DaveReid - the problem really only exists with the Garrett engines as they are unfiltered.
It is surprising how few birds actually contact the blades of a propeller as they pass through its arc. This is the result of the function of the aircraft's speed as opposed to the speed of the propeller blades and the gap in between them.
Most Garrett powered ag planes do have a very course grill installed in front of the air intake which will break foreign objects down into pieces of a couple of inches in width. This considered, it is still a testament to the engine's particle separation ability that a bird ingestion can be of little consequence to its sustained operation.
Garretts do, however, suffer from internal corrosion due to their constant ingestion of corrosive pesticides and fertilisers being applied during ag operations. It is for this reason, I believe, that the PT6 and Walter turbines are far better suited to agricultural and fire operations.