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Strategies, punctuality of arrival and ranges of migrants in the Kalahari basin, Botswana

M. HERREMANS Department of WiZdZqe and National Parks, Botswana

Avian Demography Unit, University of Cape Town, Rondebosch 7700, South Africa

The timing and intensity of the first summer rains in the Kalahari basin are highly variable between years. Avian migrants to the area follow two principal strategies. Species arriving before November typically do so before significant rain can be expected and do not use ecological conditions as a proximal cue to time arrival: their arrival was unaffected by the failure of the early rains in 1994. Species arriving after mid-November respond to the timing and extent of the early rains and were progressively more delayed in parallel to a cumulative deficit in rainfall during the early part of the 1994 wet season. The second strategy results in erratic occupation of parts of the nonbreeding range, so that during drought years, rain-dependent, late-arriving species may remain absent from the Kalahari basin, complicating the assessment of the “effective” size of their nonbreeding range.

For the duration of the breeding cycle, migrants are con- fined to territories or home ranges near their nests. In con- trast, on the nonbreeding grounds, they are not necessarily bound to a particular area but have the freedom to wander opportunistically. Although migrants may establish territo- ries during the nonbreeding season (at least temporarily and sometimes consecutively in different parts of the nonbreed- ing range), many respond to variations in local habitat qual- ity as a result of the erratic rainfall in the African savannas (Sinclair 1978, Lack 1983). Rainfall is subject to local hy- pervariation but tends to follow a similar trend of deviation from the norm at a greater geographic scale. As a conse- quence, migrants may respond by selective occupation of only part of the potential nonbreeding range in any partic- ular nonbreeding season (or part thereof), avoiding regional adverse conditions (typically drought).

The nonbreeding range in Africa of most Palaearctic-Af- rican migrants is thought to be smaller than their breeding range in the Palaearctic (Newton 1995), but the ranges in Africa of many migrants are poorly known. Also, Newton (1995) noted that movements of migrants within Africa, whereby only part of the overall nonbreeding range is oc- cupied at any one time, could lead to an overestimation of the nonbreeding range. Species that move through Africa in a number of discrete steps, and others that gradually shift farther south during several months, were mentioned spe- cifically as candidates for an overestimation of the range. Erratic occupation of parts of the range in response to local conditions was not mentioned as a factor leading to range overestimation.

The present study analyses the phenology of arrival of Palaearctic-African and intra-African migrants in the Kal- ahari basin in Botswana during the dry austral spring of

1994 as an approach to clarify migratory strategies and variations in the occupation of this highly variable and un- predictable environment. For many migrants to southern Africa, the Kalahari basin is at the edge of their distribution, and whether or not to include this c. 1 million km2 area of arid savanna in the nonbreeding range could make a differ- ence of 5 2 0 0 % in the size of this range (based on Newton 1995; the extremes are constituted by the House Martin Delichon urbica and Lesser Grey Shrike Lanius minor).

STUDY AREA

Botswana is a semiarid, subtropical country in southern Af- rica and is about 575,000 km2 in size (Department of In- formation and Broadcasting 1990). Botswana constitutes the core of the Kalahari basin, which, however, extends be- yond the borders of Botswana into eastern Namibia, the northern Cape Province (South Africa) and western Zim- babwe. Average rainfall ranges from 200 mm in the arid southwest to 700 mm in the far north (Bhalotra 1987). Rainfall is concentrated in the austral summer, mainly No- vember-March, though the timing and quantity of precip- itation are highly variable in space and time: monthly co- efficients of variation are 60-115% during the middle of the rainy season (Bhalotra 1987). The average altitude in the Kalahari basin is about 1000 m a.s.l., and at this alti- tude, the sharp rise in night temperatures from late August onwards is a more reliable indication of spring than the arrival of the rains (Herremans 1994a).

September-December 1994, the early part of the wet sea- son, were very dry months in Botswana; e.g., on average, 38% of the normal total precipitation near Gaborone occurs

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Figure 1. Average monthly rainfall in spring near Gaborone. Botswa- na. at the edge ol the Kalahari basin, and rainfall statistics for spring 1994 (up to 25 December).

during July-December, while only 5.8% of the norm was recorded up to 25 December 1994, resulting in a rainfall deficit of 161 mm by late December (Fig, 1). The first wide- spread rain over the entire Kalahari basin occurred on 26 December, when Gaborone, for example, received 100 mm.

