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An endangered population of Wattled Cranes (Grus carunculatus)
An Endangered Population of Wattled Cranes (Grus carunculatus) R. J. DOUTHWAITE Research Fellow, Kafue Basin Research Comm#tee, University o f Zambia, PO Box 2379, Lusaka, Zambia
ABSTRACT
averages 80 cm and falls between November and March. There is considerable variation in scale and The Kafue Flats, a river floodplain in southern Zambia, supports the largest known population o f Wattled Cranes. In timing, but normally the floods begin to rise in a normalyearfewer than 1,000 arepresent at highflood; but November and the flood peak passes slowly eastwards as the water subsides the population increases, and in the --reaching Itezhitezhi in early March, Lochinvar latter half o f the dry season it numbers some 3,000 birds. National Park (Fig. 2) in April, and Kafue Gorge in Following widespread flooding in 1972, at least 300 pairs late May. To the west of the survey area, the floodnested as the water fell. However, in 1973 there was little plain is dry by the end of June; in the survey area, it is flooding and few pairs bred. Many full-grown birds moult their remiges between January and April, and are then usually dry before the onset of the rains. A mosaic of old channels and lagoons remains flooded. In 1971 the flightless. The diet is largely o f rhizomes dug from soft mud. Suitable feeding grounds in the dry season are created by a flood was comparatively high, and was late in falling. falling water-leveL The population is threatened by a hydro- The flood of 1972 was slightly below average, while electric scheme that is scheduled to be completed in 1978. the flood of 1973 was well below normal. The Kafue The scheme will reduce fluctuations in the water level on the floodplain and, in so doing, reduce the area o f suitable Gorge dam reduced the rate at which the water fell, but, except at the very eastern end of the floodplain, habitat. did not produce seasonally abnormal water levels. The floodplain is fringed with open grassland, scattered thickets and trees, and is characterized by large termite mounds. Where drainage improves, INTRODUCTION woodland replaces this 'termitaria zone'. A few seasonal streams, in broad depressions, cut through The Wattled Crane (Grus carunculatus Gmelin) the woodland and termitaria zone to disappear at the occurs on wet grasslands from eastern Cape Province, edge of the floodplain. Most of the floodplain is Natal, and Transvaal, in South Africa, to northern covered with perennial grass swamp which collapses Botswana, Rhodesia, Zambia, Malawi, Angola, north as the floods recede and is then burnt off to produce in Zaire to Kasai, and Tanzania. It also occurs in new growth for the large numbers of cattle which Ethiopia (White, 1965). Generally it is scarce graze on the Flats during the dry season. The loss of (Mackworth-Praed & Grant, 1962; McLachlan & organic matter through burning may be instrumental Liversidge, 1970), but in parts of Zambia, and in producing a rich fishery on the floodplain, as then particularly on the Kafue Flats, it is at times quite dissolved oxygen concentrations are more likely to common (Benson et al., 1971). However, a hydro- remain high during the succeeding flood (Chapman electric scheme on the Kafue River may largely et al., 1971). destroy the floodplain environment of the Flats. This The Flats are renowned for their wildlife (Chapin, paper describes the significance of the area to the 1956; Darling, 1960) and have two National Parks Crane. (Fig. 2). The floodplain is characterized by the Sheppe & Osborne (1971) described the floodplain endemic, semi-aquatic Kafue Lechwe (Kobus leche and so only a brief description need be given here. kafuensis), whose population is estimated to be about The Kafue Flats is a river floodplain in southern 94,000 (__+9 per cent) (Bell et al., in press). Over 400 Zambia (15 ° 15'-16 ° 15' S; 26 ° 00'-28 ° 10' E) at an species of birds have been recorded from the area of altitude of about 1,000 metres above sea-level (Fig. 1). the Flats. Some 235 km long and up to 40 km wide, the floodIn mid-1971 a dam and hydroelectric power station plain covers an area of about 6,000 km 2. The rainfall were completed in Kafue Gorge. The dam has a low 134 Biological Conservation, Vol. 6, No. 2, April 1974--- 0 Applied Science Publishers Ltd, England, 1974---Printed in Great Britain
Douthwaite : An Endangered Population o f Wattled Cranes
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135
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-~--~-R. LUKANGA LUSAKA •
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Zambia, showing places outside the Kafue Flats survey area mentioned in the text.
storage-capacity and forms a small semi-permanent lake at the eastern end of the Flats. However, because the capacity of the dam is insufficient to meet the water demands of the power station throughout the dry season, a storage reservoir is being built at Itezhitezhi, at the upper end of the floodplain. It is scheduled to be completed in 1978. Water will be stored in this darn during the rains and released during the dry season. Thus, relative to the natural regime, flooding on the Flats will be reduced during the rains, but increased in the dry season. There will be a reduction in the area of floodplain, and presumably some encroachment of termitaria grassland at higher levels and of permanent lagoons and swamp in the lowest depressions.
METHODS
A study of the feeding ecology, distribution, and population sizes, of the more numerous avian herbivores, including the Wattled Crane, was started in late
1970. Most observations were made in Lochinvar National Park, but several aerial surveys were flown over the central section of the floodplain (Figs. 1 and 2). Usually the flights were of short duration, using a Piper Super Cub, and made in order to locate the main wildfowl concentrations. However, six censuses of the Crane population were also made from a Cessna 182, using Jolly's (1969) census method as modified by Bell et al. (in press). The survey area was divided into areas, or 'strata'. The density of Cranes within each stratum was expected to be more or less homogeneous throughout, but to be very different when compared with that in adjoining strata. High-density strata were sampled more intensively than low-density ones. Sampling was done by flying north-south transects and counting the cranes within a strip of known width. The mean number per transect strip was then multiplied by the total number of possible transects to give the population within the stratum. Cranes are conspicuous birds and pairs are widely distributed over the floodplain except at high flood. They are thus ideal subjects for this method of
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II Fig. 2. The survey area on the Kafue Flats, indicated by the small rectangle in Fig. 1, with a reconstruction o f the seasonal location o f Wattled Crane flocks under natural and average flood levels, based on observations in 1971-73. The scale at the top shows the kilometre grid.
population estimation. However, large flocks are also encountered, which are localized and often wary, flying away at the approach of the aircraft. They could not be sampled accurately by this method. Instead, an estimate of the total flock size was made and the estimate corrected for bias. Bias was determined by photographing flocks and comparing the estimate with the actual number. Inevitably some flocks would be overlooked and the population estimates are therefore probably conservative. Each census was flown at a height of 60 m and a speed of 160 kph. A strip-width of 175 m was satisfactory when counting adult Cranes but was too wide to be certain of seeing a very small chick running beneath an adult near the outer limit of the strip. A width of l l 0 m was therefore used latterly when counting breeding birds. A single bird was scored as half of one pair; three full-grown birds together were taken to be one pair and one immature bird, and four or more birds together were called a flock, Details of the censuses are given in Table I.
