I'm of the opinion that you need to tighten your security. If I leave an egg (or any potential food item) lying out in the open like that it wouldn't last an hour, day or night. Unfortunately this fox has now learned where to find an easy meal and will be in the habit of checking that wide-open nestbox regularly. If it wasn't him (or her) it could just as easily be a raccoon or any other omnivore. The way your setup looks, you're just putting bait out for any critter to take, regardless of your intention. I would fence that nestbox in ASAP, and go over my bird enclosure carefully to check for any weaknesses, because he will come back for more eggs. If he can't find them he'll look for something else, and if your birds aren't secure that might be his next step. Regardless of what you decide to do about this fox, your current setup is an attractant that is bringing wildlife right to your pen and rewarding them with food. And unless they're being brooded, collect all your eggs daily.