000 in Caprivi)and have shown a three-fold increase over the last 15years (Mendelsohn and Roberts, 1997). Ratios ofoxpeckers:cattle obtained from counts undertaken inthe Caprivi Region in 1984 (Stutterheim and Panagis,1985b) suggested East Caprivi populations of the orderof 2000±3000 birds for both species. In Namibia, theyellowbilled oxpecker is currently considered to be avulnerable Red Data species (Chris Brown, personalcommunication) and regular monitoring of the populationis desirable. We undertook counts throughout theirnorth-eastern ranges during 1997 and 1998 to assess thecurrent status of both oxpecker species and to investigateseasonal e�ects on oxpecker numbers.

2. Materials and methods

2.1. Sampling

Counts of oxpeckers were undertaken throughoutCaprivi region (east of the 21� meridian) during threeperiods representing the end of the dry season, mid wetseason and post wet season (16±26 November 1997, 13±24 February and 14±19 May 1998, respectively). Thenumbers of redbilled and yellowbilled oxpeckers on allpotential host animals observed were recorded. Cattlewere by far the most commonly observed host whilesmall numbers of nine other ungulate host species werealso observed. All cattle groups observed were inspectedon foot su�ciently closely to detect, identify and countall oxpeckers present. Groups were excluded when: (1)their behaviour was actively in¯uenced by humans, e.g.when being herded or used for ploughing, or (2) theproximity of humans was likely to have deterredoxpeckers, e.g. within or near busy villages. All gamewere similarly checked for birds except where the ani-mals had already taken ¯ight. Additional counts ofoxpeckers on game collected during the ®rst week ofDecember 1997 (C. Boix, personal communication)were combined with those collected in November. Timeof day, exact location (using a global positioning sys-tem), species and group/herd size was recorded for allpotential host animals.

2.2. Analysis

As cattle are the predominant host of oxpeckers inNamibia, previous studies have assessed the status of

the two species on the basis of oxpecker:cattle ratios(Stutterheim and Panagis, 1985b; Brown and Brown,1987). While the exclusion of other hosts will lead tounderestimates of total oxpecker numbers, estimatesderived using cattle provide a useful and simple meansof assessing changes in bird numbers over time. Toenable comparisons of our results with those obtainedpreviously, oxpecker:cattle ratios and associated esti-mates of bird abundance were calculated for eachoxpecker species. Total cattle numbers were derivedusing the most recent data available (crush pen data forJune 1996; Mendelsohn and Roberts, 1997) and thenumber of cattle multiplied by the oxpecker:cattle ratioto produce estimates of oxpecker abundance for each ofthe three counts. We have assumed that these 1996 cattledata are su�ciently representative of current cattlenumbers to obtain a reliable indication of presentpopulation sizes of oxpeckers. West Caprivi holds only8% of the total cattle in the Caprivi region (Mendelsohnand Roberts, 1997) and studies undertaken in 1983 and1984 either focussed entirely on East Caprivi (Brownand Brown, 1987; Fig. 1) or considered West Capriviseparately (Stutterheim and Panagis, 1985b). In additionto calculating an overall ratio in 1984, Stutterheim andPanagis (1985b) derived oxpecker:cattle ratios withinseven veterinary zones. More re®ned estimates of totaloxpecker numbers are obtained using this approach asaccount is taken of the fact that oxpeckers and cattle arenot distributed evenly throughout the region. Data col-lected in 1983 (Brown and Brown, 1987) were, however,not separable into zones. Furthermore, a preliminarycomparison of estimates for both 1984 and 1997±1998data showed that although estimates derived using theveterinary zones were in all cases slightly lower, therelative di�erences between them remained comparableto those obtained using the overall oxpecker:cattle ratio.Since the main purpose of this paper is to assess relativechanges in bird numbers over time, we present EastCaprivi estimates derived simply using overall oxpecker:cattle ratios for our counts and for those undertaken in1983±1984.In order to assess the e�ects of host group size on bird

presence and abundance, the ratio of oxpeckers:cattlewas regressed against cattle group size. Rough estimatesof Caprivi population sizes for the main game host specieswere calculated from data provided by Mendelsohn andRoberts (1997). Maps indicating the locations of allsightings of oxpeckers and hosts were produced usingArcview, Version 3.0a (ESRI, 1997).

