By improving the Band model (Band et al. 2007), Yui and Shimada (2013) developed a sphere shape model to estimate the number of bird-wind turbine collisions. However, given that the density distribution of birds flying through a proposed wind farm site is not uniform, a new method was developed whereby a proposed wind farm site is divided into blocks by the block count method to estimate the number of collisions in each block. First, the mean flight distances of birds passing through square blocks and circular blocks are calculated theoretically. Then, the calculated results are multiplied by the frequency of each bird species passing through the blocks in a certain period of time, which produces their total flight distances in each block. Once the total flight distances are obtained, the sphere shape model can be applied to estimate the number of bird-wind turbine collisions in each block for each bird species. Spherical models are based on the assumption that birds fly in a straight line in a wind turbine collision risk zone. We here show that spherical models and the block count method can be applied to both circling flight paths and curved flight paths. The number of collisions for circling and curved flight paths is close to the straight flight cases in every flight radius, except those under about 20 m radius, at which collision numbers of the smaller raptors become 1.3 times larger than that of straight flight paths. If the flight speed of birds decreases during the circling flight, the number of collisions rises in inverse proportion to the flight speed.
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