J. Ornithol. 139, 485-493 (1998) © Deutsche Omithologen-Gesellschaft/BlackwellWissenschafts-Verlag, Berlin ISSN 0021-8375

Monitoring the world population of the Lesser Grey ( minor) on the non-breeding grounds in southern Africa

Marc Herremans

Avian Demography Unit, University of Cape Town, Rondebosch 7700, South Africa Present Address: Royal Museum for Central Africa, Dept. Zoology, Leuvensesteenweg 13, B-3080 Tervuren, Belgium. E-mail: [email protected]

Summary

The world population of the Lesser Grey Shrike funnels in from breeding grounds in Eurasia to an area that is ten times smaller on the non-breeding grounds in the thornbelt of southern Africa, chiefly in the Kalahari basin. The species is abundant in open savanna habitat on the non-breeding grounds, where it is regularly spaced in individual territories and behaves conspicuously as a perch hunter. It has declined considerably during the last 200 years, and the non-breeding grounds offer highly favourable conditions for assessing and monitoring the world population. Extrapolation according to vegetation types of stratified survey data across southern Africa indicates a world population of 6.1 million (95% confidence 5.0-7.3 million) during the mid 1990s. However, for sensitive moni- toring, the establishment of a standard-effort index is more appropriate than the assessment of the total population. Key words: southern African thornbelt, Kalahari basin, stratified counts, Botswana

Zusammenfassung Monitoring der Weltpopulation des Schwarzstirnwiirgers (Lanius minor) im siidafrikanischen Uberwinterungsgebiet

Die Weltpopulation des Schwarzstirnwtirgers konzentriert sich im n6rdlichen Winter in dem zum eurasiatischen Brutgebiet zehnmal kleineren Gebiet der Dornbuschsteppen des stidlichen Afrikas, haupts~ichlich das Kalahad Becken. Dort kommt die Art vornehmlich in der offenen Savanne vor, wo sie territorial ist und typischerweise Stogjagd yon Ansitzen aus macht. Die Art hat in den letzten 200 Jahren stark abgenommen. Die hohe Konzentrierung im Llberwinterungsarealerlaubt ein Moni- toring der Weltpopulation. Die Weltpopulation wird nach dortigen Z~hlungen auf etwa 6,1 Millionen V6gel (5.0-7.3 Million; 95% Vertrauensbereich) gesch~tzt. Ftir ein regelm~iges Monitoring des Bestandes wird die Einrichtung von Beobachtungstransekten empfohlen.

Introduction but trends in the eastern part of the range remain unknown (Niehuis 1968, Lefranc 1978, 1993, (Laniidae) forage on large and 1995, Cramp & Perrins 1993, Glutz yon Blotz- small vertebrates, and as a group appear vulner- heim & Bauer 1993, Tucker & Heath 1994). able in the modem world; the conservation Populations of migratory shrike species are be- status of many species is of great concern, most lieved to be limited by factors on the non-breed- particularly so for northern hemisphere species ing grounds (Temple 1995a), but in the Lesser in the genus Lanius (Yosef & Lohrer 1995). Grey Shrike climatic fluctuations (particularly The Lesser Grey Shrike has declined dramati- wet summers), pollution and insecticide use on the cally over the western part of its breeding range, breeding grounds are thought to be the main which has contracted during the last 200 years, causes of poor breeding success and decline

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(Glutz yon Blotzheim & Bauer 1993. Tucker & haves conspicuously as a perch hunter from Heath 1994. Lefranc 1995), though pesticide high, exposed branches. use and drought on the non-breeding grounds The present paper presents stratified field have also been named as possible factors counts of Lesser Grey Shrikes from different (Moreau 1972, Glutz von Blotzheim & Bauer vegetation types throughout its core range in 1993. Herremans 1993 I. southern Africa; estimates of the world popula- The non-breeding range of the Lesser Grey tion are provided, and future monitoring op- Shrike is about one tenth of the size of its tions are discussed. breeding range (Dowsett 1971, Curry-Lindahl 1981. Newton 1995, Herremans 1998a in Methods press~. During the non-breeding season, the world population is concentrated in the south- Between mid-December and late February of the ern African thornbelt, centered on the Kalahari years 1993-1995, Lesser Grey Shrikes were counted basin (Dowsett 1971_ Zink 1976. Harrison et al. in southern Africa using strip-counts: birds observed 1997), where its range remarkably occupies a perched at less than, or flushed from