I just want to bring a recent experience here to inform you all of the possible dangers.
We just had our 7 year old boxer/pit mix kill two of our chickens after ten months of NO problems whatsoever. We were totally comfortable with the situation and she killed two of our babies in front of my husband before he could even react.
It's a serious shock, but looking at it realistically, we did not train her in any way. We didn't see the need because after watching her closely with them for a couple of months and her not even reacting in any way to them, we thought we were safe. Apparently not. She will never be trusted around the chickens again.
To provide balance our 4 year-old male German Pointer (bird dog) was acquired and trained to be a poultry guardian. He is having a little trouble trying to catch a raccoon making incursions near a neighbors flock but is otherwise doing very well. He is mentally stable and predicable in part owing to the experience he, the birds, and I accrued.
The dangers are well known but losses due to other causes would dwarf the losses you experienced when birds are kept free-range.
Too much emphasis on the negative without effort to control problems makes even less experienced fearful of having dogs around poultry.