|DDT in the Tropics: The Impact on Wildlife (NRI, 1994)|
|Part 2 : Scientific report|
|Effects on wildfire|
R J Douthwaite
Effects on songbirds
Effects on birds of prey
Adverse effects on birds, and particularly effects of DDE on birds of prey, contributed significantly to the decision to ban DDT in many countries. Bird studies were therefore a major concern of this project.
DDT residues can kill, cause eggshell thinning and modify the behaviour of birds.Songbird mortality provided the first evidence of adverse effects of DDT on wildlife forty-five years ago, yet there remains little information on the impact of exposure on songbird populations. Detailed monitoring schemes, such as the British Common Bird Census, were introduced to track the fate of exposed populations but were too late to measure the initial impact.In Britain, DDT was rarely implicated in bird kills, but in the United States of America, heavy mortality was associated with Dutch Elm disease control programmes, in which DDT was applied to elm trees Ulmus americana L. to control the beetle vector of the disease. More than 90 bird species were killed by these operations and population declines in some species reported but these were confirmed only for the American Robin at a local level. Hennyi found no change in adult mortality rates in 16 species comparing the periods 1916-1945 and 1946-1965 and concluded that the reported declines in the American Robin were either very localized or were compensated for by a reduction in natural mortality.
Eggshell thinning was first linked with DDT residue levels in 1967 and has since been shown to affect hatching and breeding success in a wide range of flesh- and fish-eating birds, including pelicans, cormorants, darters, herons, ibises, ducks, vultures, hawks, eagles, falcons, gulls, owls and crows. Depending upon species, DDE levels of 2-20 ppm wet weight in eggs (c. 13-130 ppm dry weight) are associated with breeding failure on a scale sufficient to cause population decline. Irrespective of species, raptor populations declined when eggshell thinning exceeded 16-18% over several years and, in some cases, were locally exterminated.
Over 635 species have been recorded in Zimbabwe of which half
(318) are known to occur around Siabuwa. The list for the Zambezi valley
includes cosmopolitan species and those endemic to local habitats, such as
mopane and miombo (Julbernardia)
woodlands, crags, riverine thicket, freshwater and their man-made derivatives; none is endemic to Zimbabwe and at risk of extinction from ground-spraying, although some, such as the Taita falcon, are local and rare and at risk of exposure to DDT residues.
Infrequent reports of songbirds dying in convulsions in areas recently sprayed for tsetse fly control are indicative of poisoning. DDE levels exceeding 8 ppm wet weight have also been reported in eggs of nine out of 14 predatory species examined. Species feeding on birds or fish were at greatest risk with highest residue levels in Peregrine and Lanner falcons, Black Sparrowhawk, African Goshawk, Fish Eagle, Wahlberg's Eagle, Pel's Fishing Owl, Darter and Grey Heron. Eggshell thinning in the Fish Eagle averaged 15.6 % (J. Tannock) and Thomson predicted the species would be extinct on Lake Kariba within 10-15 years, and falcons and hawks in less, unless the use of DDT was curtailed. Monitoring of the Fish Eagle population breeding in Matusadona National Park, on Lake Kariba, began in 1975 and nest sites of species used in falconry have been recorded by the Zimbabwe Falconers' Club since 1976. However, the impact of DDT on species populations has not been investigated either in Zimbabwe or elsewhere in the tropics.
Effects on Songbirds
Surveys of relative abundance of woodland songbirds at Siabuwa were made to identify differences between sprayed and unsprayed areas (Box 5.1).
Box 5.1 Counting Songbirds at Siabuwa
R.J. Douthwaite and H.Q.P. Crick
Surveys of relative abundance of songbirds were made in mixed mopane-Combretum-Julbernardia woodland in the northern and southern parts of the study area to identify population differences between sprayed and unsprayed areas.
In 1987, counts were made at 80 fixed points, 20 on each of two tracks passing through an area treated annually with DDT from 1984-86, and 20 on each of two tracks in an untreated area (Figure 5.1). Tracks were traversed by vehicle and sample points lay 0.4-0.5 km apart so that birds seen near one point were unlikely to be seen or heard at the next. Sampling began about 1 h after dawn, at c. 06.45 h, and continued for about 3 h. All points on one track were visited during one census. Five censuses were made along each track between 19 March and 8 April.