3. Results

Redbilled oxpeckers were observed throughout EastCaprivi, within Mahango Game Reserve and in thevicinity of the Okavango River (Figs. 1 and 2). Very few

242 A. Robertson, A.M. Jarvis / Biological Conservation 92 (2000) 241±247

yellowbilled oxpeckers were observed and these wererestricted to the southern sections of East Caprivi andalong the Kwando River, with some sightings on gamein Mahango Game Reserve in West Caprivi (Figs. 1 and2; Table 1). Low numbers of oxpeckers were observed inCaprivi Game Park due to the very low numbers ofhosts sighted. The birds were observed throughout theday and no temporal e�ects were apparent. Cattle groupsizes observed ranged between one and 175 animals andoxpeckers were observed on groups across this range,though numbers of oxpeckers per head of cattledecreased as herd size increased (p � 0:047). The num-bers of cattle examined in all three counts (Table 1) wereintermediate between those recorded by Brown andBrown (1987) at 1563, and Stutterheim and Panagis(1985b) at 4846. Numbers of oxpeckers counted were, inall cases, substantially lower than those recorded inprevious studies (Table 1) and sightings were almostexclusively of redbilled oxpeckers (95% of all birds).Consequently, comparatively low oxpecker:cattle ratioswere obtained for each of our counts.The current East Caprivi abundance estimates for

redbilled oxpeckers on cattle (3627±4902 birds) arecomparable to those of 15 years ago (Table 2) suggestingthat this population has remained essentially stable overtime. The number of yellowbilled oxpeckers on cattle,however, is now estimated to be in the region of 330birds (Table 2); substantially lower than in 1983/84(2062±2200 birds). The previous ratio of red-billed:yellowbilled oxpeckers ranged between 1.1:1 and

1.8:1 while the current ratio is around 15:1. For bothspecies, estimates were relatively consistent betweencounts and no seasonal change in bird numbers wasapparent.No yellowbilled oxpeckers were observed on cattle in

West Caprivi. The additional number of redbilledoxpeckers on cattle in this area is about 160±550 birds.Due to the infrequent sightings of other host species, datafrom the three counts were amalgamated for analysis ofnon-cattle hosts (Table 3). Redbilled oxpeckers wereobserved on ®ve of the nine species counted, while yel-lowbilled oxpeckers were observed on four. Based onthe ratios obtained, game in Caprivi probably supportaround 530 redbilled oxpeckers and 150 yellowbilledoxpeckers. Although bu�alo (Syncerus ca�er) have beenrecorded as hosts of both oxpecker species in some areas(e.g. Stutterheim and Panagis, 1985a; Hart et al., 1990),the population in Caprivi is largely transient, movingseasonally along the Okavango and Kwando River sys-tems to and from Botswana (Rodwell et al., 1995).Apart from a very few small resident herds, bu�alo areessentially sporadic hosts to oxpeckers in Caprivi andno counts of oxpeckers on bu�alo were made in thisstudy.Previous studies presented estimates of oxpecker

numbers on cattle in East Caprivi only. By includingestimated numbers of oxpeckers on cattle in WestCaprivi, and on all game hosts, current total populationestimates for Caprivi are 4319±5984 redbilled oxpeckersand 484 yellowbilled oxpeckers.

Fig. 1. Vegetation structure, rivers and protected areas in Caprivi, Namibia.

