Rate of removal of bird carcasses in arable farmland by predators and scavengers

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Abstract

Studies requiring carcasses to be retrieved to detect mortality and its causes in wildlife can be difficult to interpret due to removal of carcasses by scavengers. This paper describes two linked studies measuring rate of removal of carcasses from farmland in the UK, carried out to inform the results of a pesticide study involving carcass searching. In winter, 6% of carcasses had been removed 24 h after placement, and 32% had been removed 96 h after placement; wild bird carcasses were removed significantly faster than those of artificially reared birds. In summer, 76% of carcasses had been removed 24 h after placement and 91% after 96 h. A literature review of similar studies is given. Carcass removal rates by predators and scavengers appear to vary widely between sites and seasons, and may be high. If the results of studies involving carcass searches or surveillance are to be interpreted correctly, carcass removal rate by scavengers should be measured under similar conditions to, and contemporaneous with, those under which mortality is expected to occur.

Introduction

The presence of wildlife carcasses can be used to determine the causes of a variety of diseases including pesticide toxicity. For example, the UK's Wildlife Incident Investigation Scheme relies on members of the public finding carcasses that may have been poisoned by agricultural pesticides and reporting them to the relevant authorities (Barnett et al., 2006); the UK Department for Environment, Food and Rural Affairs surveillance scheme for avian influenza includes nature reserve staff and volunteers searching for bird carcasses at wetlands (http://www.defra.gov.uk, accessed 10 July 2007); Wobeser et al. (1982) searched wetlands to find waterfowl affected by avian cholera.

The results of carcass searches are affected by two main uncertainties:

  • Carcasses may be removed by predators, or scavengers (including domestic animals), between the time of their deaths and the time a search is made (see Table 1 for references).

  • Carcass searching is not 100% efficient, i.e., it is unlikely that every carcass present at a site at the time of a search will be found (e.g. Homan et al., 2001; Vyas, 1999; Philibert et al., 1993).

Because of these uncertainties, it can be difficult to interpret the results of carcass searches, since the proportion of animals that have died in an area which are actually found is not known. The proportion is likely to vary in time and space. If estimates of this proportion are obtained, they can be used to correct the numbers of animals found in carcass searches, and obtain a truer picture of the numbers of animals dying, or at least indicate the degree of uncertainty about the numbers dying.

This paper describes two linked studies carried out in the same geographical area, but at different times and with different combinations of variables, to assess the rate of carcass removal by animals, with the aim of reducing or quantifying the first area of uncertainty, and facilitating interpretation of the results of carcass searches carried out in the study area as part of a pesticide investigation. The results may be used to aid interpretation of other carcass searches.

While similar work has been published before, it mostly concerns events in the USA (see Table 1); very little literature is available concerning removal rate of carcasses in a European setting, where a different set of scavengers and predators is present (Pain, 1991). These are likely to include fox (Vulpes vulpes), badger (Meles meles), domestic cat (Felis catus), stoat (Mustela erminea), weasel (Mustela nivalis), mink (Mustela vison), rats (Rattus norvegicus and Rattus rattus) and, among birds, buzzard (Buteo buteo) and corvids (Corvus spp.).

Section snippets

Sites

Five clusters, each of five fields, in the Vale of York (North Yorkshire, UK) were used, making a total of 25 trial fields comprising about 258 ha. The mean field size was 10.3 ha (range 3.3–27.8 ha). The study was carried out in late February 2004. Fields of bare soil or very sparse, recently emerged crops were used (to simulate newly planted crops).

Two test carcasses were placed in each field, one on the boundary and another elsewhere on the field. This density was chosen because in previous

Study 1

The rates of disappearance of carcasses from fields, split by weeks, by species and by placement in the centre/edge of fields, are shown in Table 3.

There was no significant difference in the rates of carcass removal between the two experimental weeks (χ2=1.366, 1 df, P=0.24), or between field-centre and field-edge carcasses (χ2=0.301, 1 df, P=0.58), but there was strong evidence that the rate of removal of “wild bird” carcasses (including woodpigeons) was greater than that of reared birds

Discussion

The rates of carcass removal observed in the two studies described here are very different, despite the studies being carried out in the same area and using similar densities, substrates and carcass species. The two studies were carried out in different years, and at different times of year. The latter might affect both carcass detectability (particularly by mammalian scavengers hunting mainly by scent), and scavengers’ activity (many mammalian scavengers are less active in winter (Corbett and

Conclusions

The results of this study, and those reviewed above, show that carcass removal rates may vary widely (24–98% of carcasses remaining in place after 24 h). While there is some agreement in the magnitude of the rate of removal of carcasses under similar conditions, this is far from reliable. In particular, carcass removal rate data obtained in one season or one habitat cannot reliably be used to correct numbers of carcasses occurring in another. For this reason, studies or surveillance schemes

Acknowledgements

This study was funded by Bayer CropScience, Alfred-Nobel-Strasse 50, D-40789, Monheim am Rhein, Germany, and by Central Science Laboratory, Sand Hutton, York YO41 1LZ, UK.

Thanks to Fridlington Farms Ltd. and Messrs D. Smedley, R. Watson and J. J. Westaby for access to their land; and to Helen Thompson, Paul Irving and two anonymous referees for their comments on the manuscript.

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  • Cited by (0)

    No experimental animals were used in the work described here.

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