I would not trust 1x3 at 6 feet long with chickens on it. It bows just being carried around before you even install it! Plus, this size board is usually sold as furring strips, which are made of low quality wood and aren’t meant to be durable. They have a lot of knots and imperfections in them and snap easily.I use 2x4 and they actually prefer that over the 2" round bar. 1x3 if its laid flat shouldnt be an issue depending on length, it may bow... But youd have to have some really fat overweight birds to break it even at 6' length.