Effects of drought on birds in the Kalahari, Botswana

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Ostrich 2004, 75(4): 217–227 Printed in South Africa — All rights reserved

OSTRICH ISSN 0030–6525

Effects of drought on birds in the Kalahari, Botswana Marc Herremans Natuurpunt Studie, Kardinaal Mercierplein 1, 2300 Mechelen, Belgium e-mail: [email protected]

Results are presented from point-counts at six sites in the Kalahari in Botswana. Counts were repeated three times: during a dry season following good rains (1991), during the next wet season when rains were far below average, and the following dry season (1992) when the area became drought-stricken. Compared to the wet season, bird numbers decreased during the drought by 37–81% and species by 8–52%; compared to the previous dry season, birds decreased by 5–71% and species by 2–47%. Bird diversity (relative to numbers) tended to increase during the wet season but was little affected by drought, except in the northern Kalahari, where a greater proportion of birds moved out in response to drought. This gave the northern Kalahari the most distinct bird community during a wet cycle, but it became again typically Kalahari during the drought. Thus, the typical Kalahari bird communities expanded their range during drought into the moister periphery. Changes in numbers, most probably resulting from (local) movements were found in many species. Most confirmed earlier reports on their nomadic nature but some, like Red-crested Lophotis ruficrista and Northern Black Afrotis afraoides Korhaan, Chestnut-vented Tit-Babbler Parisoma subcaeruleum, Ant-eating Chat Myrmecocichla formicivora, Tinkling Cisticola Cisticola rufilatus, Black-chested Prinia Prinia flavicans, Marico Flycatcher Bradornis mariquensis and Brown-crowned Tchagra Tchagra australis have not been or are not widely recognised as mobile species.

Introduction The study of terrestrial bird communities has not been a very popular subject in southern Africa during the last decades. The major studies on the subject, albeit many times only qualitatively, date back to the 1950–1980s when pioneers in the field like JM Winterbottom, R Liversidge, CJ Vernon, WR Tarboton, CJ Skead, GL Maclean and RK Brooke mapped bird communities for various biomes in southern Africa. Seasonal dynamics in southern African birds have been studied in forests by Koen (1992) and Symes et al. (2002) and in woodlands in Swaziland by Monadjem (2001, 2004). R Liversidge and G Maclean provided ample information on the birds of the Kalahari National Park (now Kgalagadi Transfrontier Park), RSA, situated in the more arid part of the Kalahari basin. Both undertook studies over longer time periods of population changes (semi-quantitatively) (Maclean 1970b, Liversidge 1980, 1984) and occurrence of breeding (Maclean 1970a, 1970c) in relation to rainfall. Liversidge’s (1984) paper contained six years of data and records of over 63 000 birds, but few species were discussed separately as most were combined and discussed as feeding assemblages (raptors, granivores and insectivores, each including long-distance migrants). For the remainder of the Kalahari basin (chiefly in Botswana), little has been published. At the other end of the Kalahari, near Lake Ngami in northern Botswana, Tree (1972) reported on spectacular changes in bird populations between a very wet year and a subsequent drought. Qualitative changes in the presence of some Kalahari species beyond the normal breeding range in northern Botswana were reported by Maclean (1987). Seasonal changes in bird populations (with an emphasis on migrants) have been analysed by

Herremans (1993) for the Kalahari in Botswana, based on data collected during a consecutive dry and wet season. Rainfall is erratic in the Kalahari and although the yearly average is 200–350mm, rains are very unreliable, and annual variation is large, from nearly nothing to over 500mm. At least 20mm should fall in a fortnight to have any effect on the vegetation (Liversidge 1984). When this does not happen during the growing season in summer, the area becomes drought-stricken until substantial rain falls at some later time. Drought is a recurrent phenomenon in the Kalahari biome. In fact, if it was not for the regular and profound effects of the drought, such as the killing of large numbers of trees and shrubs, the Kalahari would be a rich and dense woodland instead of an open savanna shrubland and savanna woodland. On the other hand, birds in the Kalahari basin mainly breed during the wet season (Harrison et al. 1997), and abundant rains are a trigger for mass breeding in most species, while drought suppresses breeding (Maclean 1970a, 1970c, Lloyd 1999). This results in a paradox, whereby species that depend on the drought for maintaining the structural quality of their preferred shrubland and open savanna-woodland habitat suffer from severely reduced reproductive output and probably increased mortality during such droughts. After a series of wet years with good breeding, populations of many songbirds peak, but for many the habitat becomes critically encroached by coppicing bushes or tall grasses. During severe drought, the habitat structure improves for many species, but feeding opportunities deteriorate and breeding fails. Furthermore, the annual dry season also has an impact on many species and substantial numbers of birds migrate during the cold dry season to the

