Bias in Aerial Survey Estimates of Kangaroo Density

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Abstract
Aerial surveys of kangaroos provide population estimates which underestimate those derived from ground surveys. The degree of underestimation differs markedly between red and western grey kangaroos, between open plain and medium woodland, between sunny and overcast weather conditions and between observers. Estimates derived from aerial survey in sunny weather ranged from 74% of ground counts for red kangaroos on open plains to < 10% for grey kangaroos in medium woodland. Differences in ''sightability'' of kangaroos between observers and for the same observer in different weather conditions suggest the need for standardization of the conditions under which surveys are flown. Estimates of visibility bias currently used to convert observed aerial counts of kangaroos to absolute estimates of population size appear to overestimate the population size of red kangaroos and considerably underestimate that of western grey kangaroos. Further research is required to establish the relationship between vegetation cover and visibility bias for all harvested kangaroo species over a representative range of vegetation types.

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