Are there general guidelines to be followed that help all scarecrow devices work better?

In general, all devices work far better when they are put into use before birds? feeding or nesting habits begin. It is much harder to dissuade the birds from giving up a habit than it is to prevent the habit from developing in the first place. Also, you should remove the device just as soon as it is no longer needed. Birds, once they catch on to these deterrents, are seldom fooled a second time.