A couple of years ago, after I added Rhubarb leaves to my list of 'Poisonous plants' for chickens, my lot decided they would teach me a lesson and eat all of the new growth of rhubarb leaves in the garden
I was worried at the time but they were fine.
What is interesting about this is I've never seen them touch the older leaves - but then researching a little, it turns out that the Oxalic acid is much stronger later in the year when the leaves mature...
So do they know this and if so how? It's not like they have been bred for thousands of years with access to rhubarb leaves and learned not to eat them. I'm guessing it must be through taste and smell.