METHODS

First and last records of migrants have been collected by the Botswana Bird Club since 1978 (Anonymous 1981). From a 15-year review of the data (Herremans 1994a), the over- all median dates of first records were used as a baseline for "normal" arrival dates with which to compare punctuality of migrant arrival during 1994. Information on median ar- rival during 1994 was available for 2 7 Palaearctic-African and 19 intra-African migrant species (see Appendix). The overall median date in the sample (8 November) was used as a cut-off to distinguish between species which normally arrive in the first or second part of the season.

Only land birds were included in the analyses: substantial populations of most waterbirds (in particular waders) re-

Median arrival data

Figure 2. Punctuality of migrant arrival in Botswana during the de- layed onset of the rains in spring 1994. Palaearctic migrants. r, = 0.63, P < 0.0001: intra-African, r, = 0.44, P < 0.05. Positive values are delays in 1994 and negative values are dates before the normal values.

main in southern Africa during the austral winter (Sum- mers et al. 1995, Harrison et al. 1997), confounding the reliable establishment of arrival dates on a limited number of records (Herremans 1994a). Great Spotted Cuckoos Cla- mator glandariws in Botswana are of both African and Pa- laearctic origin, but because the majority appear to be non- breeding visitors from the northern hemisphere, they are included under Palaearctic-African migrants (Herremans 1994a). Bird names follow Maclean (1993) and seasons are austral.

RESULTS

Among the Palaearctic-African migrants arriving before 8 November, similar numbers of species were relatively early and late during 1994. Species normally arriving after that date were almost all behind the normal schedule, resulting in a significant overall trend (Fig. 2 , Table 1). There was also a significant trend among the Palaearctic-African migrants

Table 1. Punctuality of migrant arrival in Botswana during the delayed onset of the rains in spring 1994

Median arrival before Median arrival after 8 November 7 November

Palaearctic-African migrant species 1994 arrival before median (=early) 4 1 7 994 arrival after median (=late) 5 1 7

Intra-African migrant species 1994 arrival before median (=early) 7 1 1994 arrival after median (=late) 7 4

Note: Fisher exact test (null hypothesis of no difference: Palaearctic-African migrants significantly late, P = 0.03; intra-African migrants, as . ) .

1 9 9 8 S T R A T E G I E S O F M I G R A N T S I N T H E K A L A H A R l B A S I N 5 8 7

for an increasing delay in the 1994 arrival for species which normally arrive later in the season (rs = 0.67, P < 0.001 for 18 species arriving after 7 November: Fig. 2).

A significant though weaker overall trend was also found among the intra-African migrants (Fig. 2, Table 1).

DISCUSSION

An overview of migrant phenology in Botswana (Herre- mans 1994a) demonstrated that Palaearctic-African mi- grants normally arriving in late spring showed greater year- to-year variation in arrival dates and numbers, while species arriving in early spring were more punctual. Herremans (1994a) also showed that the median arrival date derived from a limited number of “first” observations in Botswana closely reflected the median arrival date calculated on the large set of data in the regional atlas (Underhill et al. 1992). Thus, the 1994 data from Botswana are similarly assumed genuinely to reflect regional trends in migrant punctuality. Comparatively later arrival records of migrants in some parts of the southern African region generally coincided with low reporting rates and, hence, with low numbers present (Harrison et al. 1997). In drought years, some oth- erwise common Palaearctic-African migrants were not re- corded at all in Botswana (Herremans et al. 1992). Similar observations on local presence or absence of some migrants in response to unusual rainfall were reported for eastern Africa by Sinclair (1978).

The occurrence or absence of early rains (August-Octo- ber) does not constitute a proximal cue for the timing of arrival in migrants normally arriving in the Kalahari basin during July-October. Before November, the monthly rainfall is, on average, less than 30 mm throughout the Kalahari basin. The chance for the total precipitation to be over 25 mm during September is less than 20%; this chance increas- es to c. 50% for October and 90% for November. Thus, eco- logically significant rainfall reliably occurs only from No- vember onwards, and migrants arriving before early Novem- ber must expect the area to be in a dry season. Therefore, an exceptionally late start of the rains, such as in 1994, did not delay their arrival. The majority (14/23) of the early arriving species are intra-African breeding visitors, but those Palaearctic-African migrants that normally arrive early were not affected by the dry conditions in 1994 either.

