Any ideas on what is munching at the leaves of my Hardy Chicago?

I would guess June Bugs, at least that's what eats mine. I went out last night and found another one, fed it to the chickens this morning. I would go outside after its dark with a flashlight and you will see the culprit =)

check for slugs as well, this is the first time I have ever had problems, they are eating my cabbage like crazy.

There are probably many choices.  I found what looked like a small tomato hornworm that had been eating some of mine yesterday (a first!), but the edges are more smooth than on your leaves.

 

Those hornworms are terrible, they can a plant a day of tomatoe or peppers.  I am glad they aren't in big numbers here and are easily killed with bt.

I've been noticing a bunch of leaf hoppers buzzing around here also.  Time to bust out the Neem...

  

Nate