Factors Affecting the Visibility of Kangaroos Counted during Aerial Surveys

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Abstract
Factors influencing helicopter and fixed-wing survey estimates of red kangaroos Macropus rufus and western gray kangaroos M. fuliginosus were examined in open habitats of Kinchega National Park, New South Wales [Australia]. Both species were censused in each season by helicopter between 1977 and 1979, at 3 survey heights, and at different times of day (early morning and late afternoon). Counts were lowest in summer and highest in winter, morning counts were higher than afternoon counts, and counts generally decreased with an increase in survey height. Ambient temperature, ranging between O.degree. C and 30.degree. C, affected the visibility of kangaroos through changes in their behavior and location with respect to vegetation cover. Multiple regression analysis indicated that if the survey temperature varied between 15.degree. C and 30.degree. C during a helicopter survey operating at a height of 91 m, the observed numbers of red kangaroos would decrease by 66%, and that for western gray kangaroos by 78%. Gray kangaroos were less visible in hot weahter than red kangaroos. Kangaroos were surveyed every season by fixed-wing aircraft between 1973 and 1979. Visibility bias due to the effects of ambient temperature was evident, producing unstable indices over time. Correction factors were developed to stabilize the indices of abundance by standardizing the effects of variable temperature.

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