A. Robertson, A.M. Jarvis / Biological Conservation 92 (2000) 241±247 243

4. Discussion

4.1. Population trends and possible limiting factors

Our comparisons of current population estimates forredbilled and yellowbilled oxpeckers in East Capriviwith those of 15 years ago suggest that redbilledoxpecker numbers have changed little while yellowbilledoxpecker numbers have declined dramatically. In abso-lute terms, the greatest number of yellowbilled oxpeck-ers observed on cattle during any one count was ninewhich is in strong contrast to two previous studies(Stutterheim and Panagis, 1985b; Brown and Brown,1987) which recorded totals of 181 and 72, respectively,on similar numbers of cattle. It therefore appears thatwhile numbers of redbilled oxpeckers have been helddown, those of yellowbilled oxpeckers have been

depressed. Whilst it would probably be unrealistic toexpect oxpecker numbers to increase proportionallywith cattle numbers (i.e. that the oxpecker:cattle ratioshould remain constant) when cattle have shown athree-fold increase over the last 15 years, some increasewould be expected assuming that bird populations werenot limited by factors other than host abundance.In order to explain the apparent change in the status

of the two oxpecker species in Caprivi, it is necessary toidentify those environmental factors which not onlylimit oxpecker numbers but also a�ect the two speciesdi�erentially. While southern African oxpecker popu-lations in general have su�ered declines as a result ofcattle dipping and spraying schemes (Stutterheim andBrooke, 1981; Stutterheim, 1982; Mundy, 1997a,b), nosuch o�cial schemes have been in operation in theCaprivi region during the period concerned (Roger

Fig. 2. Locations of game, cattle, redbilled and yellowbilled oxpeckers observed during three counts in 1997 and 1998.

244 A. Robertson, A.M. Jarvis / Biological Conservation 92 (2000) 241±247

Paskin and Ronald Mkandawira, Directorate of Veter-inary Services, personal communication). Redbilled andyellowbilled oxpeckers have similar ecological require-ments and the three factors most likely to limit birdnumbers in Caprivi are hosts, availability of nest sitesand food. Host availability can be ruled out as a limitingfactor because cattle numbers have increased. A reduc-tion in the number of natural nest cavities may haveoccurred with the loss of many large trees to burningand clearing. However, 80% of the region is comprisedof woodland vegetation (Mendelsohn and Roberts,1997) and nest site availability is unlikely to have beensigni®cantly reduced. Moreover, any changes in eithernest site availability or host numbers are expected toa�ect both oxpecker species similarly.

4.2. Tick preferences and factors which may di�erentiallyin¯uence food availability

Changes in food availability in combination with dif-ferences in foraging behaviour and tick preferencebetween the species might provide an explanation of thedi�erential population changes. Due to di�erences inbill shape, yellowbilled oxpeckers tend to `hammer' orpluck prey items from the host while redbilled oxpeckersemploy a `scissoring' motion (Stutterheim, 1981; Stut-terheim and Panagis, 1985a; Mundy, 1997b). Availableevidence suggests that the two species of oxpecker exhi-bit di�erences in prey preference. Both are known toforage on the brown ear tick (R. appendiculatus) butredbilled oxpeckers also show a preference for the bluetick (Boophilus decoloratus) while yellowbilled oxpeckersprefer the bont tick (Amblyomma variegatum) (vanSomeren, 1951; Bezuidenhout and Stutterheim, 1980;Mooring and Mundy, 1996b). Redbilled oxpeckers inKruger National Park, South Africa, were apparentlyunable to swallow engorged Amblyomma females(Bezuidenhout and Stutterheim, 1980). Both Boophilusdecoloratus and A. variegatum are found throughoutCaprivi (Walker, 1991), although, based on evidence fromZimbabwe, A. variegatum is probably largely restricted tocattle and bu�alo (Norval et al., 1994).R. appendiculatus isabsent from the region altogether (H. Heyne, Onder-stepoort Veterinary Research Centre, personal commu-nication; Walker, 1991), possibly increasing the relativeimportance of the former two species to oxpeckers inCaprivi.Two factors are likely to have important di�erential

e�ects on tick species in Caprivi. The ®rst is rainfall andsaturation levels. An investigation of the prevalence ofseveral African tick species in relation to rainfall sug-gested that Boophilus decoloratus (favoured by redbilledoxpeckers) is tolerant of drought conditions, showinghigher larval abundance in a very low (161 mm), com-pared to a higher (490 mm), rainfall year (Mooring et