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north and east of the Kalahari into more mesic vegetations (Harrison et al. 1997). These more or less regular movements and other more erratic movements are poorly mapped or understood in the Kalahari, and in many species the proportion of the birds involved is probably variable, depending on local conditions. The present paper investigates the effects of drought on bird communities in several parts of the Kalahari in Botswana. It compares the results from three bird counts at each of six sites: once during the dry season of a wet year (1991), the second time during the next wet season (1992) when the rains failed, and a third time six months later during the subsequent dry season when the Kalahari became drought-stricken. Assessments are made of the effect of the drought on bird and species numbers, the relative diversity of the bird communities and bird movements. Material and methods Point transect counts Bird communities were sampled by point transect counts, chiefly from the canopy of a vehicle along 4x4-quality tracks. Six transects were counted in the Kalahari at the end of the dry season (mid-September to early November) in 1991, and these were repeated during the next wet season (February 1992) and the next dry season (late June to midJuly 1992). These counts are a subset of the data used to assess the place of long-distance migrants in seasonal dynamics in bird populations in the Kalahari (Herremans 1993), but with an additional count in the dry season of 1992, and with analyses now pertaining to residents (including nomads) instead of well-known migrants. The transects were selected after a preliminary survey and aimed to be representative for the major vegetation types of the region (Table 1). Each transect consisted of 20 points at least 250m apart in a vegetation type as uniform and representative for the area as possible. All birds heard or seen within a cylinder of 75m radius from the observer were recorded during 10 minutes at each point, including birds flying past or overhead. Twelve points were counted in the first three hours after sunrise and eight were covered in the last two hours before sunset. Morning and evening counts were either on the same day, or on consecutive days (evening and next morning). Repeated counts were made from exactly the same points. The call of the Pear-spotted Owlet Glaucidium perlatum was imitated during the second part of each count to prompt the appearance of more unobtrusive species. If applied for a

short period at moderate volume, this method resulted in most birds in the immediate neighbourhood starting mobbing, but an influx from beyond the count perimeter to join the mobbing was never observed. When females of territorial (singing) males were not observed, they were nevertheless assumed to be present and added to the count. However, this correction was only applied for unobtrusive small species (e.g. some larks) that normally occur in pairs in home ranges less than the area surveyed from each point. Unidentified birds were recorded, but were disregarded in the analysis (on average only 0.01%). Vegetation structure Structural diversity of the vegetation was recorded during the first counts (dry season of 1991, when the veld was in good condition). The following classes were estimated: percent cover by grasses (0–25cm, 25cm–1m, >1m), percent cover by herbs (0–25cm, 25cm–1m), percent cover by bushes and trees (0–1m, 1–3.5m, 3.5–7m, 7–14m, >14m) and percent foliage. From these classes the Shannon index (H') of structural diversity was calculated (Table 1): H' = sum (pi.ln(pi)). The lower this index, the more open, low and uniform the vegetation, while a higher index indicates a structurally diverse woodland with grasses, shrubs and trees of different sizes. Illustrations approaching the above vegetation types can be found in Harrison et al. (1997) for the Nata Delta (Figure 51 Sowa Pan), Southern Kalahari savanna (Figure 44), Central Kalahari savanna-woodland (Figure 42) and Northern Kalahari woodland-savanna (Figure 41), and for Kalahari grassland in Penry (1994 — Central Kalahari Deception Valley). Diversity by rarefaction Bird diversity can intuitively be described as the chance that the next bird is of a different species than the previous one. Comparing the diversity in samples of different overall size (whether or not resulting from differences in sampling effort or survey efficiency) remains difficult, but is possible with the rarefaction diversity measurement. On the basis of the relative frequencies of species recorded, this statistic allows back-calculation of the probability that a species was already present in any smaller sample, and hence allows an estimate of the number of species in any smaller sample (Simberloff 1972, Heck et al. 1975, Magurran 1988). The estimates of species richness can then be compared for two samples at a point of common sample size (by necessity equal to or smaller than the smallest sample). Although rar-

Table 1: Localities and vegetation types for bird counts in the Kalahari (Botswana), ranked from open short grassland to more woody savannas (H' = Shannon index of structural diversity of the vegetation) Locality Nata Delta Sekoma(g) Kgoro Sekoma(s) Kutse Phuduhudu

Co-ordinates 20°22’S, 26°18’E 24°25’S, 24°05’E 25°25’S, 25°30’E 24°34’S, 24°03’E 23°22’S, 24°26’E 20°10’S, 24°37’E

Vegetation type delta grassland Kalahari grassland Acacia tortilis bushveld Southern Kalahari savanna Central Kalahari savanna Northern Kalahari savanna

H' 0.74 1.09 1.54 1.86 1.90 1.91

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Similarities Similarities in population composition between sites and seasons were calculated with Manhattan (city block) distances (Cain and Harrison 1958, Gower 1985), and a tree cluster with relative distance linking was used to visualise the groupings. Calculations were performed with Statistica 6.0 (Statsoft 2001). Rainfall Rainfall was not measured at any of the study sites (neither at any station close enough to be representative), and I can only make vague general statements about the rains during the study. During the first months of 1991, the Inter-Tropical Convergence Zone (ITCZ) moved southwards over the Kalahari several times and rains were widespread and abundant over the entire Kalahari basin. The average rainfall at 12 stations across Botswana during the wet season of 1990–1991 was 480mm, above average for the Kalahari in Botswana (250–500mm — Penry 1994). Kutse received particularly good rains (c. 600mm). The veld conditions remained excellent throughout the dry season of 1991, when the first counts were undertaken. The next summer rains (October 1991–April 1992) failed over most of the Kalahari: the ITCZ only brought widespread rains to the northern parts of Botswana, barely touching the northern Kalahari. Further south, the little rain that fell typically came as isolated storms, resulting in important local variation in rainfall and typically making the effect of the subsequent drought more severe in places than in others. This effect was probably further enhanced by longer-term carry-over effects of previous rainfall and ground moisture on vegetation. Average rainfall at 12 stations throughout Botswana in the wet season of 1991–1992 was 235mm, but these data include also stations in the more mesic north of the country where rainfall in the range 500–600mm had been normal. Overall, the wet season 1991–1992 was the driest in the Kalahari in Botswana between 1980–1994 (Herremans 1994a). The count sites near Sekoma in the southern Kalahari were the worst hit by the drought, which was already visible during the mid-summer count of early 1992. Results Species numbers and abundance A total of 9 343 birds of 134 species were counted during the 18 counts. Figure 1 presents an overview of the data structure in terms of how commonly species were encountered. No species were found at all sites during each count. The most widespread and abundant species were Scaly-feathered Finch Sporopipes squamifrons, Black-chested Prinia Prinia flavicans, Kalahari Scrub-robin Cercotrichas paena and Fawn-coloured Lark Calendulauda africanoides (group at upper right in Figure 1). Despite being widespread, the

