Okiron is right, I've got 4 Borders and only one might chase a chicken, but he'd chase anything that runs. If they are trained, or if they are the type that obsessively herd anything by just staring and doing the Border Collie down, he would just stay along the perimeter of wherever the chickens are and watch and try to keep them contained to an area. If the dog does chase chickens, you've got to come up with a way to discourage that behavior without the dog thinking that herding is a no-no all the time. Three of my Borders have a high-drive for herding sheep, but I have 8 goats and 2 sheep and they act like they don't even see them in the yard, or that they are in the way of our Frisbee game, but if I take them to a regular herding trial area, they are rabid about wanting to get in and herd some sheep.