Table 1

Numbers of redbilled and yellowbilled oxpeckers observed on cattle in

West and East Caprivi during the dry season (November 1997), mid

wet season (February 1998) and post wet season (May 1998)

Cattle

observed

Redbilled

oxpeckers

Yellowbilled

oxpeckers

West Caprivi

September 1983a ± ± ±

April 1984b 1163 82 1

November 1997 414 5 0

February 1998 482 20 0

May 1998 250 5 0

East Caprivi

September 1983a 1563 132 72

April 1984b 3683 191 181

November 1997 2001 62 1

February 1998 2601 103 7

May 1998 3375 99 9

a Brown and Brown (1987). Dashes indicate that these data were

not collected.b Stutterheim and Panagis (1985b). Their West Caprivi data do not

include counts of birds or cattle in Caprivi Game Park.

Table 2

Oxpecker:cattle ratios and abundance estimates obtained in 1983/1984 and 1997/1998 for East Caprivi

Redbilled oxpecker Yellowbilled oxpecker

Bird:cattle Abundancea Bird:cattle Abundancea

September 1983b 0.0845 3780 0.0461 2062

April 1984c 0.0519 2285 0.0491 2200

November 1997 0.0310 3837 0.0005 62d

February 1998 0.0396 4902 0.0027 334

May 1998 0.0293 3627 0.0027 334

a Estimated from bird:cattle ratio�total cattle numbers. We used the following total cattle numbers: 44 760 (1983/1984; Stutterheim and Panagis

1985b); 123 781 (1997/1998) derived from crush pen data for June 1996 (Mendelsohn and Roberts, 1997). The increase in cattle numbers from 1984

to 1996 is described in detail by Mendelsohn and Roberts (1997).b Brown and Brown (1987).c Stutterheim and Panagis (1985b).d A spurious estimate is produced from the sighting of only one bird (Table 1).

A. Robertson, A.M. Jarvis / Biological Conservation 92 (2000) 241±247 245

al., 1994). Whilst A. variegatum (favoured by yellow-billed oxpeckers) occurs in a variety of habitats, mainlywith tree and bush cover, it is absent where grass coveris short or where rainfall is below 400±750 mm (Petneyet al., 1987). Although average annual rainfall in theCaprivi region ranges between 500 mm in the south to700 mm in the north it has been lower in recent years(Mendelsohn and Roberts, 1997). In addition, riversand wetlands of the region are currently in a dry phase,with the last wet phase ending in 1981/82 (Mendelsohnand Roberts, 1997). These dry conditions are thereforelikely to have a greater negative impact on the tick speciesfavoured by yellowbilled oxpeckers.The second factor likely to a�ect tick numbers is veld

burning. This is becoming an increasing concern inCaprivi where a large proportion of the region is burnteach year (60% in 1996) to stimulate new growth forgrazing and to clear land for cultivation (Mendelsohnand Roberts, 1997). In addition, the timing of burning ischanging, with ®res no longer being restricted to thetraditionally favoured period immediately prior to therains in October/November. In 1996 a substantialincrease in the rate at which land was burnt wasobserved from August to December (Mendelsohn andRoberts, 1997). Burning is likely to have a major impacton vegetation structure and cover and thus on tick sur-vival and prevalence. The removal of ground cover hasbeen suggested as a possible control mechanism for A.variegatum (Petney et al., 1987). Additionally, the smallerrange of mammalian hosts utilised by A. variegatummay retard its recovery after veld burning (Norval et al.,1994). Spickett et al. (1992) studied the e�ects of veldburning in spring (September) in the Kruger NationalPark on 13 ixodid tick species including twoAmblyomma species and six Rhipicephalus species.While many species (including Amblyomma) showed acontinual decline in abundance in the months immedi-ately following burning, Boophilus decoloratus numbers

showed an initial decline but increased within 4 monthsto equal those in an unburnt control zone. Hence,increasing both the area and frequency of veld burningis likely not only to suppress the tick burden of hostsgenerally, but also to a�ect tick numbers and the rate ofrecovery of tick species di�erentially. Again, thesee�ects will have greater impacts on the species favouredby yellowbilled oxpeckers.

