46 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 2. 1979 A THEORETICAL ASSESSMENT OF THE ABILITY OF BIRD SPECIES TO RECOVER FROM AN IMPOSED REDUCTION IN NUMBERS, WITH PARTICULAR REFERENCE TO 1080 POISONING E. B. SPURR Protection Forestry Division, Forest Research Institute, N.Z. Forest Service, P.O. Box 31-011, Christchurch SUMMARY: A consideration of the composition of bird diets, and the list of bird species found deadl indicates that most New Zealand land bird species risk being killed by feeding directly on baits poisoned with Compound 1080 or by eating poisoned prey. Theoretically, if a population of a species is heavily reduced, its ability, or inability, to recover can be predicted from a consideration of its reproductive and dispersal capacities. Species with poor reproductive potential and poor dispersal have a high risk of non-recovery, e.g., the three species of kiwi, the takahe, kakapo, laughing owl, bush wren, rock wren, fernbird, yellowhead, stitchbird, saddleback, kokako, and New Zealand thrush. Species with either poor reproduction or poor dispersal are medium risk species, e.g., the New Zealand falcon, weka, New Zealand pigeon, kaka, kea, the three species of parakeets, the morepork, rifleman, brown creeper, whitehead, and robin. Species with good reproductive and good dispersal capacities are low risk species, e.g., the Australasian harrier, pukeko, kingfisher, welcome swallow, New Zealand pipit, grey warbler, fantail, tit, silvereye, bellbird, and tui. The implications of this classification are discussed in relation to forest management practice. INTRODUCTION suspected to have been poisoned by 1080 (Table 1). Bird populations may be reduced by, among other In some (e.g., the weka, kaka, kea, whitehead, things, pesticides (e.g., see Mills, 1973), predators robin) 1080 has been identified in the carcasses, (e.g., see Merton, 1975; Flack and Lloyd, 1978), but in others the cause of death has only been disease (e.g., see bellbirds* in Falla, Sibson and inferred, since dead birds were not found in Turbott, 1975), weather (e.g., see Bull and Dawson, unpoisoned areas of forest. The southern black- 1969), and destruction of habitat (e.g., land clear- backed gull (Douglas, 1967) and several introduced ance for farming, logging for timber). Except for species, for example, the chukor, Californian quail, the latter, the above causes of reduction usually do skylark, hedge sparrow, song thrush, blackbird, not prevent recovery. yellow hammer, chaffinch, goldfinch, redpoll, house sparrow, and the white-backed magpie have also One potential cause of reduction (the use of been found dead in areas poisoned with 1080 pesticides) has recently been highlighted by reports (Harris, 1977; Batcheler, 1978; Harrison, 1978; (summarised by Harrison, 1978) of bird deaths after personal observation). possum (Trichosurus vulpecula) poisoning operations Field trials are currently under way to assess the using sodium monofluoroacetate (Compound 1080). proportion of any population killed and the long- Compound 1080 is used mainly for control of term impact on bird numbers of any losses caused possums and other wild animals in forests and on by poisoning. However, the results of the trials may agricultural land (Batcheler, 1978). The 1080 is not be known for some time, and existing trials are incorporated into baits, usually diced carrot, but assessing only a selection of the species found in also other baits such as pollard pellets and oats, and the areas being poisoned. Some species, for example is commonly distributed from the air. The toxicity rare or endangered birds such as the kokako, and of baits ranges from 0.02 % by weight in some nocturnal birds such as the kiwi, weka and rabbit (Oryctolagus cuniculus) operations, to about morepork, require special monitoring which has not 0.09% in some possum operations. The area treated ranges from a few hectares for ground operations to about 10000 ha for aerial operations. *Latin names of all birds mentioned in the text and Several species of native birds are known or tables are included in Appendix 1. New Zealand Journal of Ecology 2: 46-63 SPURR: ABILITY OF BIRD SPECIES TO RECOVER FROM REDUCTIONS IN NUMBER 47 TABLE 1. Susceptibility of native land birds to 1080 poisoning from a consideration of the composition of their diets and of the species found dead in areas poisoned with Compound 1080. Vegetable component of dietl Bird Species species found considered none2 important minor dead Brown kiwi + Little spotted kiwi + Great spotted kiwi + Australasian harrier + +3,4 N.Z. falcon + Weka + +3,4,5 Pukeko + +3,4 Takahe + N.Z. pigeon + Kakapo + Kaka + +6 Kea + +7 Red-crowned parakeet + Yellow-crowned parakeet + Orange-fronted parakeet + Morepork + +8 Laughing owl + Kingfisher + Rifleman + +8,9 Bush wren + Rock wren + Welcome swallow + N.Z. pipit + +3 Fembird + Brown creeper + +8 Whitehead + +8,9 Yellowhead + Grey warbler + +8,9 Fantail + +8 Tit + +8,9 Robin + +8,9 Silvereye + +8,9 Stitchbird + Bellbird + Tui + Saddleback + Kokako + N.Z. thrush + Notes: 1. Data from Falla et al. (1975) unless otherwise stated in text. 2. So far as is known. 