widespread & abundant 3 NUMBER OF BIRDS RECORDED (log transformed)

efaction calculates species richness for each sample size, the outcome is based on the relative abundance of species in the sample and therefore the rarefaction curve represents a true measure of diversity. Because the rarefaction statistic allows the calculation of confidence limits, the testing of differences is also possible.

219

locally abundant widespread & common

2,5 locally common 2

widespread but sparse

1,5

restricted but numerous

1

0,5

localised uncommon

1

2 3

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

NUMBER OF TRANSECTS IN WHICH THE SPECIES WAS RECORDED Figure 1: Abundance plots of the species recorded during repeated transect counts in the Kalahari in Botswana: abundance (logarithm of the total number of birds counted) against the total number of transects in which the species was encountered

Chat Flycatcher Bradornis infuscatus was remarkably sparse (Figure 1, lower right). At the opposite end of the spectrum, the Red-headed Finch Amadina erythrocephala was localised, but unusually abundant (Figure 1, top left), most closely followed by Red-billed Quelea Quelea quelea in this respect. Red-capped Lark Calandrella cinerea, Pinkbilled Lark Spizocorys conirostris, Kittlitz’ Plover Charadrius pecuarius, Crested Francolin Dendroperdix sephaena, Shaft-tailed Whydah Vidua regia, Yellow-throated Sandgrouse Pterocles gutturalis, Magpie Shrike Corvinella melanoleuca and Lesser Masked-weaver Ploceus intermedius were found only at one or two sites, but were locally numerous. Just over one third of all species (44 species) were uncommonly recorded in only one to four of the counts (Figure 1, left bottom). Bird numbers changed inconsistently from a rich dry to a consecutive poor wet season (Figure 2a). There were two sites with little change (1–3% decrease), two with a clear decline (14–28%) and two with an increase (48–92%). At Kutse numbers (already high) nearly doubled, but this was entirely due to an influx of large flocks of Red-headed

220

Herremans

1 800

(a)

NUMBER OF BIRDS IN TRANSECT

1 600

Nata Delta Sekoma (g) Kutse Sekoma (s) Kgoro Phuduhudu

1 400 1 200 1 000 800 600 400

NUMBER OF BIRD SPECIES IN TRANSECT

200

60

(b)

50

40

30

20

10

1991 dry 1992 wet 1992 drought CONSECUTIVE DRY AND WET SEASONS Figure 2: Changes in bird populations during drought, as recorded during repeated transect counts in the Kalahari in Botswana: (a) total bird numbers, (b) number of species

Finches, with a total of 628 being recorded (from seven count points) in the summer of 1992, only 18 at four points the previous dry season and none at all during the next dry season (drought). From the wet season of 1992 to the next dry season (drought), numbers declined consistently at all

six sites; declines ranged between 37–81%. Compared to the previous dry season, birds decreased by 5–71%. The number of species generally increased from the dry season of 1991 to the next wet season (6–43%, Figure 2b). Only at one site (southern Kalahari savanna at Sekoma) did the number of species decrease (14%), paralleled by the largest decrease in the number of birds from dry to wet season (28%). After the rains failed in the wet season of 1992, species numbers dropped consistently (8–52%) when drought was apparent in the next dry season. Compared to the previous dry season, species decreased by 2–47%. Diversity The following figures each present rarefaction curves for the three seasonal counts at each of the sites, allowing comparison of changes in bird diversity from a rich dry season to a poor wet season and the subsequent drought. At the Nata Delta, bird diversity increased (not significantly) during the wet season, but declined significantly during the drought (Figure 3a). In the southern Kalahari grasslands (at Sekoma (g)), diversity increased significantly during the wet season, but despite the spectacular reduction in bird numbers and species during the drought, relative diversity remained similar (Figure 3b). At Kgoro, bird diversity increased significantly during the wet season, but remained stable during the drought (Figure 3c). In the southern Kalahari woodlandsavanna (at Sekoma (s)), diversity had already decreased during the wet season of 1992, but did not change much further during the drought (Figure 3d). In the central Kalahari woodland savanna (at Kutse) there were no changes in bird diversity during the study when the superabundance of Redheaded Finches is excluded; evidently, when the finches are included, relative diversity is somewhat depressed (Figure 3e). In the northern Kalahari woodland savanna (at Phuduhudu), diversity significantly increased during the wet season and decreased significantly during the drought (Figure 3f). Overall, there is a tendency for increased diversity during the wet season, but relatively little change during drought, except in the periphery of the Kalahari (Nata, Phuduhudu, Kgoro), where birds have only a relatively small distance to cross to reliably find richer vegetation. With these data and procedures at hand, it is also possible to compare the diversity between all the sites in the dry season, the wet season and the drought respectively, each time at a common sample size, and plot this relative diversity against the vegetation diversity (Figure 4a–c). While there is a close correlation between the bird diversity and the structural diversity of the vegetation when the veld was in good condition during the dry season of 1991, following a good year of rains (Figure 4a), this relation starts to fade during the summer of 1992 when the rains failed and during the subsequent drought (Figures 4b–c). Similarities The richer savanna woodlands of the central and northern Kalahari (Kutse and Phuduhudu) have the most distinct bird populations (Figure 5). At the other end, the grasslands (Nata and Sekoma (g)) have the most similar bird populations (i.e. have the lowest linking distances). At most sites, bird populations are more similar to each other and more

