Can the farmland bird index tell me about the effectiveness of specific agri-environmental measures?
Generally not. The farmland bird index is designed to paint a broad-scale picture of the state of farmland biodiversity as caused by the overall performance of agri-environmental schemes. To measure the effectiveness of specific, fine-scale agri-environmental measures more detailed and experimental monitoring protocols are demanded, based upon clear ecological hypotheses and tailored to the specific targets of each agri-environmental measure. These studies need a careful design with replicates, test and control plots, and measurements before and after treatments, and they will usually focus on a small set of target species (or even a single species). In some cases, however, the results of common farmland bird monitoring may be useful to assess the effectiveness of a measure, if the measure in question is being implemented at a very broad scale (e.g. reducing pesticide inputs across entire farms), and there are enough sample plots in farms where the measure is being taken to compare with those where it is not.