A regularly raised concern for wind farms is the number of species and rate of bird and bat collisions with turbines. Australian regulators require, at some operating wind farms, the monitoring of bird and bat collisions. Although monitoring is becoming more commonplace, the area recommended for searching beneath turbines is inconsistent, with many guidelines both in Australia and overseas being based on conjecture rather than empirical evidence. This has the potential to bias survey results, reduce confidence in the data collected, and preclude meaningful comparisons between sites. By having a measure of the range of the fall zone of a bird or bat, a survey can be designed that ensures that an adequate area is being searched and that the cost of searching outside the area where birds and bats can fall is minimised. This article outlines a model that describes the fall zone of bats and birds of various sizes after colliding with different sized turbines, by applying a Monte-Carlo approach to ballistics theory. The modelling results are benchmarked with data from two Australian wind farms and one from the USA, for which data were available. The results indicate the size of the search area required around already constructed turbines, and the search area required to estimate levels of background mortality for control zones for precommission mortality surveys.