within, 75 m from minor roads were counted fiom a vehicle mov- gap in the range of the resident Fiscal Shrike ing slower than 40 km/h (average speed 19.4 km/h Lanius collaris (Hen'emans 1997a). The over- during 3415 km of surveys). Thus, strips covered wheln~ing majority of moulting birds during the 15 ha per kin. The length of strips (and consequently austral mid-sunmaer occur from 19°S in Nami- the size of the sampling blocks) was not constant bia and 21°S in Botswana southwards to the (average 40.5 km), and the variance on the extrapo- northern Cape Province and the western parts of lated estimates was calculated according to the the North-Western and Northern Provinces in method of Jolly (1969), taking into account the co- variance between numbers observed and the transect South Africa (Harrison et al. 1997). Insignifi- area: cant numbers occur further north during mid- Vx = [N(N-n)/N]. (Sx2 - 2DSxy + D2Sy2) summer in southern Angola, Zambia, Malawi. Zimbabwe (except for the far south) and east- whereby wards into the central highveld and Mozam- Vx = the variance of the total number of birds esti- bique (Benson et al. 1971, Clancey 1971, Dow- mated in the stratum sett 1971. Taylor 1979, Irwin 1981. Tarboton et n = the number of transects counted al. 1987~. N = the number of transects possible in the stratum (i.e. total area/average area of transect) Working with incomplete data. Tucker & Sx2 = the variance of the number of birds seen per Heath (1994) and Kristfn & Lefranc (1997) transect estimated a population size for the Lesser Grey Sy2 = the variance of the transect area Shrike of 77.000-320,000 pairs in Europe and Sxy = covariance between the number of birds seen Russia west of the Ural. which covers about and the transect area half the present range; the former authors indi- D = the overall average density of birds observed cated that improved censusing and monitoring Lesser Grey Shrikes prefer open perches, and only of core populations in eastern and southern Eu- surveys on minor roads and tracks not paralleled by rope is required to assess the overall population power-lines or high fences were taken into account in the present study (except for the northern Nami- and its trends. Shrike studies outside the breed- bian Arid Woodland; see discussion). Roads bor- ing season are seen as a priority in shrike con- dered by fences and utility lines were excluded be- servation (Temple 1995b) and the concentrated cause shrikes are likely to occur under these condi- occurrence of the Lesser Grey Shrike in south- tions concentrated along the road verge at densities ern Africa offers the best conditions for moni- not representative for the remainder of the habitat. toring the species: it prefers the most open sa- The Lesser Grey Shrike is a very thermophyllic vanna habitat (Hen'emans 1997b p. is spaced out species (Glutz yon Blotzheim & Bauer 1993, Lefranc 1993), a feature which also holds on its subtropical regularly in individual territories (in contrast to non-breeding grounds, but it still tends to perch in the its clumped breeding distribution: Glutz yon shade during the hottest period in the middle of the Blotzheim & Bauer 1993, Tucker & Heath day, when shade-temperatures in the Katahari basin 1994, Krist/n 1995, Hoi et al. 1997) and be- typically reach the upper 30s °C or low 40s °C. The M. Herremans. The world population of the Lesser Grey Shrike 487 efficiency of a survey is defined as the fraction of lower 30% of reporting rates: Harrison et al. 1997) Lesser Grey Shrikes observed in a single survey, were included as belonging to the core range. The compared to the total number of birds known to be calculation of the size of the different vegetation present in a particular strip from a previous, more strata was made in a GIS-environment, taking into comprehensive count (or from a combination of pre- account that longitudinal parallels converge towards vious counts). Efficiency of transect surveys during the poles and grid-cells become gradually smaller the middle of the day, when birds are more reluctant towards the south of the species' range; the dif- to fly from sheltered perches, is low; therefore, sur- ference is about 10% for grid-cells at the opposite veys made between 11 a.m, and 4 p.m. were ex- latitudinal ends of the range of the Lesser Grey cluded from the analysis. Shrike in southern Africa. Despite the low travel speed during surveys and When in a pristine state, most of