Risk assessment of birds foraging terrestrially at Marion and Gough Islands to primary and secondary poisoning by rodenticides
Ross M. Wanless A B , John Cooper C D F , Martin J. Slabber E and Peter G. Ryan AA Percy FitzPatrick Institute, DST/NRF Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa.
B Seabird Division, BirdLife South Africa, PO Box 515, Randburg 2125, South Africa.
C Animal Demography Unit, Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa.
D DST/NRF Centre of Excellence for Invasion Biology, Department of Botany and Zoology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa.
E PO Box 2, Darling 7345, South Africa.
F Corresponding author. Email: John.Cooper61@gmail.com
Wildlife Research 37(6) 524-530 https://doi.org/10.1071/WR10005
Submitted: 20 January 2010 Accepted: 5 September 2010 Published: 18 October 2010
Abstract
Context.: Aerial application of poison bait pellets is an established and widely used method for removing invasive rodents and restoring insular ecological processes. However, the non-target effects of saturation poisoning require very careful consideration and precautionary risk-avoidance strategies.
Aims.: We assessed the risk of primary and secondary poisoning by rodenticides to terrestrially foraging lesser sheathbills (Chionis minor marionensis), Gough moorhens (Gallinula comeri) and Gough buntings (Rowettia goughensis) at Marion and Gough Islands.
Methods.: Birds taken into temporary captivity were offered non-toxic bait pellets dyed different colours and the carcasses of house mice (Mus musculus). In addition, dead mice were offered to these three species in the field, as well as to sub-Antarctic skuas (Catharacta antarctica) at both islands. Response to captivity was assessed by daily weighings.
Key results.: Individual birds either gained or lost mass overall during their 4–7 days in captivity. Whereas all captive birds pecked at the pellets, minimal amounts were consumed. However, Gough moorhens offered pellets in the field did consume them. Sheathbills (in captivity and in the field) and moorhens (in the field) consumed mouse carcasses, whereas buntings in captivity ate little from them. Sub-Antarctic skuas offered mouse carcasses in the field at both islands readily consumed them. At Gough Island some, but not all, skuas consumed bait in the field.
Conclusions.: Although the levels of assessed risk to primary and secondary poisoning differed among the three main species studied, it is recommended that populations for subsequent reintroduction be taken into temporary captivity before and during a poison-bait exercise as a precautionary measure. It is not deemed necessary to take sub-Antarctic skuas into captivity because they will be largely absent during a poisoning exercise in winter (the most likely period).
Implications.: Captive studies to assess susceptibility to primary and secondary poisoning are useful for determining positive risk; however, cage effects can cause false negatives by altering behaviours, and should be conducted with complimentary field trials. Where endemic species show any degree of risk (e.g. are vulnerable to the poison, regardless of how it might be ingested), precaution dictates that the risk be mitigated.
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