The study of the Cranes' feeding ecology involved both direct observation of feeding birds and examination of their excreta. The main component of the diet was normally found by watching several feeding birds in the flock and confirmed by examining excreta collected in the area. Checks were then made at irregular intervals for as long as the flock remained in the same area.
RESULTS
Bias in Estimates o f Flock Size
Bias was determined by comparing the estimated number of birds in the flock (x) with the actual number as counted from a photograph (y). The bias factor was x/y. Three flocks were estimated to contain fewer than 100 birds: the mean bias factor was 1-0. Five other flocks were estimated to range from 100 to 300 birds: the mean bias factor was 0.68. These two factors were used in correcting estimates of flock size,
Douthwaite : An Endangered Population of Wattled Cranes
TABLE I Wattled Crane Census Details Date
Strata*
6-12 October 1971 North bank: 500.5-508-5 508"5-518"0 518.0-532"5 532.5-565.0 565.0-577.5 18 April 1972 North bank : 489.7497.7 19-21 June 1972 North bank: 496"5-538.5 538.5-562.0 South bank: 496.5-519.7 519.7-530.0 530.0-547-0 1-3 November 1972 North bank: 499.0-522.0 522-0-552.7 552.7-571.0 South bank: 506-0-528.5 528.5-546.0 9 May 1973 North bank: 525.0-559.0 20-21 August 1973 North bank : 510.0-518.2 518.2-548.0 548.0-564.0 (Luwato) 548-0-564.0 (Kafue) South bank: 504.5-514.0 514.0-520.0 520.0-530.0 530.0-547-0
No. of Popu~tmn transects flown pa~s
10 12 20 14 8
27 4 151 278 0
28
715 birdst
45 22
163 24
24 21 17
105 3 83
18 40 15
0 461 71
22 10
135 64
21
408
7 27 18 14
0 151 80 25
5 11 -9
0 39 630 birds$ 20
* The east-west kilometre grid limits are given (in Fig. 2 the numbers refer to the ten-kilometre grid). The northsouth limits were the high flood-line and Kafue River, or vice versa. In August 1973, two strata with identical westeast limits were flown on the north bank--the northerly Luwato stratum included all depressions connected to Luwato Lagoon, while the southerly Kafue stratum included depressions connected to the Kafue River. t The flocks were well dispersed and could therefore be censused by this method. $ A ground count. Distribution and Population Size on the Kafue Flats The seasonal distribution of Wattled Crane flocks is indicated in Fig. 2. F r o m January through March, in both 1971 and 1972, fewer than 1,000 birds remained
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on the floodplain. Most were between Chief Muwezwa's Court and Chawembe Lagoon or in Lochinvar. In mid-January 1972, most of the 250 birds in Lochinvar left the floodplain and began to feed in the termitaria zone grassland and adjacent depression to the South of Chunga. In early February the depression became flooded and most birds then moved to the alkaline grassland below the hot springs at Bwanda. Up to 170 birds remained there until the end of March, but as the area dried out they returned to the flood-line. On 20 April, 202 were counted on the western boundary of Lochinvar near Namalio. The sequence of movements had been similar in 1971, but the wetter conditions in that year may have delayed the return of the flock to the flood-line. Up to 190 birds occupied the alkaline grasslands near the hot springs at Gwisho until mid-April. During April and May, few birds were found at Namalio, and it is presumed that the flock moved to near Muwezwa's Court, where the water level was falling, or remained at pans in the upland areas. Over 50 were present at one such pan, to the south of Bwanda, in early June, but they did not return in 1972. With the return of many birds to the floodplain, the population in April 1972 was somewhat in excess of 1,000 birds. On 18 April an estimated 715 + 156 (95 per cent level) were to the east of Muwezwa's Court, while there were about 250 in Lochinvar and scattered, small groups elsewhere. The population in the survey area was therefore of the order of 1,150 ___ 250 birds. In 1973, much of the floodplain remained dry and the Cranes stayed on it throughout the rains. A survey in March showed that large numbers were present, though to the west of grid line 510 none was seen on the north bank and very few on the south side. Later, a very large concentration was located near Namalio, 1,800 being seen in a limited ground-count while it is likely that 2,000 were present altogether. On the north bank, flocks were located south-east of Chawembe Lagoon and west of Boot Lagoon. The total population was probably of the order of 2,500. During the dry season a number of censuses were carried out (Tables I and II). They indicated that, as the floods subside, the population increases and may exceed 3,000 birds during the latter half of lhe dry season. Figure 2 is a reconstruction of the location of Wattled Crane flocks in a year of average water levels, under natural conditions. As water levels fall on the north bank, there is an easterly movement of the population towards Luwato Lagoon, while on the south bank there is a movement into the depression in Lochinvar, north of Chunga. With higher-thanaverage water levels, the seasonal movements are
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TABLE II Population Estimates o)c the Wattled Crane on the Kafue Flats During the Dry Season Pairs*
October 1971 June 1972 November 1972 May 1973 August 1973
Flocks
North bank
South bank
North bank
South bank
Population estimate
460 ± 108 187 ± 51 5 3 2 ± 141 408 ± 122 256±59
--t 191 ~ 59 199~98 --$ 59~28
364 420 567 740 938
--t 425 903 1,529 768
Unknown 1,601 2,932 3,085 2,336
Ninety-five per cent confidence limits are given. t Not censused. $ Not censused, but few pairs present. *
accelerated as the water rises and then postponed as it falls; with levels below average they are delayed, or cancelled, as the water rises, but then seasonally advanced as water levels fall. For example, the Crane flocks located in the survey of October 1971, when the water level at Nyimba was 1.5 m above normal, were distributed as shown for August-September. Similarly, in August 1973, when the water level at Nyimba was 1.0 m below normal, the flocks were located as shown for September-November. Two flights were made over the Kafue Flats to the west of the survey area: in January 1973 to Itezhitezhi, and in March 1973 to 15 km west of Namwala. No Crane was seen, though in years of flooding it is possible that displaced Cranes from the survey area move here. However, the floodplain to the west of the survey area is normally dry from June to November and therefore comparatively unsuitable. Populations Elsewhere in Zambia In the upper Kafue Basin, Logsdon (1971) and L. P. van Lavieren (in litt.) have counted Wattled Cranes on the Busanga Flats and along the River Lukanga (Fig. 1). Their population estimates were:
Busanga Flats 22-22 July 1971 145 H . S . Logsdon 1-2 September 1971 243-328 H. S. Logsdon 22 September 1971 310 H . S . Logsdon 11-12 October 1972 150-204 L. P. van Lavieren Lukanga River 7 November 1972 25 L . P . van Lavieren It is unlikely that large numbers of such birds exist elsewhere in the Kafue Basin. The Wattled Crane is common on the Bangweulu Flats (Fig. 1), although it has not been censused there. The population numbers hundreds, but is probably not as high as that on the Kafue Flats (J. J. R. Grimsdell & L. P. van Lavieren, pers. comm.)