the arid wood- the open habitat, not all birds present within the lands at the periphery of the Kalahari basin are too strip-width were observed. During 'gauge'-counts dense to be suitable for Lesser Grey Shrikes, but the the major effects on survey-efficiency were identi- species readily accepts human-made clearings. From fied: repeating counts along the same strip at differ- field notes on habitat suitability, it is estimated that only 20% of the eastern arid woodlands (chiefly in ent times of the day, in habitat with a different degree the Limpopo catchment) and 10% of the western arid of openness and at different vehicle speeds, or even woodlands (in north-central Namibia) are actually • walking to make sure as many birds as possible were suitable habitat for Lesser Grey Shrikes. Similarly, flushed and counted, revealed the efficiency range of for grid-cells in lacustrine areas, the estimated per- the different conditions during the strip-counts. Two centage of the area covered by pan surfaces, unsuit- types of population estimate are presented, one based able to shrikes, was excluded from the range. on the crude numbers counted, another based on more realistic data which were compensated for the Throughout the text, densities are expressed as estimated under-counting factor (efficiency). Overall ha/, whereby small numbers represent high den- estimates and their confidence limits were the sum of sity, and high numbers low density. estimates per stratum. The limits of the non-breeding range of the Lesser Results Grey Shrike in southern Africa were derived from Harrison et al. (1997), but because this distribution A total of 43,125 ha (2875 kin) of strip-counts, map also included birds on passage, the northern made in seven vegetation strata, in Namibia, limits of the range were established from a subset of the stone data only including records for January and Botswana and South Africa was included in the February, when birds have reached the final non- analysis (Table 1). Lesser Grey Shrikes oc- breeding destination, where they undergo a complete curred at similarly high densities in the Kalahari moult (Dowsett 1971). The boundaries of vegetation (13.3 ha/bird), on the Khomas Hochlm~d in cen- types in southern Africa relevant to birds were also tral Namibia (10.3 ha/bird) and in human-modi- based on Harrison et al. (1997), with the following fied parts of the arid woodlands (10.2 ha/bird in exceptions prompted by particularities in shrike the upper Limpopo catchment and 14.0 ha/bird abundance: i) the Namibian escarpment was split in northern Namibia). Densities in the other, into a Mopane Cotophospermum mopane dominated more peripheral vegetation types were lower. northern part and an sp. dominated southern part; ii) the Khomas Hochland (> 1400 m asl) was The core of the non-breeding range covers considered distinct from the remaining arid wood- 1580 quarter-degree grid-cells and 1.1 million lands of northern Namibia; iii) the lacustrine areas in km 2, 40-60% smaller than estimated by Dow- the Etosha and Makgadikgadi Pans areas were con- sett (1971) and Newton (1995); when only suit- sidered as a separate type, and the Cuvelai drainage able vegetation is considered, the effective non- from southern Angola into Etosha was included in breeding range, however, is ca. 20% smaller at the lacustrine zone; iv) no distinction was made be- 0.9 million km 2 (Table 1). The ca. 70 million tween southern, central and northern Kalahari vege- hectares of the more open Kalahari vegetation tations, though the last was mostly excluded as it is constitute over 60% of the range, other vegeta- outside to the core non-breeding range. For quarter- tion types each constituting less than 5% of the degree grid-cells (QDS) where vegetation types in- effective range (Table 1; Fig. 1). terdigitated at the edge of the range, only those with reposing rates (i.e. the proportion of atlas field lists Efficiency decreased by up to 50% with in- in which the species was recorded) of Lesser Grey creasing heat towards the middle of the day, by Shrikes of at least 12.5% (i.e, above the cut-off of the up to 40% with increasing driving speed (up to 488 Journal Ftir Ornithologie 139, 1998

Table 1. Population estimates of the Lesser Grey Shrike in strip-counts in vegetation strata in southern Africa. Tab. 1. Bestandsschgtzungendes Schwarzstirnw~irgernach Transektz~ihlungenin verschiedenen Vegetations- typen im sfidlichen Afrika.