Ostrich 2004, 75: 217–227

221

(a)

20

(b) 30

18

(c)

50

16

40

14 20

12

30

6

Nata 91D Nata 92W Nata 92D

4 2

25 45 65 85 105 125 145 165 185 205

(d)

50 40

10

Sekoma (g) 91D Sekoma (g) 92W Sekoma (g) 92D 30

55

80 105 130 155 180 205

(e)

50 40

30

30

20

20

NUMBER OF SPECIES EXPECTED

8



10 20 Kgoro 91D Kgoro 92W Kgoro 92D

10

110

210

310

410

510

(f) 60 50 40 30

Sekoma (s) 91D Sekoma (s) 92W Sekoma (s) 92D

10

10

25

80 155 230 305 380 455 530 605

20

Kutse 91D Kutse 92W (+RHF) Kutse 92W (-RHF) Kutse 92D 175

325

475

625

775

Phuduhudu 91D Phuduhudu 92W Phuduhudu 92D

10

925

25

150 275 400 525 650 775 900

NUMBER OF BIRDS IN SAMPLE

30 r = 0.98

20 10 0.5

1

1.5

(b) 40 30 20 10

EXPECTED NUMBER OF BIRD SPECIES E(40)

(a) 40



Figure 3: Changes in bird diversity according to season and drought. Results from the rarefaction diversity measure: each bold curve represents the expected species number at increasing sample sizes in a particular season. Thin lines are 99% confidence limits (only drawn for the season taking up the centre position): when the confidence limits of one season overlap with the bold curve of another season at a particular sample size, the differences are not significant (at 1%). a) Nata (delta grasslands); b) Sekoma (southern Kalahari grassland); c) Kgoro (Acacia tortilis bushveld); d) Sekoma (southern Kalahari woodland-savanna); e) Kutse (central Kalahari woodland-savanna); f) Phuduhudu (northern Kalahari savanna-woodland)

(c)

30

r = 0.77

20 r = 0.73

10

0.5 1 1.5 2 0.5 2 STRUCTURAL DIVERSITY OF VEGETATION (H´)

1

1.5

2

Figure 4: Relationship between the structural diversity of the vegetation (H') and the expected bird species richness (at the maximal common sample size E(x) from rarefaction) at the six sites, during (a) the dry season of 1991 (x = 150), (b) the failing wet season of 1992 (x = 220) and (c) the subsequent drought in 1992 (x = 40)

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Kutse 91D Kutse 92W Kutse 92D Nata 91D Nata 92D Nata 92W Sekoma (g) 91D Sekoma (g) 92D Sekoma (g) 92W Kgoro 91D Kgoro 91W Kgoro 92D Sekoma (s) 92D Phuduhudu 92D Sekoma (s) 91D Sekoma (s) 92W Phuduhudu 91D Phuduhudu 92W 0

20

40

60

80

100

120

(Dlink/Dmax)*100 Figure 5: Similarities in bird populations between sites and seasons; tree diagram with linking based upon Manhattan distances

distinct from the other sites regardless of season, except for Phuduhudu. This indicates that despite the general uniformity of bird communities in the Kalahari, there is still quite a substantial local specialisation. During the wet season, the bird populations at Phuduhudu in the richer woodlands in the northern Kalahari are the most distinct of all, but the bird populations remaining during the drought are very similar to those of the rest of the Kalahari (Phuduhudu 92D shifts into the Kalahari cluster). Species-specific patterns Overall, only 81 species were encountered sufficiently frequently and commonly to judge changes between seasons (flocking species recorded in relatively low numbers were also excluded here). Of these, 48 (=58%) showed changes that are considered sufficiently large to reflect a pattern rather than occasional fluctuations. The majority of species showed a consistent decline during the 1992 dry season (drought) (Table 2). In another 11 species, numbers also declined considerably during the drought, but lower numbers occurred during both dry seasons, and changes therefore rather indicate a pattern of wet season visitors to the Kalahari (Table 3). Three species increased during the drought (Table 4), of which the Fiscal Flycatcher and Common Fiscal are known dry season visitors to the Kalahari basin in varying numbers (see references in Table 4). A number of species showed inconsistent changes, the most striking of which are presented in Table 5. Fawncoloured Larks increased during the dry seasons at two sites, most notably at the eastern edge of the Kalahari in

