Owl populations and habitat: factors that could impact populations of native owls in the sugarcane growing areas in Queensland

Thomas, M.O.W.G., and Kutt, A.S. (1997) Owl populations and habitat: factors that could impact populations of native owls in the sugarcane growing areas in Queensland. Report. James Cook University, Townsville, QLD, Australia.

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[Extract] SUMMARY This study forms part of a wider research program investigating the risks to native fauna associated with the use of the second generation anticoagulant rodenticide Klerat® (0.005% brodifacoum active ingredient) in Queensland sugarcane fields. The study focuses on the five sugarcane growing areas where rodents are known to cause serious damage to crops and where Klerat® is currently or may potentially be used.

Seven of the nine species of owl which are found in Australia are known to occur within sugarcane growing areas in Queensland. A search of available literature and a lengthy consultation process with ornithologists throughout the state identified a general paucity of quantitative data relating to populations of owls in and around Queensland sugarcane growing areas apart from the Herbert River district where estimates of breeding populations of five species over the past 10 years are available. One known cause of mortality of owls in sugarcane growing areas is roadkill by collision with vehicles. One dead owl has been analysed for anticoagulant residues and was found to contain detectable levels of brodifacoum.

A number of factors were identified as potentially affecting owl populations in sugarcane growing areas and comprised: •extraordinary climatic events •fire •habitat change •human-related mortality •cane growing practices •immigration from other owl populations •mortality due to disease •agricultural practices in adjoining non-sugarcane crops •provision of owl nesting boxes •rodenticide use

The likely level of impact of each factor on owl populations in general during the study timeframe (past 25 years) was assessed. Two factors; changes to owl habitat and the use of rodenticides in sugarcane crops were identified as likely to have a high potential impact. All other factors were assessed as having a low impact apart from the effect of disease which was unable to be assessed due to a lack of available information.

The potential for secondary poisoning of owls from rodenticide use was reviewed in detail. It was concluded that there appears to be a significant potential for secondary poisoning of owls to occur in Queensland sugarcane as a result of the use of Klerat® due to the following factors: •even when Klerat® is applied at the recommended rate and using a pulse baiting technique there is the potential for high brodifacoum loadings in rodent prey to occur; •the consumption of bait does not appear to affect rodent behaviour, at least initially, therefore intoxicated rodents are probably as available to hunting owls as unaffected ones; •three species of owls are known to concentrate virtually all their hunting activity around sugarcane areas, and are selecting the same two rodent species as prey as are the targets of baiting campaigns; •four of the owl species are known to concentrate hunting effort in areas supporting large numbers of rodents and are therefore more likely to prey upon baited rodents as these areas will also attract baiting effort; and •four of the owl species concentrate their hunting effort around the margins of sugarcane blocks which is where the majority of bait is placed, particularly late in the growing season.

The Herbert River district was used as a case study for investigating the role of factors identified as potentially affecting owl populations in a substantial decline in owl numbers which has occurred in the district since 1992/93. It is likely that some, if not many of the factors listed above could have combined to Owl effect owl numbers during the decline period. However, many of these influences would have been occurring over a long time-frame and perhaps increasing their impact on owl populations gradually (e.g. road kills, habitat change) or only in the short term (e.g. disease, climate, emigration). Given the severity of the population decline observed (75-85% reduction in some species numbers), there must have been a few key factors that have created the impetus for this observed population crash. Two of these seem the most likely culprits: secondary poisoning from the introduction of Klerat® and the loss of habitat for breeding and foraging. Without further detailed information on past and present owl population numbers and data on the influence of habitat clearing and rodenticide introduction on these numbers, a definitive answer cannot be provided. However, the data presented in Young and De Lai (1997) presents a compelling case for the impact of secondary poisoning.

It was recommended that: information regarding the extent and type of natural habitats cleared for sugarcane in all districts under consideration should be obtained from the Department of Environment and the potential impact of habitat clearing on owl populations should be reviewed in light of this information; any further use of Klerat® and the potential impact of secondary poisoning on native fauna should be very closely monitored in the field on a small scale before its widespread introduction; and consideration be given to the use as an alternative in northern Queensland, other internationally accepted and low secondary poisoning risk rodenticides.

Item ID: 36275
Item Type: Report (Report)
Additional Information:

A report for the Bureau of Sugar Experiment Stations. Report No. 97/09

Funders: Bureau of Sugar Experiment Stations (BSES)
Date Deposited: 06 Dec 2016 04:56
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050205 Environmental Management @ 100%
SEO Codes: 96 ENVIRONMENT > 9606 Environmental and Natural Resource Evaluation > 960604 Environmental Management Systems @ 100%
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