At each sample point the observer counted all birds seen or heard within 100 m of the centre point during 5 min. He was free to move to increase the probability of recording secretative or silent birds. If a mixed-species flock was encountered, up to 10 min was allowed to complete enumeration.
Further point count surveys were made between 4-18 May 1988 and 18-26 April 1989 on a reduced scale. In October 1989 birds were counted during a series of eight walks through sprayed and unsprayed areas. Each transect traced a square, avoided tracks, and lasted 40 minutes. All birds seen or heard within 50 m of the observer were counted within 5 min periods. This survey reduced any seasonal and spatial biases associated with the point count surveys made earlier.
Additional surveys, based on timed counts or response to playback of song on a tape recorder, were made of selected species to obtain more precise information.
A similar number of species was recorded during surveys in the sprayed north and unsprayed south of the study area. Ignoring overflying birds, and combining point counts from 1987 and 1989, 83 species were seen in the north and 87 species in the south; 69 were common to both areas.
Depending upon the survey, 35-39 % fewer birds were seen in the north of the study area than in the south. Of the 33 most numerous species, six were commoner in the north and twenty-six in the south with one species equally abundant in both areas (Figure 5.1).
The climate was similar in the north and south, but the north was generally more hilly with sandier soils and less mopane woodland. In 1987 detailed observations were made of the vegetation to see whether differences in bird numbers were due to differences in vegetation.
Box 5.2 Vegetation Sampling
R. J. Douthwaite and H.Q.P. Crick
The woodland at each sample site was assigned to one of five types depending on the relative abundance of Julbernardia (J) and mopane (M), namely Type J: mopane absent; Type J>M: mopane scarce; Type J=M: Julbernardia and mopane both common; Type M>J: Julbernardia scarce; and Type M: Julbernardia absent. More detailed observations were then made on the vegetation at each point in two 0.01 ha plots. The densities of trees, 'small' shrubs (0.5-1 m high), 'medium' shrubs (l-2 m high), 'large' shrubs (2-3 m high) and 'very large' shrubs (>3 m high), as well as canopy cover (%) and ground cover (%) were estimated using modifications of the techniques of James and Shugart.
It was found that apart from the prevalence of mopane woodland, southern sites were also characterized by significantly less ground cover, fewer small and medium shrubs, and more trees than northern sites. Large and very large shrub densities and canopy cover were similar. The comparative lack of ground cover in the south in 1987, when the observations were made, may have been partly due to drought conditions which affected the north less.
Fewer birds were seen in Julbernardia woodland in the north than in mopane, but there was little difference in numbers in the south, although few sites dominated purely by Julbernardia were sampled (Figure 5.2).
Using data from 1987, significant correlations were found between the vegetation at sample points and numbers of some of the 19 most abundant species (Table 5.1). These were used in comparing numbers in the north and south of the study area. It was found that 9 of the 19 species were significantly less common in the north, while 10 species were equally abundant (Table 5.2). The former group mainly comprised birds feeding at or near the ground on arthropods while the latter group were mainly species feeding in the tree canopy on seeds, fruit and insects.
Many factors, including climate, food supply, availability of nesting and roosting sites, and predation can affect bird numbers. Climatic differences and differences in eight measures of woodland type and woodland structure do not explain the relative scarcity of almost half the species sampled in the north of the study area. However, other environmental differences may have been important. The choice of vegetative parameters was arbitrary and there is no indication that the measures chosen, the scale of measurement, or the number of samples taken were appropriate for every bird species. Indeed, given the varied requirements of birds, it would be remarkable if they were. For example, no direct account was taken of the availability of large seeds, which are important for doves, or of harvester termite Hodotermes mossambicus workers, which are eaten in large amounts by Red-billed Hornbills, and it is possible that the association of these key factors with the proxy measures chosen (e.g. mopane woodland, with few shrubs and low ground cover) were too weak to be correlated significantly. Unquantified natural factors, perhaps linked to the sandier soils and drier conditions under Julbernardia woodland, could therefore explain the comparative scarcity of many species in the north. However, a short-term impact of DDT on macroarthropods in sandy soil was detected (Chapter 2) and the possibility that this affected bird numbers in Julbernardia woodland, directly or indirectly, cannot be discounted.