Kgoro, where during the drought of 1992 two birds were observed belonging to the pale-plumaged race sarwensis, which breed further west on Kalahari sands. The numbers decreased during the drought at three other sites, including at both ends, the drier southern Kalahari (Sekoma) and the moister northern Kalahari (Phuduhudu). African Red-eyed Bulbul Pycnonotus nigricans data illustrate the common knowledge that it is a wet season visitor to the drier parts of the Kalahari and that there are dry season influxes into the northern and eastern periphery of the Kalahari, also beyond the breeding range. Chestnut-vented Tit-Babblers Parisoma subcaeruleum show a pattern of a dry season influx at the eastern edge of the Kalahari (Kgoro), a decline in the Kalahari during the drought (both ends — Sekoma and Phuduhudu) and an increase at Kutse (where rains had been particularly good in 1991). Marico Flycatchers Bradornis mariquensis showed important changes that, however, may still mainly be chance-related in a flocking species, except for Phuduhudu where there is a more prominent decline during the drought. Scaly-feathered Finches and Violet-eared Waxbills Granatina granatina declined throughout most of the Kalahari during the drought, but increased at the northern periphery. Discussion Methods Previous authors (Maclean 1970b, Liversidge 1984) collected huge datasets over many months in the Kgalagadi Transfrontier Park, but were somewhat concerned that they did not use a strict standard procedure for the counts. The

Ostrich 2004, 75: 217–227

223

Table 2: Species of which fewer were recorded either from when the rains failed (1992 wet) or during the drought (1992 dry)

Crested Francolin Red-crested Korhaan Northern Black Korhaan Cape Turtle-dove Emerald-spotted Wood-dove Swallow-tailed Bee-eater Acacia Pied Barbet Rufous-naped Lark Sabota Lark Ashy Tit Ant-eating Chat Kalahari Scrub-robin Yellow-bellied Eremomela Desert Cisticola Tinkling Cisticola Rattling Cisticola Black-chested Prinia Crimson-breasted Shrike Brown-crowned Tchagra Wattled Starling Cape Glossy Starling Marico Sunbird White-bellied Sunbird Red-billed Buffalo-weaver Southern Masked-weaver Lesser Masked-weaver Red-billed Quelea African Quailfinch Golden-breasted Bunting

Dendroperdix sephaena Lophotis ruficrista Afrotis afraoides Streptopelia capicola Turtur chalcospilos Merops hirundineus Tricholaema leucomelas Mirafra africana Calendulauda sabota Parus cinerascens Myrmecocichla formicivora Cercothrichas paena Eremomela icteropygialis Cisticola aridulus Cisticola rufilatus Cisticola chiniana Prinia flavicans Laniarius atrococcineus Tchagra australis Creatophora cinerea Lamprotornis nitens Cinnyris mariquensis Cinnyris talatala Bubalornis niger Ploceus velatus Ploceus intermedius Quelea quelea Ortygospiza atricollis Emberiza flaviventris

91 dry 10 61 54 130 8 13 25 29 55 22 22 221 53 90 41 34 354 23 68 26 45 45 19 9 82 12 272 61 34

92 wet 7 29 51 122 8 13 23 20 22 14 31 157 25 68 41 15 298 18 34 45 50 65 40 4 81 0 18 25 20

92 dry 0 8 18 13 0 0 9 4 18 9 9 147 19 0 2 3 115 6 4 0 12 2 1 0 0 0 0 0 0

References A S E, G, R ,S E, S E, G, L, R, S P, S E S S B, M G, S, T E, I, K, R, S G, R D, S E, G, L, N, P, Q, S O, S C, H, L, R, S C, H, R, S A, G, S A, E, G, J, L, S, U A, E, F, G, L, S E, F, G, R, S E, S

References are published sources that indicate movements or drought-related changes in numbers for the species (A = Tree 1972, B = Hawker 2003, C = Herremans 1992, D = Ashby 1992, E = Brewster 1992, F = Wilson 1984, G = Brewster 1991, H = Borello 1992, I = Brewster 1993, J = Dawson 1975, K = Brewster 1996, L = Pausch 1998, M = Muller 2002, N = Liversidge 1980, O = Liversidge 1984, P = Maclean 1970a, Q = Maclean 1970b, R = Brooke 1994, S = Harrison et al. 1997, T = Maclean 1993, U = Herremans 1994b)

Table 3: Wet season visitors

Kittlitz’ Plover Temminck’s Courser Laughing Dove Namaqua Dove Red-capped Lark Pink-billed Lark Cape Penduline Tit Capped Wheatear Barred Wren-warbler Plain-backed Pipit (*) Red-headed Finch

Charadrius pecuarius Cursorius temminckii Streptopelia senegalensis Oena capensis Calandrella cinerea Spizocorys conirostris Anthoscopus minutus Oenanthe pileata Calamonastes fasciolatus Anthus leucophrys Amadina erythrocephala

91 dry 0 0 3 3 0 0 0 3 4 0 27

92 wet 26 12 79 74 37 16 25 18 14 51 628

92 dry 0 2 1 0 0 0 1 2 1 0 0

References B, F, M C, F, H, M A, C, F, L, M A, C, F, I, J, K, L, M B, D, E, H, L, M, N J, K, L, M, N G, M B, C, D, E, F, J, K, M M C, E, F, L, M A, B, G, F, H, J, K, L, M

References are published sources that indicate movements or drought-related changes in numbers for the species (A = Brewster 1992, B = Wilson 1984, C = Brewster 1991, D = Brewster 1993, E = Brewster 1996, F = Pausch 1998, G = Muller 2002, H = Tree 1972, I = Maclean 1987, J = Maclean 1970a, K = Maclean 1970b, L = Brooke 1994, M = Harrison et al. 1997, N = Maclean 1970c) (*) The counts were conducted before Richard Liversidge described two new migratory pipit species from Kimberley: Long-tailed Pipit Anthus longicaudatus (Liversidge 1996) and Kimberley Pipit Anthus pseudosimilis (Liversidge 2002). It is unknown if (or how) the present data may relate to these new species subsequently described

