Finn will chase almost anything that moves and I did have an incident on a croft where he was very happy springing at all the sheep - Ive not repeated the visit , now older all I can say is that if I let him chase on first sight rather than calling him off then hes much harder to get back. If you ask them to stay, heel or come before they start to run they (mine is) are a lot more controllable.
I can call Finn off birds, squirrels, cats (regular animals) even during/after the chase - im not sure what he wants to do with them but after running across a road ive made a bit of a point of leave commands and general recall which although hes still keen to get back to does have him stay close by.