I am honestly surprised - you should see dead birds at-least a couple of times a week. Not annually or monthly, but weekly esp for a full wind-farm like what you are stating.

Which part of the world do you live in ? It is possible that the location of the farm was chosen wisely. Are you also sure someone is not clearing the corpses ? There are laws mandating the collection of corpses in many western nations. My personal anecdotal opinion is that data is more honest in the non-Western nations since nobody cares about bird-deaths all that much and so the deaths are not sanitized.