The global growth of wind energy has outpaced our assessment of possible impacts on wildlife. There is a pressing need for studies with pre- and post-construction data to determine whether wind facilities will have detrimental effects on susceptible avian groups such as raptors. A pre- and post-construction study was conducted to determine the impact of a windfarm on the abundance and behaviour of raptors in Wisconsin, USA. Variation in abundance and behaviour was examined both within and among years and relative to selected spatial, temporal and weather covariates. Raptor avoidance rates and indices of collision risk were calculated. Keywords: avoidance rates, birds, collision risk, displacement, flight behaviour, wildlife, wind turbines.