no, rat snakes are not poisonous.. he probably went into a 'strike mode' to scare you cause you probably gave him a bigger scare :) -- they eat rats. LOTS of rats ----unfortunately they eat eggs & chicks too...
i would think they (the snakes) are more after the eggs and warmth than a chicken (well maybe a chick or two) -- the chickens are probably not overly happy being near a snake den just on general purposes BUT i would still rather have the snakes to RATS (yecch) or mice - they do FAR more...
telling the difference isnt really all that hard.. primarily you are looking at their head and markings. its SAFER for you to Know the difference.
this may be of help http://maryeaudet.hubpages.com/hub/Rat-Snake-or-Copperhead--How-To-Tell-the-Difference
Personally i Never advocate killing a...