With mobGriefing set as true: Foxes will not be able to pick up items dropped by a player or by a berry bush. If you have a bunch of foxes (such as an enclosure) in one area, they won't be able to pick up the berry and end up clumping together and then dying if maxEntityCramming is not set to a high value.

They are able to pick up items if mobGriefing is set as false.