Migrants normally reaching the Kalahari basin in Novem- ber-December time their arrival to coincide with the start of the rains and use the availability of wet conditions as a proximal cue for arrival. Arrival seems to coincide usually with, or follow shortly after, the southwards movement into the region of large rain-generating weather systems, such as the Inter-Tropical-Convergence-Zone: lightning associated with heavy thunderstorm activity can be visible at night at distances of up to a few hundred kilometres, which might constitute a reliable cue to direct rain-dependent migrants. The later species normally arrive, the more they seem to depend on the ecological changes resulting from the rains,

hence an increased delay in arrival when the rains failed in the early part of the rainy season 1994. The two latest ar- riving intra-African migrants in the sample include Abdim’s Stork Ciconia abdimii, a nonbreeding visitor which wanders opportunistically like some Palaearctic-African migrants (Anderson 1997), and the Monotonous Lark Mirafra passer- ina, a highly erratic rains migrant to the Kalahari basin (Dean 1997).

Rain-dependent species which arrived later than normal in the Kalahari basin in the 1994-1995 wet season must have spent some time concentrated in another part of their range before eventually moving into the Kalahari basin. When the rains fail in the Kalahari and the area becomes drought stricken, these species may avoid the Kalahari basin altogether in some years. Evidently, site fidelity to the non- breeding grounds in rain-dependent opportunistic wander- ers can be expected to be low in parts of the range where the timing and amount of rainfall are unpredictable. Con- versely, the Red-backed Shrike Lanius collurio, which is the most punctual migrant in the Kalahari basin (Herremans 1994a), demonstrates the highest known site fidelity of any Palaearctic-African migrant passerine in the region (Her- remans et al. 1995).

Drought in the Sahel has been implicated in lower sur- vival and population declines of Palaearctic-African mi- grants (Winstanley et al. 1974, Kanyamibwa et aI. 1990, Marchant et al. 1990, Peach et al. 1991, Baillie & Peach 1992), but no similar studies exist for the Kalahari. Species which are early migrants to the Kalahari basin and partic- ularly those with high site fidelity (both may be coincident) are subject to the greatest local drought risks. On the other hand, species which occupy the nonbreeding grounds op- portunistically may avoid drought-stricken regions but could suffer from the consequences of having to concentrate in a much reduced effective wintering area. Which of the two strategies constitutes the greatest risk for survival is unclear at present. Available data allow the interpretation that den- sity-dependent mortality at the nonbreeding grounds in the Sahel might occur in the White Stork Ciconia ciconia (Kan- yamibwa et al. 1990) and Sedge Warbler Acrocephalus schoe- nobaenus (Baillie & Peach 1992), two late-arriving species which in southern Africa occupy highly variable ranges.

In view of the erratic dynamics of nonbreeding ranges in the Kalahari, we can revisit some of the exceptional cases mentioned in Newton (1995). where the nonbreeding range was found to be larger than the breeding range. White Stork, Great Reed Warbler Acrocephalus arundinaceus, Sedge Warbler and Olive-tree Warbler Hippolais olivetorum are all late-arriving species in southern Africa, occupying the re- gion opportunistically in response to the extent of the early rains. Consequently, their effective nonbreeding ranges have no doubt been overestimated by Newton (1995). The same applies to the Icterine Warbler Hippolais icterina, Marsh War- bler Acrocephalus palustris and Whitethroat Sylvia communis, in which the effective nonbreeding range in southern Africa is smaller in dry than in wet years, resulting in a higher breedinghonbreeding range ratio than that following from

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Moreau (1 9 72), except perhaps that the range given by Mo- reau widely underestimated the wet-cycle expansion of the ranges of the last two species (compare with distribution maps in Harrison et al. 1997).