224

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Table 4: Drought (1992 dry) visitors to the Kalahari basin

Red-faced Mousebird Fiscal Flycatcher Common Fiscal

91 dry 1 0 4

Urocolius indicus Sigelus silens Lanius collaris

92 wet 2 0 2

92 dry 54 8 13

References A, G, I B, C, D, E, F, I B, D, H, J

References are published sources that indicate movements or drought-related changes in numbers for the species (A = Herremans and Herremans-Tonnoeyr 1994, B = Brewster 1992, C = Wilson 1984, D = Anonymous 1989, E = Brewster 1993, F = Pausch 1998, G = Tyler 2001, H = Brooke 1994, I = Harrison et al. 1997, J = Herremans 1994a)

Table 5: Species with inconsistent, complex changes 91 dry Crowned Lapwing Kutse Nata Kgoro Fawn-coloured Lark Kutse Kgoro Sekoma (s) Sekoma (g) Phuduhudu Grey-backed Sparrowlark Kutse Nata African Red-eyed Bulbul Sekoma (s) Phuduhudu Chestnut-vented Tit-babbler Kutse Kgoro Sekoma (s) Phuduhudu Marico Flycatcher Kutse Kgoro Phuduhudu African Pipit Kutse Nata Kgoro Scaly-feathered Finch Kutse Kgoro Sekoma (s) Sekoma (g) Phuduhudu Violet-eared Waxbill Kutse Sekoma (s) Phuduhudu

92 wet

92 dry

0 8 0

8 12 10

22 4 0

51 10 32 16 44

27 2 10 15 47

56 12 17 3 15

0 74

93 2

5 100

20 26

38 19

1 38

4 19 33 38

1 8 18 31

40 19 7 19

12 24 35

6 40 41

13 17 11

0 28 4

18 9 2

0 12 20

220 63 70 11 12

430 73 47 15 20

331 31 29 2 136

59 27 27

70 14 36

16 3 44

Vanellus coronatus

Calendulauda africanoides

References E, H, J, L, M

A, D, L, N

Eremopterix verticalis

A, C, D, E, F, H, K, L, M, O

Pycnonotus nigricans

A, C, D, G, I, K, M

Parisoma subcaeruleum

B, G

Bradornis mariquensis

L

Anthus cinnamomeus

D, E, H, L, M

Sporopipes squamifrons

H, K, L, M

Granatina granatina

H, M

References are published sources that indicate movements or drought-related changes in numbers for the species (A = Brewster 1992, B = Wilson 1984, C = Brewster 1991, D = Brewster 1993, E = Brewster 1996, F = Pausch 1998, G = Muller 2002, H = Tree 1972, I = Maclean 1987, J = Liversidge 1984, K = Maclean 1970b, L = Brooke 1994, M = Harrison et al. 1997, N = Bishop et al. 1992, O = Maclean 1970c)

present programme used a strict procedure, but used low effort counts covering a single day at each site during three consecutive seasons; the question remains whether anything new can be detected in this way. The remark by Liversidge

(1980), that the Kalahari system is so variable that variations should be studied during many years and no conclusions be drawn from a one-year study, is most relevant here. His own data and those of Maclean (1970a, 1970b, 1970c) support

Ostrich 2004, 75: 217–227

this, but unfortunately nowadays long-term studies are limited by finance or other factors. My approach had the advantage that several sites at great distances could be compared in a short period of time, but the low intensity sampling resulted in low numbers for many species (some 40% of all), precluding any further assessment for these at species level. Furthermore, counts in different seasons inevitably have to cope with changes in bird behaviour (territoriality, singing, flocking etc.) and vegetation-dependent differences in visibility, meaning that even counts with a strict procedure result at best in semi-quantitative data. To convert these to densities has some further risks but, as an indication, it is estimated that in this study approximately 35ha were more or less effectively surveyed per transect count. The chance of detecting a bird from a stationary point decreases with distance from the observer, and current wisdom and practice for standardisation of transect counts is to calculate densities based on a species-specific detection probability function (see, for example, programme ‘distance’ at http://www.ruwpa.st-and.ac.uk/distance/). No matter how correct this approach may be in theory, in practice variances on density estimates may become so large that it is difficult to maintain statistically that any birds were indeed present at all (see, for example, also Dean and Milton 2001). Changes Liversidge (1966) recorded a drop in the bird breeding population of 31% in numbers and 28% in species between a drought and wet year in a moderately drought-sensitive area in the Eastern Cape. The Kalahari data show much larger changes, 37–81% in numbers, and are more similar to those obtained between a wet and dry cycle in the more arid southern Karoo: 70% of bird numbers and 63% of species (Dean and Milton 2001). Maclean (1970b) considered more than half the bird species in the Kalahari to be nomadic. In this study, some 60% of the species with sufficiently large samples showed considerable changes from dry to wet season or from wet season to drought. Such changes may, however, have several causes: (1) mere chance effects during sampling — the present programme with counts on a single day covering only c. 35ha may be particularly sensitive to such effects. Low density species, particularly when likely to flock at some stage during the year are especially at risk; (2) changes in conspicuousness and visibility; (3) a real decline or increase in numbers, which can be the result of mortality or breeding; (4) emigration or immigration. The first two points reflect sampling variation and error while the latter two are more interesting for population dynamics, but unfortunately with the present method mortality and emigration cannot be distinguished. Breeding was limited during the dry summer of 1992, so reproductive output will hardly have had a major impact on numbers. Chance effects have probably mostly been avoided by selecting only the 60% of species with larger samples. Changes in conspicuousness may to some extent affect the data for many species, but in the final selection (Tables 2–5) it appears that only for Crested Francolin and Barred Wren-warbler Calamonastes fasciolatus might this be the principal factor for the recorded changes. Both species are very unobtrusive when not call-