Vegetation type Kala KhHo eAWL wAWL AEsc Lacus MEsc TOTAL Number of strips sampled 26 3 13 5 7 13 4 71 Total area surveyed (hat 16245 1170 5505 2415 4995 9810 2985 43125 Number of birds counted 796 82 306 52# 38 80 8 1362 Number of birds estimated 1223 114 541 173# 66 138 18 2273 Estimated Density (ha/bird) 13.3 10.3 10.2 14.0# 75.7 71.1 165.8 Extent of stratum (number of quarter-degree grid-cells) 981 59 242 89 67 104" 38 1580 Total area ofstratum (103 ha) 68892 4203 17103 6439 4761 8654* 2748 112800 Suitability compensation no no 20% 10% no 63%* no Effective range of Lesser Grey Shrikes (103 ha) 68892 4203 3421 644 4761 5447 2748 90116 Total population estimate fcmde counts) (103 birds) 3376 295 190 14 36 44 7 3962 95% confidence interval (lower) 2454 276 147 1 18 21 0.8 2918 95% confidence interval (upper) 4297 313 233 27 55 68 14 5007 Population estimate (efficiency compensated) (103 birds) 5186 410 336 46 63 77 16 6134 95% confidence interval (lower) 4276 366 269 3 27 29 0.9 4971 95% confidence interval (upper) 6096 453 403 89 99 124 32 7296 Kala = Kalahari: KhHo = Khomas Hochland; eAWL = Eastern Arid Woodlands; wAWL = Western Arid Woodlands: Aesc = southern, Acacia-dominated Escarpment; Lacus = Lacustrine areas (Etosha and Makga- dikgadi Pans: Cuvelai drainage); Mesc = northern, Mopane-dominated Escarpment Data collected along roads with powerlines and fences * Pan-surface areas excluded on a QDS-basis; overall average suitability-compensationderived from totals

90 km/h), and by up to 70% with increasing tree-cover. Fortunately the species prefers very open habitat, and where tree-cover seriously interfered with efficiency, there were very few shrikes anyway. The average efficiency during gauge counts in the morning in central Kalahari vegetation was 71% (+ 19% s.d., range 50-- 95%, N = 9). During the heat of the day and at high speed (on tar roads), efficiency could drop below 30%, but such data have not been in- cluded in the present study. However, data with an estimated low efficiency due to tree-cover were included. The efficiency compensation factor for each strip count was estimated by comparing the conditions during each survey with those that would have the most serious impact (heat, speed, tree-cover). The average of Fig. 1. Vegetation types determiningthe core range of the estimated (and applied) efficiencies for all the Lesser Grey Shrike at the non-breedinggrounds in counts used in the calculations was 65% + 11% southern Africa: 1) Kalahari, 2) Khomas Hochland, 3) eastern Arid Woodlands, 4) western Arid Wood- s.d., range 30-100%, n = 71). lands, 5) Lacustrine, 6) Acacia escarpment, 7) Mopane The crude data result in a total population escarpment. estimate of nearly 4 million birds (95% con- Abb. 1. Vegetationstypen des Kerngebiets des fidence limits 2.9-5.0 million), but a more real- Schwarzstirnwttrgers im stidlichen Afrika: 1) Kala- istic estimate at 6.1 million birds (95% con- hari, 2) Khomas Hochland, 3) 6stliche trockene Aka- fidence limits 5.0-7.3 million) follows from the ziensavannen, 4) westliche trockene Akaziensavan- nen, 5) Lacustrine, 6) Akazienb6schung, 7) Mopane efficiency compensated approach (Table 1). B6schung. About 85% of birds are in the Kalahari vegeta- M. Herremans. The world population of the Lesser Grey Shrike 489 tion type, with another ca. 7% on the Khomas areas cleared by humans (mostly for agricul- Hochland and ca. 6% in human-modified habi- ture) are suitable. The patches of suitable and tat in the arid woodlands. The other vegetation unsuitable habitat are not randomly mixed in types contribute marginally to the total estimate these vegetation types, but are concentrated and and the numbers are insignificant relative to the clumped. As the strip-counts were only con- confidence limits on the Kalahari estimate. ducted in areas containing suitable habitat, the overall area of these vegetation types needed to be adjusted for the proportion which actually Discussion comprised suitable habitat. The areas of pan surface were estimated from maps in the case of The strip-counts covered 0.04% of the non- the Lacustrine vegetation type, and an esti- breeding range; potential sources of error dur- mated 10% and 20% of the western and eastern ing the extrapolations are considered below. A Arid Woodlands, respectively, was considered crucial factor for the extrapolation of stratified sufficiently cleared to provide shrike