Aerial line transect methods have been successfully used to count animals in open country; we describe their application for assessing red deer (Cervus elaphus) numbers in Scotland. The results of a pilot study in the north of Scotland are compared to a census count carried out in the same area in the previous year. The aerial line transect method gave an estimate of 2240 red deer in May 1995. This agreed well with the census count giving 2270 in April 1994. The line transect estimate had a coefficient of variation of 25%, whereas no measure of precision is available for the census count. Separate estimates for the number of stags and the number of hinds and calves were obtained with both methods. We suggest improvements in the practical application of the aerial line transect method to increase precision of the abundance estimates. Aerial line transect surveys require fewer person-days of effort than census counts, and are appreciably more cost effective for assessing deer numbers over a large area. We present three case studies for applying aerial line transect methods in Scotland. The case studies compare the cost and effort for an aerial line transect estimate with those of a conventional ground-based census count in West Sutherland and North Ross (Red Deer Commission's counting blocks). Obtaining an estimate of red deer numbers in North Ross with a coefficient of variation of 10% would take 33% of the time of a census count, require 50% of the people and take about 60% of the resources required for a ground-based census count. A joint estimate for North Ross and West Sutherland would reduce the time demand for the line transect estimate (10% coefficient of variation) to about 18% of that required by a census and the resources to 33%.
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