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ing, and the numbers in the samples are fairly low. Furthermore, for neither of the species did I find literature references indicating the possibility of movements. Similar consideration could be made for Tinkling Cisticola Cisticola rufilatu, Rufous-naped Lark Mirafra africana and Browncrowned Tchagra Tchagra australis, except that here good numbers were recorded during the 1991 dry season as well, making the decline probably real. Furthermore, the cisticola responds well to Pearl-spotted Owlet calls and is thus relatively easy to record; it also fits the pattern of congeneric species, which all lose the grassy component of their habitat during drought in the Kalahari. I consider it unlikely that these species suffered such a high mortality rate so soon into a drought period; the drought continued well into 1994, and if mortality would already have such an impact during the first months of a drought, there would not be many left during prolonged droughts. I suggest that these species should be watched more closely in future for (local) movements. The Ant-eating Chat Myrmecocichla formicivora is another species for which I did not find literature evidence for movements, but because it is conspicuous and responds well to Pearl-spotted Owlet calls, the counts are reliable, and movements are a likely explanation. The korhaans also form an intriguing case. Red-crested Korhaan Lophotis ruficrista can be very unobtrusive in dense shrubland when not calling, and I accept that I failed to record them during the drought when not calling. However, Tree (1972), during months of observation from a bush camp, also noted that this species became much less common in the northern Kalahari in a severe drought. It is likely that some move into nearby moister woodlands during dry cycles. Northern Black Korhaans Afrotis afraoides were mostly recorded at the grassland sites; when these become bare during drought, the species would still be fairly conspicuous, so the changes are thought to be real, and the birds most likely move to denser shrubland habitat during drought. Possibly, with time, the great majority of species in the Kalahari will at one or another time or place be found to undertake some (local) movements. For most of the other species, there is a good deal of literature (see Tables 1–4) confirming that they indeed undertake movements during droughts. Not surprisingly, the grasslands, which have the lowest vegetation structural diversity, and during drought have the largest proportional loss of structure in the vegetation, also lost the most birds during the drought. This was evident at the worst hit area (Sekoma) in the southern Kalahari. Bird populations at Kutse in the central Kalahari, on the other hand, where there was a carry-over of very abundant rains the previous season, remained much more stable with important influxes of species (e.g. Chestnut-vented Tit-babbler). The particular situation of the Red-faced Mousebirds concentrating during the drought here on Grewia berries has been dealt with separately (Herremans and HerremansTonnoeyr 1994). Probably many species that are forced to leave an area through drought explore the Kalahari over relatively short distances in search of a better patch. The nature of locally variable rainfall during dry cycles with localised storms is helpful in this respect. Even during the worst droughts there are somewhere greener places where rains have been bet-

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ter or are better captured in the landscape (drainage lines); birds just have to find them. Dean and Milton (2001) also point out the importance of drainage lines with richer vegetation for local movements at landscape level, particularly during droughts. Curiously enough, the largest changes and most profound impact on avian diversity come from the northern periphery of the Kalahari (Nata and Phuduhudu). These sites are closest to reliably more lush and moister vegetations and more birds seem to have adapted here to regular tactics of moving in and out. The northern Kalahari has the most distinct bird community during a wet cycle, caused by an influx of species from the moister woodlands further north, but at the start of droughts, these more mesic elements move out again, and the bird community becomes entirely Kalahari (Figure 5), also enhanced by influxes from typical Kalahari birds (e.g. Table 5: Scaly-feathered Finch, Violet-eared Waxbill). Consequently, the typical Kalahari bird community expands its range during drought into the moister periphery. Remarkably, even some of the more typical Kalahari birds appear to be more mobile in the northern and more lush part of the Kalahari, and undergo unexpectedly substantial decreases during drought at Phuduhudu (Laughing Dove Streptopelia senegalensis, Cape Turtledove Streptopelia capicola, Namaqua Dove Oena capensis, Acacia Pied Barbet Tricholaema leucomelas, Fawncoloured Lark, Chestnut-vented Tit-babbler, Kalahari Scrubrobin, Marico Flycatcher, Black-chested Prinia, Browncrowned Tchagra). Probably because better habitat can so reliably be found somewhere relatively close to the northern periphery of the Kalahari, it pays off for these populations to leave their territories and go and look for greener pastures at a lower threshold of drought. A similar effect could also happen at the eastern periphery of the Kalahari with movements into the eastern Hardveld of Botswana, but the sites in this study were not optimally chosen to demonstrate this (although the influx of species into Kgoro in the wet season is of note in this context). Acknowledgements — I am grateful to the referees Ara Monadjem and Stephanie Tyler for their improvements.