Box 5.3 Feeding behaviour and diet
R.J. Douthwaite and C.C.D. Tingle
Methods Gut contents of birds collected were preserved in 70% ethanol. Food remains were sorted and identified under a binocular microscope, with reference to the pitfall trap collections of invertebrates from mopane woodland. In general, identification was only possible to order or family, but certain insects could be identified to species.
Results During October to November the seven species were separated clearly by feeding site and method (Table 5.3), and less clearly by diet (Table 5.4). Two species, the White-headed Black Chat and White-browed Sparrow-weaver, often fed together, in open ground, but the sparrow-weaver was mainly granivorous, whereas the chat, like the five remaining species, was largely insectivorous.
Feeding sites of Red-billed Wood-hoopoe and White-headed Black Chat were sometimes treated, or accidentally contaminated, with DDT. Of the remaining species, feeding sites of the White browed Sparrow-weaver and Black Tit were generally closest to, and the Striped Kingfisher's most distant from, spray deposits.
Beetles predominated numerically in the guts of Black Tit, Chinspot Batis, White Helmet Shrike and Red-billed Wood-hoopoe. Weevils were particularly important to Black Tits, whereas Chinspots took more chrysomelids. Ants, especially Camponotus spp., [Formicidae: Formicinae] and Pheidole spp. [Myrmicinael predominated in the diet of the White-Headed Black Chat in both July and November. However, termites, especially Odontotermes spp. [Isoptera: Termitidae], important in July, were largely replaced by beetles in November. Grasshoppers, mainly acridids, were the main component in the diet of the Striped Kingfisher.
Predators, potentially more heavily contaminated with residues, figured most prominently in the diets of White-headed Black Chat (ants) and Striped Kingfisher (mantids and arachnids).
Box 5.4 DDT residues
Methods Samples of Red-billed Wood-hoopoe (RbWh), White-headed Black Chat (WhBC), Black Tit (BT), Chinspot Batis (CsB), Striped Kingfisher (SK), White Helmet Shrike (WHS) and Whitebrowed Sparrow-weaver (WbSw) were shot in sprayed areas 1-3 and 12-17 months after treatment. Samples of White-headed Black Chat, Chinspot Batis and White-browed Sparrowweaver were also taken from unsprayed woodland.
Carcasses, minus feathers, viscera, beaks, tarsi and feet, were stored in 10% formalin before being homogenized in a blender and then deep frozen, pending analysis for DDT residues. Carcass fat content and residues of o,p' and p,p' isomers of DDT, DDD and DDE were determined at the Natural Resources Institute, except for samples of White-headed Black Chat and Chinspot Batis collected in July 1987, which were analysed under contract by ADAS. Analytical methods used by the two laboratories were similar. Residues were extracted with hexane in Soxhlet apparatus for 4-6 h. The extracts were concentrated and redissolved in cyclohexane/ethyl acetate. Extracts were cleaned at the Natural Resources Institute by gel permeation chromatography (GPC), elusion with ether hexane in 10 g columns of florisil and elusion with hexane in 10 g columns of 6% deactivated neutral alumina. Clean-up by ADAS was by GPC and Sep-Pak cartridges. Clean extracts were examined by gas liquid chromatographs (GLC) fitted with electron capture detectors to determine the pesticide residues present. Confirmation of a small number of analyses was obtained by mass spectral detection. Both laboratories recovered over 85% of residues in sample checks.
Interpretation of results Unaltered DDT, DDD or DDE were found in every sample. The origin of DDD in wildlife specimens is controversial but some, if not most, is derived from DDT postmortem. Conversion continues when tissue is stored in formaldehyde but the amount converted equals about 1.11 times the level of DDD. Corrected figures of DDT, shown as 'DDT', are therefore used below.
Technical DDT contains about 92% unaltered DDT, which degrades to DDE. The higher the proportion of unaltered DDT in S DDT in animal tissue, the closer the temporal, spatial or trophic distance to a spray deposit and the greater the risk, because unaltered DDT is more toxic than DDE. The effects of DDT and DDE may be additive, but in experimentally poisoned cowbirds Molothrus ater, one part of DDT was equivalent to about 6.64 parts of DDE. This ratio has been used to convert DDE residue levels to toxicological equivalents of DDT. The total number of toxicological equivalents of DDT is expressed as 'S DDT' in Figures 5.3-5.5. The distribution of residue levels in a large number of samples is almost invariably negatively skewed. Values were therefore transformed logarithmically to facilitate statistical treatment; means are geometric and levels are given in ppm of extractable lipid. Sample variances were compared by the F test and means by the T test.