Brelsford's (1947) largest count was 200, while Mwenya (1973) found it in flocks of up to 100. The floodplains of the upper Zambezi in Western Province support modest numbers, especially towards the Angolan border (S. Revell, pers. comm.), though no large flocks have been recorded there.
BREEDING AND MOULT
The Wattled Crane starts to breed as the water level is falling, and the surveys of 1972 were made at the beginning and end of the breeding season. In June, four nests with eggs were seen, all on the southern side; in November, 28 young and two nests with eggs were seen. All the nests were sited on featureless grass plains, some apparently in shallow water. The most important breeding grounds were on the plains to the north of Namalio, and between Chawembe and Luwato Lagoons. No pair was seen east of Luwato Lagoon on the north side, or east of Magoye River on the south side. On the north bank, flood levels affect the breeding population in the same way as they affect the non-breeding flocks: the higher the water level is, the more westerly will be the population. Thus, the most easterly chicks were seen
~J
Fig. 3. A pair of Wattled Cranes on the floodplain of the Kafue Flats, Zambia, in the dry season. They stand slightly over a metre high but considerably taller when alerted. _j
Douthwaite : An Endangered Population o f Wattled Cranes
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TABLE III Production o f Young Wattled Cranes in Relation to Peak Flood Levels at Nyimba Peak water Year
1971 1972 1973
level (m)
6"9 6"4 5.0
Survey
October November August
No. o f pairs seen
64'5 75 66
Pairs with young or nests
Estimated production o f young
No.
Per cent
N. bank
S. bank
Total
18 30 2
28 40 3
129 229 7
* 56 2
-285 9
* Not surveyed.
at grid line 547 in the October 1971 survey (Nyimba water level: 3.9 m) and at 560.5 in the November 1972 survey (Nyimba water level: 3.35 m). The number of pairs attempting to nest depends upon the amount of flooding (Table III). In 1972, after an average flood, it is likely that more than 300 pairs, or over 40 per cent of all pairs, attempted to breed. In 1973, after negligible flooding, only nine pairs b r e d - - s o m e three per cent of all pairs. Two eggs are frequently laid, but only one chick is ever reared. Family groups do not, in the main, join the flocks. In February 1973, a count amongst the widely dispersed 'flock' near the western boundary of Lochinvar revealed three juveniles amongst 114 birds (2.6 per cent), while there were eight juveniles in 33 birds (24.2 per cent) scattered on the eastern boundary of the Park. Similarly, the numbers of young in the flocks on the alkaline grasslands in Lochinvar were very low: 5-2 per cent in 1971 and 3.6 per cent in 1972.
Fig. 5.
A pair o f Wattled Cranes in flight: their wings span some 2'5 metres.
Flightless birds, moulting their remiges, were frequently seen from January into April of 1973, either alone or in small groups.
FOOD
The diets of Wattled Cranes in the four main habitats which they utilized in the Lochinvar area are described below. 1. Floodplain
Fig. 4. Wattled Cranes and their juvenile. The young bird lacks the black cap o f the adults. In the photograph the adults stand about one metre high. Red-billed Teal (Anas erythrorhynchos), Knob-billed Duck (Sarkidiornis melanotos), a Spur-winged Goose (Plectropterus gambensis) and a Spoonbill (Platalea alba) are seen in the foreground.
The floodplain was utilized throughout the year, but with average or high flood-levels most Cranes moved to other habitats in January, remaining there until April. In 1971 and 1972, some 50 to 100 remained on the floodplain, scattered along the waterline. They probably fed largely on rhizomes, though the following items were recorded: Nymphaea sp. rhizomes
Cyperaceae rhizomes--probably Cyperus eseulentus
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Echinochloa pyramidalis 'seed' Paspalum orbiculare 'seed' P. orbiculare leaf Pisces: Mastacembalus mellandi Insecta: Termitidae--Hodotermes mossambicus workers Orthoptera. In 1973 the Cranes remained on the floodplain throughout the period January-April. In January and February they ate large numbers of Coleoptera-especially fossorial Heteronychus spp. and Copridae. Rhizomes remained important items of food, and were especially so in March and April when the sedge fields on the western boundary were shallowly inundated. The following food items were recorded:
Eleocharis dulcis rhizomes Cyperus esculentus rhizomes C. usitatus rhizomes Nymphaea sp. rhizomes Panicum subalbidum 'seed' Insecta: Coleoptera--Copridae Heteronychus spp. including H. rusticus. As the floods subsided in 1971 and 1972, the Cranes fed largely on the rhizomes of Eleocharis dulcis and Cyperus esculentus. In the former year the feeding grounds remained moist for most of the dry season, but in the latter year, and in 1970 and 1973, they dried out and the ground became hard. The rhizomes of water-lilies of the genus Nymphaea, especially N. capensis, then became the staple food--particularly in the period October through December. In November and early December of 1972, large numbers of tenebrionid ground-beetles were eaten in one of the few areas of unburnt, dry floodplain grassland. The Cranes fed on the beetles in the early morning, returning to forage in the lily lagoons for the remainder of the day. The dry season foods were:
Nymphaea spp. rhizomes Cyperus eseulentus rhizomes Eleocharis dulcis rhizomes lnsecta: Coleoptera--Tenebrionidae Copridae Orthoptera. 2. Termitaria Zone Grassland In 1971 and 1972, ripening grass 'seed' formed the bulk of the diet at the end of January, although some birds were noted probing in dung which contained numerous coprid beetles (especially Onthophagus emeritus). The following items of food were recorded:
Panicum coloratura 'seed' P. subalbidum 'seed' P. schinzii 'seed' Digitaria ternata 'seed' Echinochloa colonum 'seed' Insecta: Coleoptera--Onthophagus emeritus. 3. Alkaline Grassland In 1971 and 1972, the alkaline grasslands near the hot springs at Bwanda and Gwisho were occupied from mid-February to mid-April. The main food was the rhizome of the sedge Cyperus usitatus, but in February, and again in April, some grass 'seed' (of Echinochloa colonum and Eriochloa nubica) and a small amount of grass leaf, were also eaten. The importance of grass 'seed' in April reflected the hardening of the ground and the increasing difficulty of probing for rhizomes. Sequential utilization of the rhizomes by a total of four species of birds occurred as the ground hardened. Thus, after heavy rain, especially in March, several hundred Spur-winged Geese (Plectropterus gambensis) would feed, tearing up the waterlogged turf with their beaks to reach the rhizomes. They were displaced as the primary feeders on rhizomes first by the Wattled Crane and then, at the end of April, by a flock of about 100 Crowned Cranes (Balearica pavonina). The Crowned Cranes left for the floodplain in July but were succeeded by flocks of Guineafowl (Numida meleagris), which, like the Crowned Cranes, scratched the rhizomes up with their feet. Guineafowl fed on the rhizomes until the end of the dry season. 