Most studies of wind farm – bird interactions examine the effects of disturbance on bird use of a wind farm site, and mortality through collision of birds with turbine blades. Here we describe an approach of field study and modelling that can be used to predict collision mortality risk, illustrated by two examples of studies at proposed wind farm sites in Scotland involving greylag goose and hen harrier. We use the modelling method developed by W. Band to estimate the number of bird collisions over a period of time. The calculation is in two stages: number of birds colliding per annum = number flying through rotor (Stage 1) x probability of bird flying through rotor being hit (Stage 2). Vantage point observational studies at a proposed wind farm site are used to gather information on bird use of the site and the frequency of bird flights in the area swept by turbine blades (Stage 1). For Stage 2 the probability of collision depends on the size of the bird (both length and wingspan), the breadth and pitch of the turbine blades, the rotation speed of the turbine, and the flight speed of the bird. Combining Stage 1 with Stage 2 gives a predicted collision mortality rate that assumes birds take no action to avoid collision. In practice, birds probably show a very high degree of collision avoidance, which dramatically lowers predicted mortality. For the collision risk model to predict accurate measures of collision mortality it is essential that more information is collected on avoidance.
This is a book chapter in Birds and Wind Farms: Risk Assessment and Mitigation.