DDT residues dissolve in body fat and are harmless unless the fat is metabolized. Laboratory work has shown that residue levels in the brain or carcass lipid of birds killed by DDT are distinct from those surviving the experimental dose. However, a range of environmental factors will determine whether critical levels are reached in the wild. The need to metabolize fat varies seasonally, with temperature and the availability of food, and differs between the sexes especially during the breeding season. Females 'dump' residues into the egg, just as lactating mammals shed burdens in milk. Body weightand associated fatnessis a major factor in determining which birds of a given species die under pesticide exposure. Heavy birds tend to lose more weight and die later than light birds. Drought, and its effects on the food supply, may therefore be a significant factor affecting risk in the present context. Previous exposure to DDT may also be important, as the ability to detoxify residues, which varies between species, is increased by previous exposure.
Conclusions The highest proportions of unaltered DDT (corrected for 000) in S DDT were found in Red-billed Woodhoopoe (mean 77%), White-headed Black Chat (62%), Black Tit (52"/o) and White-browed Sparrow-weaver (50%) within 1-3 months of treatment, consistent with their feeding close to spray deposits. The lowest proportion at this time was found in Chinspot Batis (23%).
The lowest proportions of unaltered DDT were found in Striped Kingfisher (2%) and Chinspot Batis (7%) in October, 15-16 months after treatment. Levels in three sedentary species from the unsprayed area were indicative of windborne contamination. Residues in White-browed Sparrowweavers contained 10-18% unaltered DDT in March, July and October, compared with 25% in Chinspot Batis and 52"/o in White-headed Black Chat in July.
The risk posed by residues varied by an order of magnitude between species within a treatment area at any one time (Figures 5.3-5.5). Residue burdens fell by an order of magnitude in the year following spraying operations (Figure 5.5). At any one time, exposure in birds in sprayed areas was about two orders of magnitude higher than in birds in unsprayed areas with background contamination. Exposure in the White-headed Black Chat 1-3 months after treatment varied by an order of magnitude dependent upon diet.
Effects of DDT on selected species
Seven common species, which could be readily sampled, were studied in detail to assess whether feeding habits and DDT residue accumulation could explain their relative abundance in the sprayed and unsprayed areas.
The Red-billed Wood-hoopoe (Plate 18) is usually common where large trees occur, living in noisy territorial groups of a breeding pair and up to 15 non-reproductives, which help the pair raise young. Rainfall is the main determinant of breeding success and survival is closely related to territory quality. Tree cavities, where roosting and nesting birds are safe from predation by genets and driver ants, are the most important territorial resource. Both sexes, and especially the male, are strongly philopatric.
Three surveys of occurrence were made in the study area, in July 1987, October 1988 and October 1989. After birds were located, pre-recorded calls were played back on a tape recorder to lure them closer for more accurate counting. In July 1987, flocks were significantly larger in the unsprayed area than in the area treated annually since 1984 (Mann & Whitney, p = 0.003), but sighting frequency was not recorded (Table 5.5). Flock size had altered little by October 1988 in either the unsprayed area or the area treated annually since 1984. Flock size in an area first treated in July 1987 remained similar to the unsprayed area, but flocks were encountered less often. One year later, in October 1989, flocks were fewer and smaller and the species was almost as scarce as in the area treated annually since 1984. By contrast, there had been little change in occurrence in the unsprayed area.
Birds, shot in October and November, two months after spraying ended, had fed exclusively upon arthropods, mainly beetles and termites (Table 5.4). These had been taken from crevices in tree trunks, large branches and from the ground (Table 5.3).
DDT residue levels in some of the birds were amongst the highest recorded for any species and there can be little doubt that the population decline after spraying was due to a lethal accumulation of DDT residues (Figure 5.4).
