No, sunflowers are not poison to chickens. I would recommend you plant the seeds where the stems will be out of reach of those sharp little beaks. Nibbling the leaves won't kill the plant, but pecking holes in the stem will. Also, if they are the tall variety, plan to stake each one with a bamboo or dowel stake. The heads get very heavy and a good wind or rain will blow them over easily.
You said you were a vegetarian and your friends thought you were insane? Someone on this forum (I can't remember who) replied this way to the same issue:
"Good garden 'o peas, ma'am...WE'RE ALL INSANE! That's why we're on this website talking to each other!"
It was so funny and so true I've never forgotten it. My theory is: Say NO to drugs, say YES to chickens!