Has anyone thought that maybe the problem is that with the decline of natural food, vegetation & insects for all birds, that maybe they are dying from the same diseases as humans? Namely that they eat junk food which does not nourish and sustain their tiny bodies. Chips and Mcdonalds from car parks.! That is never right.
I leave 8 fat balls on my trees and have to replenish them every two days. I think starvation amongst the bird population is to blame. The more food i put out the more they eat.
For a test i put meat out for the magpies. I used to have two in my garden. One morning i woke up to find 9 sitting on the branch outside my window. The word had got round :-)