There are also other cases where Newton’s (1995) un- amended use of Moreau’s (19 72) nonbreeding distributions was inappropriate in the light of more recent knowledge. Breeding and nonbreeding “ranges” are in fact fundamen- tally different concepts. The latter (sensu Moreau 1972) reg- ularly partly confounded the final migratory destination and transitional areas used on passage or as stopover sites. To make a valid comparison of range sizes (and certainly eco- logical implications such as carrying capacity), only the area in which birds take up temporary residence (e.g. during moulting, either at stopover sites or at the final nonbreeding destination) should be considered for comparison with the breeding range. A most dramatic case in point is the River Warbler Locustella puviatilis, for which Newton (1995) cal- culated a nonbreeding range of 2.43 million km2, while the species has to date been documented on the final nonbreed- ing grounds during the moulting season (mid January-early March) at only a handful of localities (admittedly, these range from South Africa, Botswana and Zambia to Kenya: Tucker 1978, Herremans 1994b, Hockey et al. 1995); de- lineating a nonbreeding range for this species represents an overestimation of the state of knowledge. The Lesser Grey Shrike is calculated to have a breeding range 7.4 times larg- er than its nonbreeding range, the most dramatic range ra- tio of all (Newton 1995). However, only insignificant num- bers are found north of 20”s during the austral midsummer (Dowsett 1971, M. Herremans, unpubl. data), and the ef- fective range during moult is close to 1.1 million km’, mean- ing that the breeding range is about 10 times larger than the nonbreeding range. In the European Bee-eater Merops apiaster (range ratio 0.96), the assessment of the nonbreed- ing range suffered from overlooking the existence of a sep- arate breeding population in southern Africa which occurs during the austral summer to the south of the range of the Palaearctic-African migrants (Brooke & Herroelen 1988, Fry et al. 1992, Underhill 1997).

Although Newton’s finding that the nonbreeding range of Palaearctic-African migrants appears to be smaller than their breeding range is further corroborated here, the anal- ysis is unsatisfactory because the concept of nonbreeding range is poorly defined, and the range dynamics and “effec- tive ranges” in Africa are poorly documented. The Olive-tree Warbler is a case in point. The species has a restricted breed- ing range and is rare, with a population numbering about 25,000 pairs (Cramp & Brooks 1992, Tucker & Heath 1994). Although, over the years, it may well have been re- corded during the austral summer in Africa over an area of 1.65 million km2 (Newton 1995), how so few birds can ecologically “occupy” such a range is more difficult to ap- preciate. Probably a more meaningful approach is to aban- don the crude comparison of “ranges” altogether and rather focus on habitat occupation, territory size and densities at both ends (and intermittent stages) of the migratory route

(see Bruderer & Bruderer 1994 for data on the Red-backed Shrike).

I was supported in Botswana by VVOB (Vlaamse Vereniging voor Ontwikkelingssamenwerking en technische Bijstand). I am pleased to acknowledge the Botswana Bird Club and the principal recent contributors to the migrant scheme: C. A. Brewster, D. R. and D. Bishop, W. D. and R. M. Borello. G. McGowan. I? Springer and E. Pryce. W. D. Borello and G. van Regenmortel provided rainfall data.

REFERENCES

Anderson, M.D. 1997. Abdim’s Stork. In Harrison, J.A., Allan. D.G., Underhill. L.G.. Herremans, M.. Tree, A.J., Parker, V. & Brown, C.J. (eds) The Atlas of Southern African Birds, Vol. 1: 84. Johannesburg: BirdLife South Africa.

Anonymous. 1981. Records: Migrant dates. Babbler 1: 15-16. Baillie, S.R. & Peach, W.L. 1992. Population limitations in Pa-

laearctic-African migrant passerines. Ibis l .34(Suppl. l): 120- 132.

Bhalotra. Y.P.R. 1987. Climate of Botswana, Part 11, Elements of climate. 2. Temperature. Gaborone: Department of Meteorologi- cal Services.

Brooke. R.K. & Herroelen. I? 1988. The nonbreeding range of southern African bred European Bee-eaters Merops apiaster. Os- trich 59: 63-66.

Bruderer. B. & Bruderer. H. 1994. Numbers of Redbacked Shrikes Lanius collurio in southern Africa. Bull. Br. Ornithol. Club 114: 192-201.

Cramp, S. & Brooks, D.J, 1992. The Birds of the Western Palcarctic. Vol. VI, Warblers. Oxford: Oxford University Press.

