Environmentalists have continually argued for the use of alternatives to fossil fuels to generate energy. Harnessing wind power by setting up wind farms had been one alternative proposed and implemented. As it is now known that these wind farms lead to the slicing and dicing of migratory birds and the decimation of the local bat population, we could say that environmentalists have committed the:

Mistake of ignoring secondary effects

Explanation:

Whenever there arises an adverse impact of a policy and it's implementation, owing to ignorance of secondary consequences, it is termed as ignoring secondary effects.

In short, it refers to assessing and viewing only the positive aspects of a policy or a move, meanwhile not taking into consideration the other adverse consequences which are also associated with the same policy.

In the given case, the environmentalists have only considered the generation of alternative sources of energy via windmills which will lead to preservation of fossil fuels. The proposed policy has been implemented without taking into account it's flip side i.e the harm it causes to bat population and migratory birds.

Thus, it can be stated that the environmentalists herein only considered the favorable outcome of a policy implementation and ignored the adverse effect of the same. Hence, they are said to have committed the mistake of ignoring the secondary effects.