4. Upland Pans Wattled Cranes were seen at pans early in the dry season, particularly from April to June. Their main food was water-lily rhizomes, but fragments of mollusc shell were also found in their excreta. The food of the Wattled Crane in Lochinvar thus consisted mainly of rhizomes, but grass 'seed' and insects were also important at times. It was evident that flooding, the hardness of the ground, and the annual cycle of rhizome production, influenced rhizome availability and therefore the composition of the diet. There was a progressive movement from the water-lily lagoons to the floodplain sedge-fields and then to upland sedge-fields as the water level rose. Though induced by water level changes, these movements were marked by an increasing dependence on non-rhizomatous food sources which were brought on by periods of rhizome shortages. The importance of Coleoptera in early 1973, and of grass 'seed' in January and February 1972, occurred during the sedge growing-season (December-February). Insects
Douthwaite : An Endangered Population of Wattled Cranes
were again important when the extent of moist ground became restricted; this was after the rains but before the floods began to subside (April-May), and at low water when little rain falls (December 1972). Few observations were made on the Crane's feeding ecology elsewhere. However, several of them were seen eating Eleocharis rhizomes on the Busanga Flats in July 1973, and it is likely that Eleocharis also provides the main food source on the north bank of the Kafue Flats, as it is abundant there whereas water-lilies are not. In both areas Cranes have been seen feeding on burnt ground that has long been dry, and it is evident that they are able to dig out rhizomes provided these are near the surface of non-compacted soils. DISCUSSION
As far as is known, the largest concentration of Wattled Cranes in Africa occurs on the Kafue Flats in the latter half of the dry season. But this population is threatened by two developments: one, that of mining gypsum near Gwisho in Lochinvar National Park, has less serious implications for the Wattled Crane than for the viability of this small Park. The other, that of the completion of a hydroelectric power project in the lower Kafue Basin, is likely to induce fundamental changes in the existing floodplain environment. With the completion of the storage reservoir at Itezhitezhi, the floods will tend to rise later, less and fall more slowly: the area of floodplain will be diminished, and suitable breeding and feeding grounds for the Wattled Crane will become restricted. It is impossible to predict the changes in the water regime in detail. However, unless flood levels at Nyimba regularly exceed five metres' depth, it is likely that the Crane population will be greatly diminished. Though confidence limits cannot be put to the total population estimates made in this paper, it is likely that future trends could be monitored by a repetition of my methods. The size of the breeding population is probably related to water levels on the Flats, but the size o f the non-breeding flocks rather to water levels both on the Flats and elsewhere. The impact of the hydroelectric scheme will therefore be most clearly seen through changes in the breeding population. Because the breeding season is so extended, no single survey would include the total breeding population. If future surveys are to be comparable, they would be better made at a particular water level than in a particular month. 3.81 m of water at Nyimba was the lowest level recorded every year between 1962 and 1972, and would therefore be a suitable level at which to make Crane censuses. A census of the north bank between Chawembe and
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Luwato Lagoons would be sufficient to reveal population changes. The food of the non-breeding flocks in Lochinvar consisted mainly of rhizomes. Grass 'seed' became important from late January to mid-April in the wetter years, and some animal matter was consumed. However, McLachlan & Liversidge (1970) describe the diet as 'small reptiles, frogs, insects and sometimes grain', while Mackworth-Praed & Grant (1962) mention 'reptiles, fish and insects of all kinds' and that the bird occasionally 'does some damage to mealies'. The difference between the present study and the earlier accounts of diet is almost certainly due to the different environments in which the observations were made. Both reptiles and frogs are uncommon on the floodplain, and high insect populations in the grass and litter layers are also unusual--probably because most grass is destroyed as soon as it is dry enough to burn. In Lochinvar, the diet of birds in April and May approximated most closely to the published accounts. Then, the flood-line was stable and the fringing grassland unburnt; rhizomes were scarce either because they were beneath the water or because the ground was too dry and hard for them to be dug out, while insect populations in the grassland were still comparatively high and a little grass 'seed' was still available. Nevertheless, in a normal year, April-May was probably the time of greatest food scarcity for the Cranes.
ACKNOWLEDGEMENTS
My work was financed by the National Council for Scientific Research as part of the University of Zambia's Kafue Basin Research Programme. I should like to thank Dr S. M. Silangwa and Dr A. R. Quartermain, successive Chairmen of the Kafue Basin Research Committee, for their support and for the latter's constructive criticism of this paper. I am also grateful to the Department of Wildlife, Fisheries and National Parks, Zambia, whose cooperation, particularly with the aerial surveys, facilitated this study. Plant determinations were made by Mr R. B. Drummond, the beetles were identified by Dr J. Bryden, while Mr L. P. van Lavieren and M r A. S. Muyundu assisted in a variety of ways: I would like to thank them all for their help.
References BELL, R. H. V., GRIMSDELL,J. J. R., LAVIEREN,L. P. VAN t~ SAYER,J. A. (in press). Census of the Kafue Lechwe by a modified method of aerial stratified sampling. E. Aft. Wildl. J.
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BENSON, C. W., BROOKE, R. K., DOWSETT, R. J. & IRWIN, M. P. S. (1971). The Birds of Zambia. Collins, Glasgow: 414 pp., illustr.
BRELSEORD,V. (1947). Notes on the Birds of the Lake Bangweulu Area in Northern Rhodesia. Ibis, 43, pp. 57-77. CHAPIN, J. P. (1956). A visit to Lochinvar Ranch, Northern Rhodesia. Oryx, 3(6), p. 329.
CHAPMAN, D. W., MILLER, W. H., DUDLEY, R. G. & SCULLY,R. J. (1971). Ecology of Fishes in the Kafue River. University of Idaho, Moscow, USA: 66pp., mimeographed. DARLING, F. F. (1960). Wildh'fe in an African Territory. Oxford University Press, London: 160 pp. JOLLY, G. M. (1969). The treatment of errors in aerial counts of wildlife populations. E. Afr. Agric. For. J., 34, Special Issue, pp. 50-5.
