Get a big tea bottle. Its for making Ice Tea in the summer. Its 2 gallons or so? You take the lid and put a U joint pvc pipe in the middle of it. I will try to make a drawing or picture later. You then toss something with meat into the jar with water, let it rot. The flys are attracted to the smell fly through the U joint and are stuck. They WILL not fly into darkness on purpose, unless following food. The U joint makes it so when they are inside the bottle all they see is a dark spot above them where the U joint enters. U joint blocks the light. While some may fly out randomly the majority will sit in that bottle an rot with their maggots. Just make sure to keep adding water as it evaporates.
Bubba
PS Place it AWAY from your birds, it will attract flys from all over the place. Your birds might sit outside it and just pick them off as they go towards the bottle. So you may have to secure it, birds love to jump on stuff.