3. Batcheler (1978). 4. Harris (1977). 5. McIntosh et al. (1966). 6. Anon (1978). 7. Douglas (1976). 8. Personal observation. 9. Harrison (1978). 48 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 2. 1979 yet been attempted. Thus, I believe it appropriate Harrison, 1978), whitehead (Falla et al., 1975; St to use data available now to attempt to predict the Paul, 1976), yellowhead (Child, 1978), grey warbler (Gibb, in Harrison, 1978), tit (Riney et al., 1959), ability of species to recover from an imposed and robin (St Paul, 1976), do feed on vegetable reduction in numbers, such as might occur from matter (and sometimes from the ground) and other 1080 poisoning for control of possums in forests. insectivorous species might do the same. For I have restricted the assessment .to 38 species of example, the fantail has been seen to take insects native land (forest and forest margin) birds. Fresh- and grit from the ground (St Paul, 1975) and might water birds and the two migrants, the shining also take vegetable matter. Thus, the predominantly cuckoo and the long-tailed cuckoo, have been insectivorous species might feed on poisoned baits, excluded. particularly the small fragments or "chaff", from either the ground or, more probably, up in the leaves SUSCEPTIBILITY TO 1080 POISONING and branches. The propensity to take baits is Before assessing species' ability to recover from probably increased by poisoning in winter when a reduction in numbers, I have attempted to identify food, particularly insect life, is likely to be in short the land birds likely to be reduced by 1080 supply. poisoning. To discover how birds obtain 1080 poison, it will Consideration of feeding habits be necessary either to observe birds feeding, or to The first step is to determine which species might perform autopsies on the recovered carcasses. The obtain poison by feeding on poisoned baits or on former approach has so far provided little informa- other animals that have eaten baits. Those which tion, e.g., Pracy (1958) found that "Definite interest feed on vegetable matter (particularly fruits, shoots, was shown by robins and wekas, but baits were etc.) may be in danger of feeding on baits given only incidental attention by other birds. (particularly carrot, but also grain-based baits) used pigeons, parakeets, tuis, bellbirds, fantails, kakas, for possum control. For example, Bull (1959) tomtits, and whiteheads showed little interest warned that the presence of vegetable matter in the irrespective of whether baits were placed in trees or diet of kiwis should be taken into account by on the ground". However, keas (Douglas, 1967), anyone planning to use poisoned baits for control whiteheads and silvereyes have been seen feeding of noxious (or wild) animals. According to Fleming directly on carrot baits on the ground. Carrot has (1974), the little spotted kiwi may feed on fruit been identified from the gizzards of introduced more than do the other kiwi species. It may there- blackbirds and chaffinches (Spurr, unpubl.), but the fore be more likely to eat poisoned baits. Reid gizzards of poisoned insectivorous birds have usually (1978) reported recently that the little spotted kiwi been empty at autopsy. has drastically declined in numbers. Estimation of lethal dose Vegetable matter is eaten by all of our land birds The second step in identifying species at risk from except the birds of prey and some insectivorous 1080 poisoning is to establish whether the amount birds (Table I). The Australasian harrier, New Zealand falcon, laughing owl, morepork, and king- TABLE 2. Quantity of carrot bait. containing 0.09% fisher would be unlikely to ingest poisoned baits 1080, required to kill various bird species assuming an directly, but might eat dead or sub-lethally poisoned LD50 of 3.0 mg 1080 / kg bird's body weight. rodents or small birds. Dead mice (Mus musculus) have been found on the forest floor after appl:cation Name of Weightl mg 1080 g carrot of 1080 poison. Sub-lethally poisoned rodents and species (g) for LD50 for LD50 birds would be prime targets for predators. This is Rifleman 7 .02 .02 possibly how the morepork listed in Table 1 was killed. Grey warbler 7 .02 .02 Several so-called insectivorous species have been Tit 11 .03 .03 found dead after poison operations (Table 1), so Silvereye 12 .04 .04 they must have eaten either poisoned insects or Whitehead 17 .05 .06 baits. In support of the former, many insects and Chaffinch 21 .06 .06 slugs have been found on both carrot and pollard- Robin 30 .09 .10 based baits (Batcheler, 1978) and it is known Blackbird 90 .27 .30 that insects generally are very sensitive to 1080 (Chenoweth, 1949). However, some predominantly Note: 1.