not his fault, but now we know the flock is at risk, it can be prevented. If the dog was able to get on the property and no attempt was made to put the birds inside after the attacked started that day, then yes, some responsibility falls on the owner of the poultry, and I believe the dog should have been taken back to the owners who should've been informed. If it continued, then the law could get involved and the owners reprimanded.
If my birds were being continuously attacked during the day, I would protect them, move them inside and deal with the predator. If I left them exposed knowing there was a predator around, I would consider my own actions to be neglectful. Lethal force is only required if the animal cant be got under control.