Dean, W.R.D. 1997. Monotonous Lark. In Harrison, J.A., Allan. D.G., Underhill, L.G., Herremans, M.. Tree. A.J.. Parker, V. & Brown. C.J. (eds) The Atlas of Southern African Birds, Vol 2: 6. Johannesburg: BirdLife South Africa.

Department of Information and Broadcasting. 1990. Facls on Bot- swana. Gaborone: Government Printer.

Dowsett, R. J. 1971. The Lesser Grey Shrike Lanius minor in Africa. Ostrich 42: 259-270.

Fry, C.H.. Fry, K. & Harris, A. 1992. Kingfishers, Bee-eaters and Rollers. Halfway House, Johannesburg, South Africa: Russel Friedman.

Harrison, J.A.. Allan, D.G.. Underhill, L.G., Herremans, M., Tree. A.J., Parker, V & Brown, C.J. 1997. The Atlas of Southern Af- rican Birds, Vols 1-2. Johannesburg: BirdLife South Africa.

Herremans, M. 1994a. Fifteen years of migrant phenology records in Botswana. A summary and prospects. Babbler 28: 47-68,

Herremans, M. 1994b. Major concentration of River Warblers Lo- custella fluviatilis wintering in northern Botswana. Bull. Br. Or- nithol. Club 114: 24-26.

Herremans. M., Brewster, C. & Herremans, I). 1992. Migrant phe- nology in Botswana: 1990-1992. Babbler 24: 37-46.

Herremans, M.. Herremans-Tonnoeyr, 1). & Borello, W.D. 199 5. Non-breeding site-fidelity of Redbacked Shrikes Lanius collurio in Botswana. Ostrich 66: 145-147.

Hockey, P.A.R. and the Rarities Committee. 1995. Rare birds in South Africa. 1991-1992. Birding in Southern Africa 47: 14- 19.

Kanyamibwa, S., Schierer, A., Pradel. R. & Lebreton, J.D. 1990. Changes in adult annual survival rates in a western European population of the White Stork Ciconia ciconia. Ibis 132: 27-35.

1 9 9 8 S T R A T E G I E S O F M I G R A N T S [ N T H E K A L A H A R I B A S I N 5 8 9

Lack. P.C. 1983. The movements of Palaearctic landbird migrants in Tsavo East National Park, Kenya. J. Anim. Ecol. 52: 513-524.

Maclean, G.L. 1993. Roberts' Birds of Southern Africa. Cape Town: john Voelcker Bird Book Fund.

Marchant, J.H.. Hudson, R., Carter, S.P. & Whittington. P.A. 1990. Population Trends in British Breeding Birds. Tring: British Trust for OrnithologyiNature Conservancy Council.

Morcau. R.E. 19 72. The Palaearctic-African Bird Migration Sys- tems. London: Academic Press.

Newton. I . 199 5. Relationship between breeding and wintering ranges in Palaearctic-African migrants. Ibis 1 3 7: 241-249.

Peach, W.J., Raillie, S.R. & Underhill, L.G. 1991. Survival of British Sedge Warblers Acrocephalus schoenobaenus in relation to west Af- rican rainfall. Ibis 133: 300-305.

Sinclair, A.K.E. 1978. Factors affecting the food supply and breed- ing season of resident birds and movements of Palaearctic mi- grants in a tropical African savannah. Ibis 120: 480-497.

Summers, R.W., Underhill, L.G. & Prys-Jones, R.P. 1995. Why do

young waders in southern Africa delay their first return migra- tion to the breeding grounds? Ardea 8 3: 3 51-3 5 7.

Tucker, G.M. & Heath, M.F. 1994. Birds in Europe: Their conser- vation status. BirdLife Conservation Series No. 3. Cambridge: BirdLife International.

Tucker, 1.J. 1978. A River Warbler Locustella juvintilis 'wintering' and moulting in Zambia. Bull. Br. Ornithol. Club 98: 2-4.

Underhill. L.G. 199 7. European Bee-eater. In Harrison, J.A.. Allan. D.G.. Underhill, L.G., Herremans, M.. Tree, A.J., Parker, V. & Brown, C.]. (eds) The Atlas of Southern African Birds, Vol. 1: 660. Johannesburg: BirdLife South Africa.