habitat, counts is the distribution pattern of the : but these estimates may be inaccurate. In view the more uniformly spread these are over the of the low densities of shrikes in the Lacustrine habitat, the more repeatable and reliable the zone and the relatively small area of arid wood- counts and the extrapolations become. Lesser land in northern Namibia, the errors have a Grey Shrikes maintain individual territories marginal effect on the overall estimate. How- when at the final non-breeding destination in ever, for the eastern Arid Woodlands, the effect southern Africa, an essential, though not could be larger: for each 10% error in the esti- necessarily sufficient, condition for regular mate of available habitat, the difference would spacing. be 175,130 birds more or less (ca. 3% of the The Kalahm-i is a complex mosaic of many total estimate). types of wood- and shrubland (see e.g. Peary For the western Arid Woodlands in Namibia 1994 for habitat illustrations), and not all are count-data could only be collected along tar occupied at similar densities by Lesser Grey roads bordered by open verges with utility lines Shrikes. In general, dense shrub and woodland, and tall fences. Lesser Grey Shrikes are at- and monotypic stands of the broad-leaved, de- tracted to these open hunting posts, but whether ciduous Terrninalia sericea and Terminalia that merely makes them more visible or also prunioides are avoided. The size of such pat- attracts them from beyond the 75 m strip-width ches of unsuitable habitat appears to have been boundary is unknown. The average density in sufficiently small (compared to the transect this vegetation type (14 ha/bird) is, however, size) and randomly distributed, not to make not unrealistically high and even lower than in further stratification within the Kalahari a suitable parts of the eastern Arid Woodlands. necessity; preference and avoidance of some The data for the western Arid Woodlands habitats within the Kalahari is already ac- should nevertheless be viewed with some cau- counted for in the variation in densities found tion, but fortunately the contribution of this area between strip-counts. At the western edge of to the total is limited. the range, Lesser Grey Shrikes penetrate the interface between the Namibian escarpment The sample size from the Khomas Hochland and the Namib desert, where they occupy pat- is small, and the risk exists that the surveys may ches with taller vegetation, typically along fiver not be representative. However, other sources courses. Such clumped distribution results in also indicate that Lesser Grey Shrikes are par- unstable densities in strip-counts and conse- ticularly common in this area (Rowan et al. quently the confidence levels on the estimates 1964, Becker 1974, Loske 1985, Tree 1989), are wide. These areas are, however, relatively and although in need of further quantitative small and hold only a small fraction of the confirmation, the density data for this area ap- overall population. pear realistic. In the Lacustrine vegetation type, large areas The accuracy of the assessments of the non- of pan surface are totally unsuitable for Lesser breeding range and the vegetation zones is an- Grey Shrikes, while in the Arid Woodland only other potential source of error. However, the 490 Journal ffir Ornithologie 139, 1998 drop-off of the numbers of Lesser Grey Shrikes other visual advertising tool (Sloane 1991; Le- outside the preferred vegetation types is rapid, franc 1997), but this behaviour is rare in the and only the establishment of the boundaries of Lesser Grey Shrike (Lefranc 1997) and has the Kalahari could have a serious effect on the never been reported on the non-breeding estimates. Most of the Kalahari is well defined grounds. Despite the unusual conspicuousness by soil conditions and the transitional zone is for a small bird, the efficiency of furthermore small compared to the total area of counts was unexpectedly low (on average 71% this vegetation. in repeated gange-counts), which warrants the A limiting factor determining the northern application of an efficiency compensation to and eastern boundaries of the non-breeding the data. There may be uncertainty as to the range of the Lesser Grey Shrike could be in- accuracy of the efficiency compensation factor creased grass cover and the consequently re- estimated for each strip-count. Improving this duced prey visibility in the moister