this is a pretty nice little book, and has a good list of "chicken resistant plants" (p. 141) -- but a lot of the trick is the have the ratio of plants to chickens right, so even if they nip a few leaves, they move on to nibble something else, rather than staying and chowing down.
http://www.timberpress.com/books/free_range_chicken_gardens/bloom/9781604692372
other plants my flock ignores: scented-leaf geraniums, calla lilies, foxglove, thyme, oregano, borage, love-in-the-mist...
edit: all the plants in my garden are also ones that deer don't like to eat, since they are a far bigger plant-predator around here than the chickens are!