LOGSDON, H. S. (1971). Preliminary Results of Aerial Surveys on Busanga Plain. Dept of Wildlife, Chilanga, Zambia: 4 pp., mimeographed. MACKWORTH-PRAED, C. W. & GRANT, C. H. B. (1962). Birds of the Southern Third of Africa, Vol. I. LongmanN, London: xxiv + 688 pp., illustr. MCLACHLAN, G. R. & LIVERSlDGE, R. (1970). Roberts' Birds of South Africa, 3rd edition. Trustees of the John Voelcker Bird Book Fund, Cape Town: xxxii -+- 643 pp., illustr. MWENYA, A. N. (1973). Ornithological notes from southeast of Lake Bangweulu. Puku, 7, pp. 151-61. SHEPPE, W. & OSaORNE, T. (1971). Patterns of use of a flood plain by Zambian mammals. Ecological Monographs, 41(3), pp. 179-205. WHITE, C. M. N. (1965). A Revised Check List of African Non-Passerine Birds. Government Printer, Lusaka, Zambia: vi + 299 pp.
ABSTRACT
averages 80 cm and falls between November and March. There is considerable variation in scale and The Kafue Flats, a river floodplain in southern Zambia, supports the largest known population o f Wattled Cranes. In timing, but normally the floods begin to rise in a normalyearfewer than 1,000 arepresent at highflood; but November and the flood peak passes slowly eastwards as the water subsides the population increases, and in the --reaching Itezhitezhi in early March, Lochinvar latter half o f the dry season it numbers some 3,000 birds. National Park (Fig. 2) in April, and Kafue Gorge in Following widespread flooding in 1972, at least 300 pairs late May. To the west of the survey area, the floodnested as the water fell. However, in 1973 there was little plain is dry by the end of June; in the survey area, it is flooding and few pairs bred. Many full-grown birds moult their remiges between January and April, and are then usually dry before the onset of the rains. A mosaic of old channels and lagoons remains flooded. In 1971 the flightless. The diet is largely o f rhizomes dug from soft mud. Suitable feeding grounds in the dry season are created by a flood was comparatively high, and was late in falling. falling water-leveL The population is threatened by a hydro- The flood of 1972 was slightly below average, while electric scheme that is scheduled to be completed in 1978. the flood of 1973 was well below normal. The Kafue The scheme will reduce fluctuations in the water level on the floodplain and, in so doing, reduce the area o f suitable Gorge dam reduced the rate at which the water fell, but, except at the very eastern end of the floodplain, habitat. did not produce seasonally abnormal water levels. The floodplain is fringed with open grassland, scattered thickets and trees, and is characterized by large termite mounds. Where drainage improves, INTRODUCTION woodland replaces this 'termitaria zone'. A few seasonal streams, in broad depressions, cut through The Wattled Crane (Grus carunculatus Gmelin) the woodland and termitaria zone to disappear at the occurs on wet grasslands from eastern Cape Province, edge of the floodplain. Most of the floodplain is Natal, and Transvaal, in South Africa, to northern covered with perennial grass swamp which collapses Botswana, Rhodesia, Zambia, Malawi, Angola, north as the floods recede and is then burnt off to produce in Zaire to Kasai, and Tanzania. It also occurs in new growth for the large numbers of cattle which Ethiopia (White, 1965). Generally it is scarce graze on the Flats during the dry season. The loss of (Mackworth-Praed & Grant, 1962; McLachlan & organic matter through burning may be instrumental Liversidge, 1970), but in parts of Zambia, and in producing a rich fishery on the floodplain, as then particularly on the Kafue Flats, it is at times quite dissolved oxygen concentrations are more likely to common (Benson et al., 1971). However, a hydro- remain high during the succeeding flood (Chapman electric scheme on the Kafue River may largely et al., 1971). destroy the floodplain environment of the Flats. This The Flats are renowned for their wildlife (Chapin, paper describes the significance of the area to the 1956; Darling, 1960) and have two National Parks Crane. (Fig. 2). The floodplain is characterized by the Sheppe & Osborne (1971) described the floodplain endemic, semi-aquatic Kafue Lechwe (Kobus leche and so only a brief description need be given here. kafuensis), whose population is estimated to be about The Kafue Flats is a river floodplain in southern 94,000 (__+9 per cent) (Bell et al., in press). Over 400 Zambia (15 ° 15'-16 ° 15' S; 26 ° 00'-28 ° 10' E) at an species of birds have been recorded from the area of altitude of about 1,000 metres above sea-level (Fig. 1). the Flats. Some 235 km long and up to 40 km wide, the floodIn mid-1971 a dam and hydroelectric power station plain covers an area of about 6,000 km 2. The rainfall were completed in Kafue Gorge. The dam has a low 134 Biological Conservation, Vol. 6, No. 2, April 1974--- 0 Applied Science Publishers Ltd, England, 1974---Printed in Great Britain
Douthwaite : An Endangered Population o f Wattled Cranes
L
200
km
135
I
FLATS
BUSANGA
WESTERN
FLATS ,' ITEZHITEZHI
I~o VlNCE/ NAM~VA '",;; ~LA ,
-~--~-R. LUKANGA LUSAKA •
KAFUEFLATS (and survey a r ~
GORGE
,,
Fig. 1.
Zambia, showing places outside the Kafue Flats survey area mentioned in the text.
storage-capacity and forms a small semi-permanent lake at the eastern end of the Flats. However, because the capacity of the dam is insufficient to meet the water demands of the power station throughout the dry season, a storage reservoir is being built at Itezhitezhi, at the upper end of the floodplain. It is scheduled to be completed in 1978. Water will be stored in this darn during the rains and released during the dry season. Thus, relative to the natural regime, flooding on the Flats will be reduced during the rains, but increased in the dry season. There will be a reduction in the area of floodplain, and presumably some encroachment of termitaria grassland at higher levels and of permanent lagoons and swamp in the lowest depressions.