White-headed Black Chat
The White-headed Black Chat (Plate 19) is an arboreal thrush, virtually endemic to mopane and miombo woodland in southern Africa. It lives in groups of 1-6, breeds co-operatively, and feeds largely on arthropods taken from the ground or from tree trunks. The species is resident but the degree of philopatry is unknown. In the Zambezi valley, it is common and conspicuous in old woodland with tall, bare-limbed trees, an open canopy, little understorey and patchy ground cover. Territory size at Siabuwa was estimated to be 5-25 ha.
The species is territorial and birds responded conspicuously to playback of their song on a tape recorder. Surveys of relative abundance were made in treated and untreated woodland in six widely separated areas of north-west Zimbabwe. Numbers were invariably lower in the treated area, than in the adjacent untreated area even when no treatment had been applied for seven years.
Population changes at Siabuwa were monitored from July 1987 to January 1991 and related to spray treatments (Figure 5.7). In an area treated in 1987, 1988 and 1989 numbers fell by 88% over 33 months following the first spray, mainly due to a reduction in the number of occupied sites (Figure 5.6). Numbers in the unsprayed area fell by 13% during the same period. In January 1991, numbers were increasing again in areas treated in 1987 and in 1987-89. Isolated groups were also present in an area treated six times between 1984 and 1989.
In a replicate study, numbers also declined in the Omay Communal Area following treatment with DDT.4
Food supply, breeding success and DDT residue accumulation were investigated as possible causes of decline at Siabuwa.
Ants, especially Camponotus spp., were numerically predominant in the diet in both July and November. Termites, which were also important in July, were largely replaced by beetles in November (Table 5.4).
Ant and termite activity at sprayed sites in the study area was as great as, or greater than that at unsprayed sites. Ants (Camponotus spp.) from sprayed sites held mean residue levels of 8.7 ppm (max. 218 ppm) dry wt. S DDT, of which 67% was unaltered DDT. Termites and beetles had mean residue levels of 3.3 ppm (max. 14 ppm) and 0.9 ppm (max. 8 ppm) S DDT, of which 44% and 37% was unaltered DDT respectively.
Fledging success of White-headed Black Chats in adjacent sprayed and unsprayed areas was similar. In November 1987, fledged young were seen in 10 out of 14 groups in the area sprayed for the first time earlier that year, and in three groups found in the area treated annually since 1984. After breeding in 1988, young were seen in 38% of groups (n = 21) in the area first treated in 1987, and in 36% of groups (n = 28) in the unsprayed area.
Residues of DDT, DDD and DDE were found in all 23 carcasses examined. Birds collected in the dry season (July) from an area sprayed one month before contained up to 2206 ppm DDT, 367 ppm DDD and 578 ppm DDE extractable lipid (86, 17 and 27 ppm dry weight respectively) and there can be little doubt that these would have proved fatal for some. Residue levels in birds collected in the early rains (November) from a recently sprayed area in Omay were much lower, probably because birds were feeding largely on beetles, especially weevils, instead of ants and termites. Although there was little risk of poisoning at the time, the population subsequently declined, probably due to re-exposure to high residue levels with another seasonal shift in diet.
It was concluded that spraying operations with DDT have had a severe, and perhaps prolonged, impact on the White-headed Black Chat population of north-west Zimbabwe. The initial decline at Siabuwa was due to a lethal accumulation of DDT residues from prey, especially Camponotus spp. ants, rather than from reduced availability of prey or lower fledging success, but why numbers should continue to decline for 2-3 years after spraying ended is unclear. However, evidence suggests some ant species remain heavily contaminated with DDT two years after treatment.
The Black Tit is a common, widespread resident of the moister woodlands of southern Africa. It is philopatric, living in groups of 2-5 birds which breed co-operatively in territories of 25-48 ha.
Numbers in the north and south of the study area were similar (Table 5.2). In October, birds had fed largely on beetles gleaned from tree branches, mainly found above the target zone for DDT applications (Tables 5.3-5.4). DDT residue levels generally were significantly below those found in Red-billed Wood-hoopoes and no effect of exposure was apparent (Figures 5.4-5.5). The most heavily contaminated bird, shot one month after spraying ended, contained 935 ppm DDT, 176 ppm DDD and 432 ppm DDE ppm extractable lipid (= 1195 ppm DDT equivalents), half the amount found in the most heavily contaminated Red-billed Wood-hoopoe and White-headed Black Chat.
