Probably all kinds of things, lucky punks! Whatever wiggles in front of their wee faces. Most likely small bloodworms and even more likely, daphnia.
I have that 40 gallon tub out back that I've used to grow daphnia before and once, by accident, a fry came along with the daphnia culture. I didn't know what kind of fish it was,it was sooo tiny, so I just left it out there and it absolutely flourished with little to no help from me. I never did a water change on that tub, at the most I replaced the evaporation with old dirty fish water from my inside tanks. I couldn't find him the last couple of weeks so I figured I( was too late, the cold must have got him (he'd been in there since spring!) I started to clean out the tub to start over and I found him alive at the bottom! I pulled him out, and the plants, and gave him to a friend of mine to go in her molly tank (turns out he's a gambusia (sp?) ) It had dropped below 40 this week and he still looked great.
Long story short, the fish flourished living outside on his own, even long after he ate all of my daphnia!