METHODS
A study of the feeding ecology, distribution, and population sizes, of the more numerous avian herbivores, including the Wattled Crane, was started in late
1970. Most observations were made in Lochinvar National Park, but several aerial surveys were flown over the central section of the floodplain (Figs. 1 and 2). Usually the flights were of short duration, using a Piper Super Cub, and made in order to locate the main wildfowl concentrations. However, six censuses of the Crane population were also made from a Cessna 182, using Jolly's (1969) census method as modified by Bell et al. (in press). The survey area was divided into areas, or 'strata'. The density of Cranes within each stratum was expected to be more or less homogeneous throughout, but to be very different when compared with that in adjoining strata. High-density strata were sampled more intensively than low-density ones. Sampling was done by flying north-south transects and counting the cranes within a strip of known width. The mean number per transect strip was then multiplied by the total number of possible transects to give the population within the stratum. Cranes are conspicuous birds and pairs are widely distributed over the floodplain except at high flood. They are thus ideal subjects for this method of
Biological Conservation
136
r,,I,,,,,,,,, 48
I, 49
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II Fig. 2. The survey area on the Kafue Flats, indicated by the small rectangle in Fig. 1, with a reconstruction o f the seasonal location o f Wattled Crane flocks under natural and average flood levels, based on observations in 1971-73. The scale at the top shows the kilometre grid.
population estimation. However, large flocks are also encountered, which are localized and often wary, flying away at the approach of the aircraft. They could not be sampled accurately by this method. Instead, an estimate of the total flock size was made and the estimate corrected for bias. Bias was determined by photographing flocks and comparing the estimate with the actual number. Inevitably some flocks would be overlooked and the population estimates are therefore probably conservative. Each census was flown at a height of 60 m and a speed of 160 kph. A strip-width of 175 m was satisfactory when counting adult Cranes but was too wide to be certain of seeing a very small chick running beneath an adult near the outer limit of the strip. A width of l l 0 m was therefore used latterly when counting breeding birds. A single bird was scored as half of one pair; three full-grown birds together were taken to be one pair and one immature bird, and four or more birds together were called a flock, Details of the censuses are given in Table I.
The study of the Cranes' feeding ecology involved both direct observation of feeding birds and examination of their excreta. The main component of the diet was normally found by watching several feeding birds in the flock and confirmed by examining excreta collected in the area. Checks were then made at irregular intervals for as long as the flock remained in the same area.
RESULTS
Bias in Estimates o f Flock Size
Bias was determined by comparing the estimated number of birds in the flock (x) with the actual number as counted from a photograph (y). The bias factor was x/y. Three flocks were estimated to contain fewer than 100 birds: the mean bias factor was 1-0. Five other flocks were estimated to range from 100 to 300 birds: the mean bias factor was 0.68. These two factors were used in correcting estimates of flock size,
Douthwaite : An Endangered Population of Wattled Cranes
TABLE I Wattled Crane Census Details Date
Strata*
6-12 October 1971 North bank: 500.5-508-5 508"5-518"0 518.0-532"5 532.5-565.0 565.0-577.5 18 April 1972 North bank : 489.7497.7 19-21 June 1972 North bank: 496"5-538.5 538.5-562.0 South bank: 496.5-519.7 519.7-530.0 530.0-547-0 1-3 November 1972 North bank: 499.0-522.0 522-0-552.7 552.7-571.0 South bank: 506-0-528.5 528.5-546.0 9 May 1973 North bank: 525.0-559.0 20-21 August 1973 North bank : 510.0-518.2 518.2-548.0 548.0-564.0 (Luwato) 548-0-564.0 (Kafue) South bank: 504.5-514.0 514.0-520.0 520.0-530.0 530.0-547-0
No. of Popu~tmn transects flown pa~s
10 12 20 14 8
27 4 151 278 0
28
715 birdst
45 22
163 24
24 21 17
105 3 83
18 40 15
0 461 71
22 10
135 64
21
408
7 27 18 14
0 151 80 25
5 11 -9
0 39 630 birds$ 20
* The east-west kilometre grid limits are given (in Fig. 2 the numbers refer to the ten-kilometre grid). The northsouth limits were the high flood-line and Kafue River, or vice versa. In August 1973, two strata with identical westeast limits were flown on the north bank--the northerly Luwato stratum included all depressions connected to Luwato Lagoon, while the southerly Kafue stratum included depressions connected to the Kafue River. t The flocks were well dispersed and could therefore be censused by this method. $ A ground count. Distribution and Population Size on the Kafue Flats The seasonal distribution of Wattled Crane flocks is indicated in Fig. 2. F r o m January through March, in both 1971 and 1972, fewer than 1,000 birds remained
137
on the floodplain. Most were between Chief Muwezwa's Court and Chawembe Lagoon or in Lochinvar. In mid-January 1972, most of the 250 birds in Lochinvar left the floodplain and began to feed in the termitaria zone grassland and adjacent depression to the South of Chunga. In early February the depression became flooded and most birds then moved to the alkaline grassland below the hot springs at Bwanda. Up to 170 birds remained there until the end of March, but as the area dried out they returned to the flood-line. On 20 April, 202 were counted on the western boundary of Lochinvar near Namalio. The sequence of movements had been similar in 1971, but the wetter conditions in that year may have delayed the return of the flock to the flood-line. Up to 190 birds occupied the alkaline grasslands near the hot springs at Gwisho until mid-April. During April and May, few birds were found at Namalio, and it is presumed that the flock moved to near Muwezwa's Court, where the water level was falling, or remained at pans in the upland areas. Over 50 were present at one such pan, to the south of Bwanda, in early June, but they did not return in 1972. With the return of many birds to the floodplain, the population in April 1972 was somewhat in excess of 1,000 birds. On 18 April an estimated 715 + 156 (95 per cent level) were to the east of Muwezwa's Court, while there were about 250 in Lochinvar and scattered, small groups elsewhere. The population in the survey area was therefore of the order of 1,150 ___ 250 birds. In 1973, much of the floodplain remained dry and the Cranes stayed on it throughout the rains. A survey in March showed that large numbers were present, though to the west of grid line 510 none was seen on the north bank and very few on the south side. Later, a very large concentration was located near Namalio, 1,800 being seen in a limited ground-count while it is likely that 2,000 were present altogether. On the north bank, flocks were located south-east of Chawembe Lagoon and west of Boot Lagoon. The total population was probably of the order of 2,500. During the dry season a number of censuses were carried out (Tables I and II). They indicated that, as the floods subside, the population increases and may exceed 3,000 birds during the latter half of lhe dry season. Figure 2 is a reconstruction of the location of Wattled Crane flocks in a year of average water levels, under natural conditions. As water levels fall on the north bank, there is an easterly movement of the population towards Luwato Lagoon, while on the south bank there is a movement into the depression in Lochinvar, north of Chunga. With higher-thanaverage water levels, the seasonal movements are
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TABLE II Population Estimates o)c the Wattled Crane on the Kafue Flats During the Dry Season Pairs*
October 1971 June 1972 November 1972 May 1973 August 1973
Flocks
North bank
South bank
North bank
South bank
Population estimate
460 ± 108 187 ± 51 5 3 2 ± 141 408 ± 122 256±59
--t 191 ~ 59 199~98 --$ 59~28
364 420 567 740 938
--t 425 903 1,529 768
Unknown 1,601 2,932 3,085 2,336
Ninety-five per cent confidence limits are given. t Not censused. $ Not censused, but few pairs present. *
accelerated as the water rises and then postponed as it falls; with levels below average they are delayed, or cancelled, as the water rises, but then seasonally advanced as water levels fall. For example, the Crane flocks located in the survey of October 1971, when the water level at Nyimba was 1.5 m above normal, were distributed as shown for August-September. Similarly, in August 1973, when the water level at Nyimba was 1.0 m below normal, the flocks were located as shown for September-November. Two flights were made over the Kafue Flats to the west of the survey area: in January 1973 to Itezhitezhi, and in March 1973 to 15 km west of Namwala. No Crane was seen, though in years of flooding it is possible that displaced Cranes from the survey area move here. However, the floodplain to the west of the survey area is normally dry from June to November and therefore comparatively unsuitable. Populations Elsewhere in Zambia In the upper Kafue Basin, Logsdon (1971) and L. P. van Lavieren (in litt.) have counted Wattled Cranes on the Busanga Flats and along the River Lukanga (Fig. 1). Their population estimates were:
Busanga Flats 22-22 July 1971 145 H . S . Logsdon 1-2 September 1971 243-328 H. S. Logsdon 22 September 1971 310 H . S . Logsdon 11-12 October 1972 150-204 L. P. van Lavieren Lukanga River 7 November 1972 25 L . P . van Lavieren It is unlikely that large numbers of such birds exist elsewhere in the Kafue Basin. The Wattled Crane is common on the Bangweulu Flats (Fig. 1), although it has not been censused there. The population numbers hundreds, but is probably not as high as that on the Kafue Flats (J. J. R. Grimsdell & L. P. van Lavieren, pers. comm.)
