Figure S1. Effect of assuming a constant removal probability when it is actually decreasing or increasing in time. Proportion of remaining carcasses (solid black lines) and the corresponding daily removal probabilities (dotted black lines) for four simulation scenarios. To obtain the proportions of remaining carcasses and the removal probabilities , Weibull distributions for the persistence times were assumed. For simulating decreasing removal probabilities with time, a was set to 0.7, and for increasing removal probabilities a was set to 1.3. k was then obtained using the mean assumed persistence time T: k ¼ ðCð1 þ ð1=aÞÞ=T. The proportion of remaining carcasses per day is S(t) ¼ e [-(kt) a ] and the daily removal probability (1-s(t))¼1-e [-(k(tþ1)) a ] /e [-(kt) a ] , where s is the probability that a carcass remains ('survives') one day. Grey solid lines are the proportion of remaining carcasses under the (false) assumption of a constant removal rate (grey dotted lines). 350 Ó WILDLIFE BIOLOGY 17:4 (2011)

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Environmental impacts of wind energy facilities increasingly cause concern, a central issue being bats and birds killed by rotor blades. Two approaches have been employed to assess collision rates: carcass searches and surveys of animals prone to collisions. Carcass searches can provide an estimate for the actual number of animals being killed but they offer little information on the relation between collision rates and, for example, weather parameters due to the time of death not being precisely known. In contrast, a density index of animals exposed to collision is sufficient to analyse the parameters influencing the collision rate. However, quantification of the collision rate from animal density indices (e.g. acoustic bat activity or bird migration traffic rates) remains difficult. We combine carcass search data with animal density indices in a mixture model to investigate collision rates. In a simulation study we show that the collision rates estimated by our model were at least as precise as conventional estimates based solely on carcass search data. Furthermore, if certain conditions are met, the model can be used to predict the collision rate from density indices alone, without data from carcass searches. This can reduce the time and effort required to estimate collision rates. We applied the model to bat carcass search data obtained at 30 wind turbines in 15 wind facilities in Germany. We used acoustic bat activity and wind speed as predictors for the collision rate. The model estimates correlated well with conventional estimators. Our model can be used to predict the average collision rate. It enables an analysis of the effect of parameters such as rotor diameter or turbine type on the collision rate. The model can also be used in turbine-specific curtailment algorithms that predict the collision rate and reduce this rate with a minimal loss of energy production.