Because of a fox, we have our birds all contained. One was brave enough (most likely because of kits) to take many muscovy drakes in broad daylight when we were all in the yard. She did this in grazed pasture. She also came back through the winter any time of day to check things out. I only saw her twice, but saw evidence of her presence more often.
You might want to get a wounded rabbit call and sit out there waiting for it...or use some birds (safely in a separate compartment) in a live trap. It is not trapping season for foxes nor are their pelts worth a lot right now anyway, but perhaps you could talk with a trapper?
Foxes will know the routines and watch and wait out everything when routines change and so will quickly adapt. It could be a couple weeks between each strike, repeated throughout a day or week, or any time day or night. They are determined, especially with hungry mouths to feed.
My mom claims that no dog can outsmart/outwit a fox. She also said that they can squeeze through 2" holes (I can attest to that). I have also read that they can scale a 6'fence. If you have an outdoor pen, it should have a 2' hardware cloth wire skirt around the base at ground level as well to keep it from digging underneath.
They aren't called sly for nothing. Good luck!