areas of the aspect of the surveys would, however, imply region (Herremans 1997b). The species is something as elaborate as the establishment of nevertheless widespread, though sparse in the a density-probability function (e.g. Andersen et grassland biome in South Africa, where suit- al. 1985) for each of the various possible micro- able pockets of Acacia sp. occur tHarrison et al. habitats, according to variations in shrike per- 1997L Density data tbr this biome were not ching behaviour during the day, and relative to collected, but the reporting rates were generally road-type and driving speed, which would be low to marginal (mostly below 2%, at most into unpractical for surveys of this scale. Improve- the 2-12.5% category Harrison et al. 1997), ment of the accuracy of the estimates, including and numbers in this area are unlikely to be addressing the above mentioned data-deficien- substantial compared to the core range. Al- cies would be a laborious process, which would though the species does occur locally to the not necessarily result in a much more reliable north of the cut-off boundary used here to estimate with a sufficiently reduced confidence define the core range, it is only found locally in range to warrant the extra effort. patches of unusually open parts of the northern Although the present study presents a first Kalahari and onto the central watershed in Zim- estimate of the world population of the Lesser babwe (such as along grassy pans. road verges Grey Shrike based on field counts, the error- and clearings for agriculture coppicing with margin on the total estimate is too wide to thomhnshes). As with the grassland biome such become the basis of a meaningful monitoring of a patchy presence makes a stratified approach population trends in future: the 95% confidence with limited coverage of the surveys an unsuit- limits span c. 40% of the total estimate. Even in able technique to assess the population in these the Kalahari, the vegetation type with the most areas accurately. The numbers in these areas uniform densities, the 95% confidence range is must be insignificant compared to the core still 35% of the total estimate. Simulations population, however. showed that to increase the accuracy range to Lesser Grey Shrikes have strikingly pied 10%, c. 150 strip-counts would be necessary, plumage. In contrast to behaviour on the breed- covering 6150 km or 0.13% of this single vege- ing grounds, where they frequently hover dur- tation type, compared to 0.02% now; a further ing bunting (Lefranc 1993. Glutz von Blotz- reduction of the margin of the confidence range heim & Bauer 1993), they do not hover while to 1% of the total would require about 2500 moulting on the non-breeding grounds. Except strip-counts (i.e. 102,500 kin, 2.25% of the for rare hawking of flying insects, they hunt by range in the Kalahari). Only the former, requir- the 'sit and wait' technique, and pounce on the ing c. 55 days of surveys by a single team, is ground, mostly from high, exposed perches, technically feasible. Thus, despite the unique where they spend most of the day conspicu- concentration and conspicuousness of the ously visible. Exposed perching has probably Lesser Grey Shrike on the non-breeding also a function in enhancing visibility as a grounds, and the almost ideal regular spacing in method of territorial advertisement. Many La- most of the range in the Kalahari savanna habi- nius shrikes also make use of food larders, an- tat which is sufficiently uniform in its diversity, M. Herremans - The world population of the Lesser Grey Shrike 491 estimating the world population within reliable cally, congregating at places with the best veld and adequately narrow limits that would allow conditions, competition would become very a sensitive population trend assessment is not important in the small "good" areas remaining feasible; it is only barely feasible for the part of during drought periods, when large areas of the the population occupying the Kalahari vegeta- range would be less suitable, and the dynamics tion type. Consequently, a better strategy is to can be expected to become density-dependent focus future monitoring of this species on a (see also discussion in Herremans 1998a in number of fixed transects in the core range in press). Because of the low numbers ringed the Kalahari from which a more sensitive index (Dowsett 1981) site fidelity is not proven in this