The White-browed Sparrow-weaver is a sedentary species, living in
groups comprising a breeding pair and up to nine non-reproductives. These breed
co-operatively and defend territories some 50 m in diameter centred on a
conspicuous group of nests built of grass in a tree (Figure 5.8). The
White-browed Sparrow-weaver was the most abundant species in the study area
accounting for about 25% of all birds.
Point count surveys indicated more birds were present in the south of the study area than in the north. This was confirmed by a survey of colonies made in December 1989. The presence or absence of colonies in plots 100 m in diameter was recorded at regular intervals along tracks through the study area. Colony density was estimated as 60/ km2 in the south and 48/km2 in the north. However, colonies were restricted to mopane woodland which was more widespread in the south (Figure 5.9). Estimated densities in mopane woodland in the north and south were 74 and 71 colonies/km2, respectively.
Colony size varied with area and time, but not with spray treatment. In 1987 colonies in the north were twice the size of those in the south (means 15.5 and 8 nests/colony), probably due to more severe drought in the south. Two years later, following good rains, average colony size in the north and south was similar (16 and 17 nests/colony respectively). It was concluded that the White-browed Sparrow-weaver population was unaffected by spray treatments.
Residue levels in birds sampled at intervals from 3 to 17 months after spraying were highest at 3 months, falling almost to background levels 12 months after treatment (Figures 5.3-5.5). The most heavily contaminated bird contained 1083 mg DDT-equivalents/kg carcass lipid, less than half the amount found in White-headed Black Chats and Red-billed Wood-hoopoes, whose populations collapsed after spraying. White-browed Sparrow-weavers and White-headed Black Chats occur together and both species feed on the ground. Their differing exposure to DDT is probably related to diet. The White-browed Sparrow-weaver eats some arthropods, especially termites, but over 90% of the items eaten were seeds. The White-headed Black Chat, on the other hand, feeds entirely on invertebrates, especially ants.
The Striped Kingfisher breeds co-operatively and males predominate in the population. It occurs in open wooded grassland, pouncing from a perch onto prey, which in October consisted largely of grasshoppers, spiders and solifugids (Tables 5.3-5.4).
During most of the study Striped Kingfishers appeared equally abundant in the north and south of the study area. However, in October 1989, kingfishers were much more conspicuous, and were especially noisy, in the north (Figure 5.10). Breeding was probably in progress and all three females shot in the the sprayed area had large oocytes (< 3-6 mm in diameter) and enlarged oviducts. None of the seven males, on the other hand, had enlarged testes (< 2-3 mm in length). The difference in apparent numbers between the north and south of the study area at this time cannot be explained satisfactorily but may have been due to local suitability for breeding. An effect of spraying seems unlikely given the diet and relatively low residue burden found in birds from sprayed areas (Figure 5.4).
The Chinspot is a common flycatcher in the open, deciduous woodlands of southern Africa where it captures insects from the tree canopy by hawking-gleaning.
Numbers in the north and south of the study area were similar (Table 5.2). In October, birds had fed largely on small beetles (Table 5.4) and their DDT residue burden was relatively low; there was little risk numbers would be adversely affected by spraying operations (Figure 5.4).
The White Helmet Shrike is resident and territorial, occurring in flocks of 2-17 birds and breeding co-operatively.
The relative abundance of White Helmet Shrikes in the north and south of the study area varied widely between surveys (Figure 5.11). A more intensive study made in May 1989, using the playback of tape-recorded calls, showed that there was little difference between areas: flocks were sighted on average every 24 min., or 2.6 playback attempts in the north, compared with 28 min., or 3.3 playback attempts in the south (n = 14 flocks in each area).
Flock size and age structure were recorded in April-May 1988 and May 1989 (Figure 5.1 1). Variation in flock size and proportion of young birds was greater between years than between spray treatments.
White Helmet Shrikes feed at all levels, but especially low down, gleaning foliage for beetles, ants and grasshoppers (Tables 5.3 and 5.4). DDT residue burdens were lower than in any other species and no effect of spraying was evident (Figures 5.4 and 5.5).
