A Correction Factor for Counting Emus from the Air, and its Application to Counts in Western Australia

Abstract

We used the mathematics of the mark-recapture model to derive a factor correcting counts of emus surveyed from the air. The emus were neither marked nor recaptured, the correction factor being derived from the number of emu groups counted independently by two observers simultaneously scanning the same transect. The analysis suggests that about 68% of emu groups on the transect are counted by a given observer during a standard survey, and that his counts must therefore be multiplied by 1.47 before they estimate true density of groups. Having determined independently the mean size of emu groups as 3.75 at that time of the year, we applied this factor to counts from a survey of 1480000 km2 in Western Australia. Overall density was 0.074 emus km-2, being highest in the pastoral zone at 0.103 km-2 and lowest in unoccupied land at 0.008 km-2. The difference probably reflects availability of drinking water.