of relative change can be established, particu- species (Curry-Lindahl 1981), but from fea- larly if environmental conditions such as rain- tures of its arrival pattern (i.e. early, fast and fall and vegetation condition are also monitored punctual: Herremans 1994) site fidelity may be along these transects. expected to be high (Herremans 1998a in First-year Lesser Grey Shrikes can be recog- press), as in its congener the Red-backed Shrike nized in the field in southern Africa on plumage Lanius collurio (Herremans et al. 1995). (e.g. Cramp & Perrins 1993, Glutz von Blotz- Because clearing of the Arid Woodlands at heim & Bauer 1993) up to mid-January, and the edge of the Kalahari has created suitable including age-structure in a monitoring pro- habitat in a less arid region, the species may gram could also reveal aspects of breeding suc- currently have better opportunities to avoid cess and survival. However, in the early part of widespread, serious drought by occupying the wet season improvement of veld conditions these less xeric peripheral areas. However, even induced by rainfall can be very patchy in the if clearance of the Arid Woodlands in future Kalahari (particularly in years with below aver- made this vegetation type almost entirely avail- age rainfall) and an increased risk of counting able to Lesser Grey Shrikes, it would only be transects through locally drought-stricken areas able to accommodate c. 35% of the Kalahari could increase the variability of count results population. and reduce the reliability of a trend index. Al- Increased pesticide use was mentioned by though we agree with Dowsett (1971) in his Glutz yon Blotzheim & Bauer (1993) as another criticism of Moreau (1972) that drought on the threat to the species on the non-breeding non-breeding grounds is unlikely to be a major grounds; however, this is not an issue as the factor per se affecting this xerophilic species, it widespread use of pesticides in tsetse control should be noted that local severe drought con- operations in Botswana (Davies 1980, Merron ditions during surveys in February 1994 in half- 1992) was restricted to the Okavango basin, an degree grid-cells 2424A and 2424B in Bot- area only visited for a very short period on swana resulted in the lowest densities (30-51 passage by Lesser Grey Shrikes, while the long- ha/bird) recorded in the core of the range in the term effects of pesticide use on insectivorous Kalahari. Sensitivity to local drought would birds in this area also appear to be minimal increase if Lesser Grey Shrikes were highly (Douthwaite 1980, 1982, Douthwaite et al. faithful to previous non-breeding territories. If 1981, Herremans 1998b). Pesticide use is wide- the majority of Lesser Grey Shrikes showed spread in locust control in the Karoo, but this high site fidelity, population dynamics could be vegetation type is avoided by Lesser Grey expected to be most severely affected by sto- Shrikes (Harrison et al. 1997). The largest im- chastic drought occurrences and hence they pact on the species in the Kalahari basin may, would be density independent; the variations in however, be the increasing habitat loss follow- density on the non-breeding grounds would be ing from bush encroachment in response to insignificant compared to the local variations in widespread, recurrent overgrazing, which may vetd conditions and the resulting capacity to also affect prey abundance (Herremans 1998b). sustain shrikes. If Lesser Grey Shrike numbers The present exercise of estimating the total were restricted by the availability of quality population of the Lesser Grey Shrike also has habitat in the Kalahari basin and the species implications for a wider context of bird popula- were to occupy the Kalahari very opportunisti- tion assessments in southern Africa. If it is not 492 Journal f'tir Ornithologie 139, 1998