References Anonymous 1989. List of migrant bird species occurring in Botswana. Babbler 18: 51–52 Ashby N 1992. Some records from Nata. Babbler 24: 30–31 Bishop D, Borello WD and Herremans M 1992. Interesting and unusual sightings. Babbler 24: 59–61 Borello WD 1992. An incidence of mass sunbird migration in northern Botswana. Babbler 23: 18–21 Brewster CA 1991. Birds of the Gumare area, north-west Botswana. Babbler 21&22: 12–61 Brewster CA 1992. Notes on the effects of drought on birds in the Sefhare area from February–September 1992. Babbler 24: 31–32 Brewster CA 1993. Notes on the birds of the Tswapong South area. Babbler 25: 6–20 Brewster CA 1996. Fluctuations in bird numbers on fallow land in the Sefhare area of eastern Botswana. Babbler 31: 17–21 Brooke RK 1994. Sub-speciation studies and our knowledge of migration and other movements in southern African birds. Ostrich 65: 49–53 Cain AJ and Harrison GA 1958. An analysis of the taxonomists’

Herremans

judgement of affinity. Proceedings of the Zoological Society of London 131: 85–98 Dawson JL 1975. The birds of Kutse Game Reserve. Botswana Notes and Records 7: 141–150 Dean WRJ and Milton SJ 2001. The density and stability of birds in shrubland and drainage line woodland in the southern Karoo, South Africa. Ostrich 72: 185–192 Gower JC 1985. Measures of similarity, dissimilarity and distance. In: Johnson NL, Kotz S and Read CB (eds) Encyclopaedia of Statistics, Vol. 5. pp 397–405. Wiley, New York Harrison JA, Allan DG, Underhill LG, Herremans M, Tree AJ, Parker V and Brown CJ 1997. The Atlas of Southern African Birds, Vols 1 & 2. BirdLife South Africa, Johannesburg Hawker R 2003. More Kalahari species in Maun. Babbler 43: 47 Heck KL, Van Belle G and Simberloff D 1975. Explicit calculation of the rarefaction diversity measurement and the determination of sufficient sample size. Ecology 56: 1459–1461 Herremans M 1992. Indirect evidence for the existence of movements of sunbirds in Botswana. Babbler 24: 4–9 Herremans M 1993. Seasonal dynamics in sub-Kalahari bird communities with emphasis on migrants. In: Wilson RT (ed) Birds and the African Environment. Proceedings of the 8th Pan-African Ornithological Congress. pp 555–564. RMCA, Tervuren Herremans M 1994a. Fifteen years of migrant phenology records in Botswana: a summary and prospects. Babbler 28: 47–68 Herremans M 1994b. Partial migration in the Masked Weaver Ploceus velatus in south-eastern Botswana. Ostrich 65: 79–85 Herremans M and Herremans-Tonnoeyr D 1994. Evidence for the existence of movements in the Red-faced Mousebird Colius indicus in the Kalahari. Babbler 26–27: 7–10 Koen JH 1992. Medium-term fluctuations of birds and their potential food resources in the Knysna forest. Ostrich 63: 21–30 Liversidge R 1966. Fluctuations in a breeding population in the Eastern Cape. Ostrich supplement 6: 419–424 Liversidge R 1980. Seasonal changes in the use of avian habitat in southern Africa. Acta Congressus Internationalis Ornithologici 1980(2): 1019–1024 Liversidge R 1984. Changes in the avian population of the Kalahari National Park. In: Ledger J (ed) Proceedings of the 5th PanAfrican Ornithological Congress. pp 163–177. SAOS, Johannesburg Liversidge R 1996. A new species of pipit in southern Africa. Bulletin of the British Ornithologists’ Club 116: 211–215 Liversidge R 2002. The Kimberley pipit: a new African species. Bulletin of the British Ornithologists’ Club 122: 93–109 Lloyd P 1999. Rainfall as a breeding stimulus and clutch size determinant in South African arid-zone birds. Ibis 141: 637–643 Maclean GL 1970a. The breeding seasons of birds in the southwestern Kalahari. Ostrich Supplement 8: 179–192 Maclean GL 1970b. An analysis of the avifauna of the southern Kalahari Gemsbok National Park. Zoologica Africana 5: 249–273 Maclean GL 1970c. The biology of the larks (Alaudidae) of the Kalahari sandveld. Zoologica Africana 5: 7–39 Maclean GL 1987. Seasonal changes in the bird life of north-eastern Botswana. Bokmakierie 39: 109–111 Maclean GL 1993. Roberts’ Birds of Southern Africa. 6th edn. John Voelcker Bird Book Fund, Cape Town Magurran AE 1988. Ecological diversity and its measurement. Croom Helm, London Monadjem A 2001. Population fluctuations of birds in Acacia savanna in Swaziland. Durban Museum Novitates 26: 6–9 Monadjem A 2004. Population fluctuations of birds in riverine forest and broadleaved woodland in Swaziland. Durban Museum Novitates 29: 50–56 Muller M 2002. Influx of Kalahari specials and other bird news in Maun. Babbler 41: 39–40

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Pausch E 1998. Birds of Kutse Game Reserve, Botswana. Babbler 33: 5–16 Penry H 1994. Bird Atlas of Botswana. University of Natal Press, Pietermaritzburg Simberloff D 1972. Properties of the rarefaction diversity measurement. American Naturalist 106: 414–418 StatSoft, Inc. 2001. Statistica 6.0 (data analysis software system). StatSoft, Tulsa Symes CT, Wirminghaus JO and Downs CT 2002. Species richness and seasonality of forest avifauna in three South African Afromontane forests. Ostrich 73: 106–113

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Tree AJ 1972. Ornithological comparison between differing dry seasons at a pan in Botswana. Ostrich 43: 165–168 Tyler SJ 2001. Local movements by Red-faced Mousebirds Colius indicus at Ruretse. Babbler 38: 17–18 Wilson JR 1984. The avifauna of the Lobatse area, south-east Botswana. Babbler 8: 17–45

Received May 2004, accepted June 2004 Editor: MD Anderson