If there was also leaf damage then slugs would be my first guess. But without that then the lizards and/or turtles come first to mind as they are nibblers. Rabbits or groundhogs wouldn't stop with just a nibble. Squirrels and deer will also nibble. Dave

Caterpillars, prolly cabbage looper. Bt should be effective or sevin.

Keep an eye out for those tiny green cabbage moth caterpillars too. They are so hard to see, but their droppings often are easily seen. I good spraying with Bt, the natural bacteria that kills only caterpillars will take care of them. They could be grasshoppers for sure too. If it's the caterpillars, the Bt will take care of them. Keep us posted! Good luck identifying the culprit, hah.

I was going to ask if you had deer in the area but you already mentioned it a couple of entries ago. When I looked at how neatly all of the leaves of your broccoli were nipped off the first thing I thought of was deer. We live way out in the sticks, so to speak, and have a lot of deer. This evening I saw 6 does out nipping off the tops of my wildflowers. This spring a doe and her yearling fawn managed to break through a weak spot in the 8' fence we have a around the garden. All of my broccoli and brussels sprouts were nipped off as cleanly as yours appear to have been. I'm betting your local deer are the culprits. The cages you built should take care of the problem nicely. Good luck with your next crop of broccoli. Actually, if you haven't pulled the damaged plants yet they may recover. Mine did and I'm harvesting wonderful plate sized heads of broccoli now. Gail