possible to estimate the world population of dosulfan sprayed for tsetse fly control. Environm. Lesser Grey Shrike within narrow limits despite Pollut. Set. A. 22: 273-279. the uniquely suitable predisposition of its dis- Douthwaite, R.J. (1982): Food and feeding behaviour of the Little Bee-eater Merops pusillus in relation tribution pattern and behaviour on the non- to tsetse fly control by insecticides. Biol. Conserv. breeding grounds to stratified counts, what 23: 71-78. widespread bird species could such an ap- Douthwaite, R.J., Fox, P.J., Matthiessen, P. & Rus- proach be used for satisfactorily? Yet, total sell-Smith, A. (1981): The environmental impact population estimates and proportions derived of aerosols of endosulfan, applied for tsetse fly therefrom (e.g. the 1% norm) are currently control in the Okavango Delta, Botswana. ODA, widely in use. and are for instance the standard London. Dowsett, R.J. (1971): The Lesser Grey Shrike Lanius in waterbird conservation policy and practice minor in Africa. Ostrich 42: 259-270. (e.g. Anon. nd.. Rose & Scott 1994), while. Glutz von Blotzheim, U.N. & Bauer, K.M (1993): except under conditions of almost complete Handbuch der VOgel Mitteleuropas. Band 13/II coverage (which are far from being achieved in Passeriformes (4. Tell). Wiesbaden. southern Africa: e.g. Dodman & Taylor. 1996), Harrison, J.A., Allan, D.G., Underhill, L.G., Herre- this approach is no doubt scientifically unsatis- roans, M., Tree, A.J., Parker, V. & Brown, C.J. factory, particularly as many of the species in- (1997): The Atlas of Southern African Birds. Vols 1-2. Johannesburg. volved have highly clumped distributions. I-Ierremans, M. (1993): Seasonal dynamics in subKa- lahari bird communities with emphasis on mi- grants. In Wilson, R.T. (ed.): Birds and the African environment: Proceedings of the Eighth Pan-Afri- Acknowledgements can Ornithological Congress, 555-564. Mus6e Part of this study relied on the invaluable and unconditional Royal de l'Afrique centrale, Tervuren. efforts of all volunteers who cooperated with the Southern I-Ierremans, M. (1994): Fifteen years of migrant phe- African Bird Atlas Project. Diane Herremans' help as an nology records in Botswana: a summary and pro- observer, recorder and friend during the counts was indispen- sable. Wendy Borello provided some of the shrike counts spects. Babbler 28: 47-68. from the Kalahari in Botwana. The author was supported in Herremans, M. (1997a): Lesser Grey Shrike. In Har- Botswana by VVOB (Vlaamse Vereniging voor Ontwikke- rison, J.A., Allan, D.G., Underhill, L.G., Herre- lingssamenwerking en technische Bijstand) and by the De- mans, M., Tree, AJ., Parker, V. and Brown, C.J. partment of Wildlife and National Parks. Marieke Berkvens (eds.): The atlas of southern African Birds, Vol. II., assisted with the preparation of the Zusammenfassung. 406. Johannesburg. Herremans, M. (1997b): Habitat segregation of male and female Red-backed Shrikes Lanius collurio References and Lesser Grey Shrikes Lanius minor in the Ka- lahari basin, Botswana. J. Avian Biol. 28: 240-248. Anonym (nd): The Ramsar Convention. Gland. Herremans, M. (1998a in press): Strategies, punctua- Andersen, D.E., Rongstad, OJ. & Mytton, W.R. lity of arrival, and ranges of migrants in the Kala- (i985): Line transect analysis of raptor abundance hari basin, Botswana. Ibis 140. along roads. Wildl. Soc. Bull. 13: 533-539. Herremans, M~ (1998b in press): Becker, P. (1974): Beobachmngen an pal~arktischen of birds in Botswana in relation to land use. Biolo- Zugv~geln in ihrem Winterquartier Stidwestafrika. gical Conservation. Wissenschaftliche Forschung in S.W.A., 12. Herremans, M., Herremans-Tonnoeyr, D. & Borello, Benson, C.W., Brooke, R.K., Dowsett, RJ. &. Irwin, W.D. (1995). Non-breeding site-fidelity of Red- M.P.S ( 1971 ): The birds of Zambia. London. backed Shrikes Lanius collurio in Botswana. Clancey, P.A. (1971): A Handlist of the birds of Ostrich 66: 145-147. southern Mozambique. Louren~o Marques. Hoi, H., Valera, F., Krisffn, A. & K6nig, C. (1997): Cramp, S. & Pert'ins, C.M (1993): The birds of the Reproductive strategies in the Lesser Grey Shrike western Palearctic. Vol. VII. Oxford. (Lanius minor). Proc. Abstr. 1st. Europ. Ornith. Curry=Lindahl, K. (1981): Bird migration in Africa. Conf., Bologna: 122. Vol. 1. London. Irwin, M.P.S. (1981): The birds of Zimbabwe. Salis- Davies, J.E. (1980): The history of tsetse fly control bury. in Botswana. Mann. Jolly, G.M. (1969): Sampling methods for aerial cen- Dodman, T. & Taylor, V. (1996): African waterfowl suses of wildlife populations. East African Agri- census. Wageningen. cultural & Forestry Journal - special issue: 4&49. Donthwaite, R.J, (1980): Occurrence of birds in Aca- Kristin, A. (1995): Why the Lesser Grey Shrike (La- cia woodland in northern Botswana related to en- nius minor) survives in Slovakia: food and habitat M. Herremans • The world population of the Lesser Grey Shrike 493

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