Brelsford's (1947) largest count was 200, while Mwenya (1973) found it in flocks of up to 100. The floodplains of the upper Zambezi in Western Province support modest numbers, especially towards the Angolan border (S. Revell, pers. comm.), though no large flocks have been recorded there.
BREEDING AND MOULT
The Wattled Crane starts to breed as the water level is falling, and the surveys of 1972 were made at the beginning and end of the breeding season. In June, four nests with eggs were seen, all on the southern side; in November, 28 young and two nests with eggs were seen. All the nests were sited on featureless grass plains, some apparently in shallow water. The most important breeding grounds were on the plains to the north of Namalio, and between Chawembe and Luwato Lagoons. No pair was seen east of Luwato Lagoon on the north side, or east of Magoye River on the south side. On the north bank, flood levels affect the breeding population in the same way as they affect the non-breeding flocks: the higher the water level is, the more westerly will be the population. Thus, the most easterly chicks were seen
~J
Fig. 3. A pair of Wattled Cranes on the floodplain of the Kafue Flats, Zambia, in the dry season. They stand slightly over a metre high but considerably taller when alerted. _j
Douthwaite : An Endangered Population o f Wattled Cranes
139
TABLE III Production o f Young Wattled Cranes in Relation to Peak Flood Levels at Nyimba Peak water Year
1971 1972 1973
level (m)
6"9 6"4 5.0
Survey
October November August
No. o f pairs seen
64'5 75 66
Pairs with young or nests
Estimated production o f young
No.
Per cent
N. bank
S. bank
Total
18 30 2
28 40 3
129 229 7
* 56 2
-285 9
* Not surveyed.
at grid line 547 in the October 1971 survey (Nyimba water level: 3.9 m) and at 560.5 in the November 1972 survey (Nyimba water level: 3.35 m). The number of pairs attempting to nest depends upon the amount of flooding (Table III). In 1972, after an average flood, it is likely that more than 300 pairs, or over 40 per cent of all pairs, attempted to breed. In 1973, after negligible flooding, only nine pairs b r e d - - s o m e three per cent of all pairs. Two eggs are frequently laid, but only one chick is ever reared. Family groups do not, in the main, join the flocks. In February 1973, a count amongst the widely dispersed 'flock' near the western boundary of Lochinvar revealed three juveniles amongst 114 birds (2.6 per cent), while there were eight juveniles in 33 birds (24.2 per cent) scattered on the eastern boundary of the Park. Similarly, the numbers of young in the flocks on the alkaline grasslands in Lochinvar were very low: 5-2 per cent in 1971 and 3.6 per cent in 1972.
Fig. 5.
A pair o f Wattled Cranes in flight: their wings span some 2'5 metres.
Flightless birds, moulting their remiges, were frequently seen from January into April of 1973, either alone or in small groups.
FOOD
The diets of Wattled Cranes in the four main habitats which they utilized in the Lochinvar area are described below. 1. Floodplain
Fig. 4. Wattled Cranes and their juvenile. The young bird lacks the black cap o f the adults. In the photograph the adults stand about one metre high. Red-billed Teal (Anas erythrorhynchos), Knob-billed Duck (Sarkidiornis melanotos), a Spur-winged Goose (Plectropterus gambensis) and a Spoonbill (Platalea alba) are seen in the foreground.