Exposure to DDT residue levels in the species studied was related mainly to feeding station and diet. Highest levels were found in Red-billed Wood-hoopoes and Whiteheaded Black Chats, both of which eat arthropods from tree trunks likely to be sprayed with DDT. Both also roost and nest in tree holes at risk of treatment. Intermediate residue levels were found in the Black Tit, which takes arthropods mostly from branches above the spray target, and in the White-browed Sparrow-weaver, which often feeds alongside the White-headed Black Chat, but eats seeds rather than arthropods. Lower levels were found in species feeding on arthropods associated with grassland (Striped Kingfisher) and foliage of trees and bushes (Chinspot Batis and White Helmet Shrike).
The Red-billed Wood-hoopoe and White-headed Black Chat declined in the wake of spraying operations but numbers of the other five species studied in detail did not vary with treatment. Rainfall and habitat differences were probably more important than residue contamination in determining abundance.
The number of other species eating heavily contaminated arthropods is probably small. However, both the Red-billed Hornbill and Fork-tailed Drongo are opportunists and likely to eat spray-affected animals: both were much less common in the north of the study area and numbers may have been reduced by DDT. Four woodpecker species, Bearded, Cardinal, Bennett's and Golden-tailed, also feed on arthropods from tree trunks. Their numbers in sprayed and unsprayed areas appeared similar, although the 1987 census results, corrected for differences in vegetation, indicated Bearded and Cardinal woodpeckers were significantly less common in the sprayed area. Two resident doves, the Cape Turtle Dove and Emerald-spotted Wood-dove, and one immigrant species, the Laughing Dove, first recorded after the good rains of 1987/88, were much less common in the north of the study area and may have been affected by DDT, although seeds comprise a large part of their diet. The Arrow-marked Babbler and Red-capped Robin, which feed on invertebrates in riverine thickets, were uncommon and only seen in unsprayed areas.
In conclusion, populations of two common woodland songbirds were severely affected, but not exterminated, by spraying operations. Several other species were probably affected by ground-spraying but it is clear that rainfall and habitat are more important determinants of bird numbers generally. The population dynamics of affected species and their long-term exposure to DDT residues have not been studied, making prognoses for population recovery uncertain. The persistence of DDT in ants, and relative scarcity of White-headed Black Chats in superficially suitable woodland seven years after treatment, suggest recovery in this species may take ten years or more.
Effects on Birds of Prey
The study, begun in 1988, was a co-operative venture with the Zimbabwe Falconers' Club, Falcon College and Peterhouse School.
The African Goshawk (Plate 20) is a common raptor in riverine woodland, preying mainly on birds. Pairs at Siabuwa occupied small home ranges and bred between November and January.
Searches for nests were made through thick bush along streams during the breeding seasons of 1988/89,1989/90 and 1990/91. By January 1991, only two of the ten known sites in the north of the study area remained in use, one in an area treated annually since 1984 and the other in an area treated since 1987 (Table 5.6). At least four sites were deserted during the study period. By contrast, nine of the 11 sites known in the southern, unsprayed area were still in use, and not more than two were abandoned during the study period.
Eggs were collected from seven nests and shell fragments from another three. Egg contents were analysed for DDT residues and the thickness of eggshells measured using a microscope stage micrometer accurate to + 0.004 mm. Clutch means are used in calculations where more than one egg was collected from a nest.
Both S DDT and DDE content were related to number of past spray treatments (Table 5.7). High residue levels were found in heavily treated areas and low levels in unsprayed areas. Eggshell thickness was correlated significantly with DDE content (rS = -0.72, p < 0.05) (Figure 5.12). Eggshells from heavily sprayed areas averaged 22% thinner than shells from unsprayed areas and addled eggs, with cracked shells, were found in nests in both heavily sprayed and less sprayed areas (Figure 5.13).
Remains of 27 prey items were recovered and identified. Nineteen (70%) were birds, five (18%) mammals and three (11%) lizards. Significantly, remains of Red-billed Wood-hoopoe and other species suspected of accumulating high residue levels (e.g.
Red-billed Hornbill and doves) were found, demonstrating a short food chain between the spray deposit via bark, invertebrates and insectivorous bird, to avian predator. There can be little doubt that breeding failure and population decline at Siabuwa occurred as a result of DDE contamination arising from tsetse spraying operations.