5. Conclusion

Available evidence suggests that the two oxpeckerspecies have di�erent food preferences. The preferredprey of yellowbilled oxpeckers appears less tolerant ofdry conditions and veld burning than that of redbilledoxpeckers. We therefore suggest that the suppression ofoxpecker populations in Caprivi and the apparent dra-matic decline of the yellowbilled oxpecker are related tochanges in the prevalence of preferred tick species,resulting from the combined e�ects of the drier condi-tions currently prevailing and the increased incidence ofburning. Reductions in oxpecker numbers may havearisen as a result of emigration, increased mortality and/or reduced breeding success. It seems likely that thecurrent limits of distributions of the two oxpeckers inEast Caprivi are related to the distributions of theirpreferred tick prey. However, further work is clearlyrequired to test these hypotheses and to ascertain con-clusively the factors responsible for the decline in theyellowbilled oxpecker population. Speci®cally, moreinformation is needed on the relative abundance of dif-ferent tick species on cattle and prey preferences of eachoxpecker species in Caprivi. Direct assessment of thee�ects of burning on populations of the con®rmedfavoured tick species is also required. If current trendscontinue, yellowbilled oxpeckers will disappear fromCaprivi.

Table 3

Numbers of oxpeckers and known game hosts observed over three counts in Caprivi during 1997 and 1998

Game Redbilled oxpecker Yellowbilled oxpecker

Estimated

Caprivi

populationa

No.

observed

Birds Bird:host Estimated

Caprivi

population

Birds Bird:host Estimated

Caprivi

population

Impala (Aepyceros melampus) 412 256 0 0 0 0 0 0

Hippopotamus (Hippopotamus amphibius) 1607 130 28 0.22 354 7 0.05 80

Greater kudu (Tragelaphus strepsiceros) 323 79 3 0.04 13 3 0.04 13

Burchell's zebra (Equus burchelli) 633 58 0 0 0 0 0 0

Sable (Hippotragus niger) 446 47 9 0.19 85 3 0.06 27

Roan (Hippotragus equinus) 164 17 0 0 0 0 0 0

Blue wildebeest (Connochaetes taurinus) 58 6 2 0.33 19 0 0 0

Gira�e (Gira�a camelopardalis) 76 5 4 0.80 61 2 0.40 30

Warthog (Phacochoerus aethiopicus) 303 3 0 0 0 0 0 0

a Estimated from Mendelsohn and Roberts (1997). Their data suggest a maximum of 1700 bu�alo (Syncerus ca�er) in Caprivi; but this popula-

tion is largely transient and no counts were made (see text). Eland (Taurotragus oryx) have also been recorded in very low numbers.

246 A. Robertson, A.M. Jarvis / Biological Conservation 92 (2000) 241±247

Acknowledgements

This work was funded by GTZ (Deutsche Gesell-schaft fuÈ r Technische Zusammenarbeit) through theNamibia's National Biodiversity Programme. We aregrateful to Roger Paskin (State veterinarian, Windhoek)and Ronald Mkandawira (State veterinarian, Caprivi)for information on dipping and spraying practices inCaprivi and to JohnMendelsohn and Carole Roberts forproviding supplementary data on cattle and game num-bers. Christian Boix provided additional observations ofoxpeckers on game and Heloise Heyne (OnderstepoortVeterinary Research Centre) supplied valuable infor-mation on ticks. We thank Chris Brown, Phil Hockey,John Mendelsohn, Peter Mundy, Rob Simmons and ananonymous reviewer for comments on an earlier draftof the manuscript.

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