The floodplain was utilized throughout the year, but with average or high flood-levels most Cranes moved to other habitats in January, remaining there until April. In 1971 and 1972, some 50 to 100 remained on the floodplain, scattered along the waterline. They probably fed largely on rhizomes, though the following items were recorded: Nymphaea sp. rhizomes
Cyperaceae rhizomes--probably Cyperus eseulentus
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Echinochloa pyramidalis 'seed' Paspalum orbiculare 'seed' P. orbiculare leaf Pisces: Mastacembalus mellandi Insecta: Termitidae--Hodotermes mossambicus workers Orthoptera. In 1973 the Cranes remained on the floodplain throughout the period January-April. In January and February they ate large numbers of Coleoptera-especially fossorial Heteronychus spp. and Copridae. Rhizomes remained important items of food, and were especially so in March and April when the sedge fields on the western boundary were shallowly inundated. The following food items were recorded:
Eleocharis dulcis rhizomes Cyperus esculentus rhizomes C. usitatus rhizomes Nymphaea sp. rhizomes Panicum subalbidum 'seed' Insecta: Coleoptera--Copridae Heteronychus spp. including H. rusticus. As the floods subsided in 1971 and 1972, the Cranes fed largely on the rhizomes of Eleocharis dulcis and Cyperus esculentus. In the former year the feeding grounds remained moist for most of the dry season, but in the latter year, and in 1970 and 1973, they dried out and the ground became hard. The rhizomes of water-lilies of the genus Nymphaea, especially N. capensis, then became the staple food--particularly in the period October through December. In November and early December of 1972, large numbers of tenebrionid ground-beetles were eaten in one of the few areas of unburnt, dry floodplain grassland. The Cranes fed on the beetles in the early morning, returning to forage in the lily lagoons for the remainder of the day. The dry season foods were:
Nymphaea spp. rhizomes Cyperus eseulentus rhizomes Eleocharis dulcis rhizomes lnsecta: Coleoptera--Tenebrionidae Copridae Orthoptera. 2. Termitaria Zone Grassland In 1971 and 1972, ripening grass 'seed' formed the bulk of the diet at the end of January, although some birds were noted probing in dung which contained numerous coprid beetles (especially Onthophagus emeritus). The following items of food were recorded:
Panicum coloratura 'seed' P. subalbidum 'seed' P. schinzii 'seed' Digitaria ternata 'seed' Echinochloa colonum 'seed' Insecta: Coleoptera--Onthophagus emeritus. 3. Alkaline Grassland In 1971 and 1972, the alkaline grasslands near the hot springs at Bwanda and Gwisho were occupied from mid-February to mid-April. The main food was the rhizome of the sedge Cyperus usitatus, but in February, and again in April, some grass 'seed' (of Echinochloa colonum and Eriochloa nubica) and a small amount of grass leaf, were also eaten. The importance of grass 'seed' in April reflected the hardening of the ground and the increasing difficulty of probing for rhizomes. Sequential utilization of the rhizomes by a total of four species of birds occurred as the ground hardened. Thus, after heavy rain, especially in March, several hundred Spur-winged Geese (Plectropterus gambensis) would feed, tearing up the waterlogged turf with their beaks to reach the rhizomes. They were displaced as the primary feeders on rhizomes first by the Wattled Crane and then, at the end of April, by a flock of about 100 Crowned Cranes (Balearica pavonina). The Crowned Cranes left for the floodplain in July but were succeeded by flocks of Guineafowl (Numida meleagris), which, like the Crowned Cranes, scratched the rhizomes up with their feet. Guineafowl fed on the rhizomes until the end of the dry season. 4. Upland Pans Wattled Cranes were seen at pans early in the dry season, particularly from April to June. Their main food was water-lily rhizomes, but fragments of mollusc shell were also found in their excreta. The food of the Wattled Crane in Lochinvar thus consisted mainly of rhizomes, but grass 'seed' and insects were also important at times. It was evident that flooding, the hardness of the ground, and the annual cycle of rhizome production, influenced rhizome availability and therefore the composition of the diet. There was a progressive movement from the water-lily lagoons to the floodplain sedge-fields and then to upland sedge-fields as the water level rose. Though induced by water level changes, these movements were marked by an increasing dependence on non-rhizomatous food sources which were brought on by periods of rhizome shortages. The importance of Coleoptera in early 1973, and of grass 'seed' in January and February 1972, occurred during the sedge growing-season (December-February). Insects
Douthwaite : An Endangered Population of Wattled Cranes
were again important when the extent of moist ground became restricted; this was after the rains but before the floods began to subside (April-May), and at low water when little rain falls (December 1972). Few observations were made on the Crane's feeding ecology elsewhere. However, several of them were seen eating Eleocharis rhizomes on the Busanga Flats in July 1973, and it is likely that Eleocharis also provides the main food source on the north bank of the Kafue Flats, as it is abundant there whereas water-lilies are not. In both areas Cranes have been seen feeding on burnt ground that has long been dry, and it is evident that they are able to dig out rhizomes provided these are near the surface of non-compacted soils. DISCUSSION
As far as is known, the largest concentration of Wattled Cranes in Africa occurs on the Kafue Flats in the latter half of the dry season. But this population is threatened by two developments: one, that of mining gypsum near Gwisho in Lochinvar National Park, has less serious implications for the Wattled Crane than for the viability of this small Park. The other, that of the completion of a hydroelectric power project in the lower Kafue Basin, is likely to induce fundamental changes in the existing floodplain environment. With the completion of the storage reservoir at Itezhitezhi, the floods will tend to rise later, less and fall more slowly: the area of floodplain will be diminished, and suitable breeding and feeding grounds for the Wattled Crane will become restricted. It is impossible to predict the changes in the water regime in detail. However, unless flood levels at Nyimba regularly exceed five metres' depth, it is likely that the Crane population will be greatly diminished. Though confidence limits cannot be put to the total population estimates made in this paper, it is likely that future trends could be monitored by a repetition of my methods. The size of the breeding population is probably related to water levels on the Flats, but the size o f the non-breeding flocks rather to water levels both on the Flats and elsewhere. The impact of the hydroelectric scheme will therefore be most clearly seen through changes in the breeding population. Because the breeding season is so extended, no single survey would include the total breeding population. If future surveys are to be comparable, they would be better made at a particular water level than in a particular month. 3.81 m of water at Nyimba was the lowest level recorded every year between 1962 and 1972, and would therefore be a suitable level at which to make Crane censuses. A census of the north bank between Chawembe and
141
Luwato Lagoons would be sufficient to reveal population changes. The food of the non-breeding flocks in Lochinvar consisted mainly of rhizomes. Grass 'seed' became important from late January to mid-April in the wetter years, and some animal matter was consumed. However, McLachlan & Liversidge (1970) describe the diet as 'small reptiles, frogs, insects and sometimes grain', while Mackworth-Praed & Grant (1962) mention 'reptiles, fish and insects of all kinds' and that the bird occasionally 'does some damage to mealies'. The difference between the present study and the earlier accounts of diet is almost certainly due to the different environments in which the observations were made. Both reptiles and frogs are uncommon on the floodplain, and high insect populations in the grass and litter layers are also unusual--probably because most grass is destroyed as soon as it is dry enough to burn. In Lochinvar, the diet of birds in April and May approximated most closely to the published accounts. Then, the flood-line was stable and the fringing grassland unburnt; rhizomes were scarce either because they were beneath the water or because the ground was too dry and hard for them to be dug out, while insect populations in the grassland were still comparatively high and a little grass 'seed' was still available. Nevertheless, in a normal year, April-May was probably the time of greatest food scarcity for the Cranes.
ACKNOWLEDGEMENTS
My work was financed by the National Council for Scientific Research as part of the University of Zambia's Kafue Basin Research Programme. I should like to thank Dr S. M. Silangwa and Dr A. R. Quartermain, successive Chairmen of the Kafue Basin Research Committee, for their support and for the latter's constructive criticism of this paper. I am also grateful to the Department of Wildlife, Fisheries and National Parks, Zambia, whose cooperation, particularly with the aerial surveys, facilitated this study. Plant determinations were made by Mr R. B. Drummond, the beetles were identified by Dr J. Bryden, while Mr L. P. van Lavieren and M r A. S. Muyundu assisted in a variety of ways: I would like to thank them all for their help.
References BELL, R. H. V., GRIMSDELL,J. J. R., LAVIEREN,L. P. VAN t~ SAYER,J. A. (in press). Census of the Kafue Lechwe by a modified method of aerial stratified sampling. E. Aft. Wildl. J.
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