The population dynamics of the African Goshawk have not been studied sufficiently to make a confident prognosis for recovery, but given the relatively rapid disappearance of DDT residues from the physical environment and potential prey species, recovery may be faster than that of the goshawk's counterpart, the European Sparrowhawk, which has taken 20 years or more to recover in the areas worst affected by diedrin and DDT.
A separate national survey of breeding success and DDT contamination in the Peregrine Falcon found little evidence of failure (Box 5.5). However, residue levels sufficient to cause significant eggshell thinning were found in an egg from the Chizarira National Park, sprayed for the third and last time in 1984, suggesting tsetse spraying operations are likely to cause local breeding failure in this species.
Box 5.5 DDT residues and status of the Peregrine falcon in Zimbabwe R. Hartley
In 1990, the Zimbabwe Falconer's Club, Department of National Parks and Wildlife Management and US Peregrine Fund Inc., assessed the impact of DDT on breeding success in the Peregrine Falcon. 3'
Single intact eggs were removed under licence from 11 sites, and eggshells from recent hatchings from another two. Seven of the sites were in areas treated with DDT for tsetse fly control. Residues analysis was carried out at the Institute of Terrestrial Ecology, Monks Wood. Every egg contained DDE, but only one, from Chizarira National Park contained enough (22 ppm wet weight) to cause significant eggshell thinning. The adjacent area was last treated in 1984, for the third time. The next highest level (8 ppm) was found in an egg from the Chirisa Safari Area nearby. Measurement of eggshell thickness remains to be done.
The number of known Peregrine eyries in Zimbabwe has increased from 10 in 1971, to 47 in 1984, 107 in 1990 and 116 in 1991, doubtless reflecting successful development of the ZFC database rather than a change in Peregrine population. Similarly, the number of known Taita Falcon eyries, Zimbabwe's rarest breeding falcon, has increased from 2 in 1983 to 16 in 1991.
Scientific names of birds mentioned in the text
African Golden Oriole Oriolus auratus
African Goshawk Accipiter tachiro
American Robin Turdus migratorius
Arrow-marked Babbler Turdoides jardineii
Bearded Woodpecker Thripias namaquns
Bennett's Woodpecker Campethera bennettli
Black Sparrowhawk Accipiter melanoleucus
Black Tit Parus niger
Black-crowned Tchagra Tchagra senegala
Black-eyed Bulbul Pycnonotus barbatus
Blue Waxbill Uraeginthus angolensis
Cape Turtle Dove Streptopelia capicola
Cardinal Woodpecker Dendropicos fuscescens
Chinspot Batis Batis molitor
Crested Barbet Trachyphonus vaillantii
Crowned Hornbill Tockus alboterminatus
Darter Anhinga rufa
Emerald-spotted Wood Dove Turtur chalcospilos
European Sparrowhawk Accipiter nisus
Fish Eagle Haliaeetus vocifer
Fork-tailed Drongo Dicrurus adsimilis
Golden-breasted Bunting Emberiza flaviventris
Golden-tailed Woodpecker Campethera abingoni
Grey Heron Ardea cinerea
Grey Hornbill Tockus nasutus
Grey-headed Sparrow Passer griseus
Lanner Falcon Falco biarmicus
Laughing Dove Streptopelia senegalensis
Long-billed Crombec Sylvietta rufescens
Pel's Fishing Owl Scotopelia peli
Peregrine Falcon Falco peregrinus
Puffback Shrike Dryoscopus cubla
Racket-tailed Roller Coracias spatulata
Red-billed Hornbill Tockus erythrorhynchus
Red-billed Wood-hoopoe Phoeniculus purpureus
Red-capped Robin Cossypha natalensis
Rock Bunting Emberiza tahapisi
Scimitar-billed Wood-hoopoe Rhinopomastus cyanomelas
Short-tailed Cisticola Cisticola fulvicapilla
Striped Kingfisher Halcyon chelicuti
Tawny-flanked Prinia Prinia subflava
Three-streaked Tchagra Tchagra australis
Wahlberg's Eagle Aquila wahlbergi
White Helmet Shrike Prionops plumata
White-browed Sparrow-weaver Plocepasser mahali
White-headed Black Chat Thamnolaea arnoti
Yellow-eyed Canary Serinus mozambicus
Yellow-throated Sparrow Petronia superciliaris
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