i would look for a worm of the same color... usually hanging on the undersides in daylight

and i would also.. gently.. search down into the freshest.. softest.. most luscious leaves... which are already being damages ..

have you ever seen a JB on it???

ken

Ken
I have never seen anything on the plant. Something just seems to be feeding on it when I'm not looking.

I did a quick check of the plants and didn't see anything there. The plants are pretty much lost now, I would just like to know what I'm dealing with.

That looks like the damage seen when the Imported Cabbageworm or the Cabbage Lopper, both the larva of small moths. Both of these are a light green color that is very close to the color of the cabbage leaves and they can be difficult to see.
Although somewhat damaged I would not consider them to be lost at all, and often people will harvest Broccoli and soak the heads in salt water to remove these wee buggers.
Look carefully at those leaves, again. If there truely are none there then the birds are doing pest control for you.

Here is a link that might be useful: Cabbage worm damage images