J. Ornithol. 139, 485-493 (1998) © Deutsche Omithologen-Gesellschaft/BlackwellWissenschafts-Verlag, Berlin ISSN 0021-8375 Monitoring the world population of the Lesser Grey Shrike (Lanius 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 birds (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, Shrikes (Laniidae) forage on large insects 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 U.S. Copyright Clearance Center Code Statement: 0021-8375/98/13904-0485 $11.00/0 486 Journal ftir Ornithologie 139, 1998 (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.