Underhill. L.G., Prys-Jones, R.l?, Harrison, J.A. & Martinez, P. 1992. Seasonal patterns of occurrence of Palaearctic migrants in south- ern Africa using atlas data. Ibis 134(Suppl. 1): 99-108.

Winstanley, D.R., Spencer, R.E. & Williamson, K. 1974. Where have all the Whitethroats gone? Bird Study 21: 1-14.

Submittted 20 February 1997: revision accepted 5 June 1997

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APPENDIX

Migrant arrival data in Botswana

Difference Normal arrival date 1994-1995 arrival (days)

Palaearctic-African migrants White Stork Ciconia ciconia Steppe Eagle Aquila nipalensis Steppe Buzzard Buteo buteo vulpinus Montagu’s Harrier Circus pygargus Hobby Falcon Falco subbuteo Lesser Kestrel Falro naurnanni Great Spotted Cuckoo Clomator glandarius European Swift Apus apus European Bee-eater Merops apiaster Blue-cheeked Bee-eater Merops persicus European Roller Coracias garrulus European Swallow Hirundo rustica House Martin Delichon urbica Sand Martin Riparia riparia European Golden Oriole Oriolus oriolus Garden Warbler Sylvia borin Whitethroat Sylvia cornmutiis Icterine Warbler Hippolais icterina Olive-tree Warbler Hippolais olivetorurn Great Reed Warbler Acrocephalus arundinaceus Marsh Warbler Acrocephalus palustris Sedge Warbler Acrocephalus schoenobaenus Willow Warbler Phylloscopus trochilus Spotted Flycatcher Muscicapa striata Yellow Wagtail Motacilla Java Lesser Grey Shrike Lanius minor Red-backed Shrike Lanius collurio

Intra-African migrants Abdim’s Stork Ciconia abdirnii Ycllow-billed Kite Milvus migrans parasitus Wahlberg’s Eagle Aquila wahlbergi African Cuckoo Cuculus gularis Red-chested Cuckoo Cueulus solitarius Hack Cuckoo Cuculus rlamosus Striped Cuckoo Clamator levaillantii jacobin Cuckoo Clamator jacobinus Klaas’s Cuckoo Chrysococcyx klaas Diederik Cuckoo Chrgsococcyx caprius White-rumped Swift Apus cuffer Woodland Kingfisher Halcyon senegalensis Monotonous Lark Mirafra passerina Red-breasted Swallow Hirundo sernirufa Greater Striped Swallow Hirundo cucullata Lesser Striped Swallow Hirundo abyssinicu unitatis Black Cuckoo-shrike Campephaga flaw Paradise Flycatcher Terpsiphone viridis Plum-coloured Starling Cinnyricinclus leucogaster

25 November 26 October 26 October

2 December 4 December 9 November 2 December

30 October 10 October 10 November 9 December

12 October 2 November 1 November

1 3 November 20 November 28 November 28 November 17 December 1 December

18 December 28 November 1 7 October 6 November 8 November

10 November 13 November

10 December 3 September

26 September 25 October 4 November

30 October 1 3 November 1 4 November 28 October 20 October 1 5 September 23 November 4 December

28 August 24 September 20 August 31 October 31 October 26 October

16 February 2 December

19 October 25 December 1 7 December 2 1 November 1 3 January 1 6 November 16 October 12 November 8 Ianuary 9 October

1 5 October 12 October 30 November I 8 November 20 December 4 January

14 January 2 4 December 1 4 January 12 December 19 October 23 November 1 7 November 1 3 November 26 November

30 December 4 September 2 October 2 November

31 October 1 9 November 23 October 16 November 1 January

30 October 26 August 1 4 December 18 January 1 August

18 September 14 August 6 November

23 October 22 October

+82 +36

-7 +23 +13 1-12 +42 + I 7

+6 +2

+31 -3

-18 -20 + 17

-2 +22 + 37 +28 + L 3 +27 + I 4

+2 + 1 7

+9 + 3

+13

+ 20 + 1 + 6 +8 -4

+ 2 0 -21

+2 +65 +10 -20 +21 +45 -27

-6 - 6 +6 -8 -4