Now, I made doubly sure that tractor was reinforced with extra screws and staples. There was no way they could have chewed their way out or pressed themselves through the wire 1" chicken wire that wrapped around the tractor. What I did overlook was that they were still small enough (beware, Jersey Woolies look much bigger because of that poofy fur than what they actually are) to squeeze through the 2" chicken wire that I used to cover the bottom of the tractor. This would not have been an issue had the tractor been sitting on perfectly level ground, but unbeknownst to me, one corner had been set over a small hole in between tufts of grass. Upon inspection, I saw the 2" hole in the chicken wire that had been made slightly larger from two little bodies squeezing through it, and that's when my heart sank. So, my advice to other inexperienced bunny folks out there would be to always use hardware cloth or small animal cage wire when dealing with rabbits.
I called Lee out for a bunny search party. We found ZuZu right away, who was fine other than looking a little dirty, hopping around in the back section of the yard behind the sheds with her papa, Bugsy. I scooped her up and carried her back to her mama and siblings, when I caught a glimpse of white fur on the ground. Then I saw a little white foot. Then I saw the stray mama cat and her babies finishing up what used to be our little Tribble, only feet away from a full bowl of cat food! I burst into tears.
We buried Tribbles remains, and talked about what we should do. The mother cat is quite a hunter, she's been keeping our garden gopher free, but we decided that wasn't a good enough trade-off to risk our bunnies being harmed. She and the kittens would have to go right away. Unfortunately, it's not realistic to attempt to break a cat of a habit like hunting and eating prey, so we just came to terms with the fact that she would make a better fit in a home where there weren't small animal pets around. Luckily I have found a no-kill shelter that will spay her and spay/neuter the kittens before adopting them out, so things will work out in the end.