I thought they were poisonous and was horrified one spring to look out and see that all the leaves had been gobbled in a couple of hours. In previous years, the hens had not touched them. Expected the hens to all pop their clogs but they were fine.
Read at a later date (but I don't remember where) that hens would eat rhubarb at certain times of the year in order to self-medicate and that they were probably a natural wormer.