I have a friend who keeps a free ranging flock and has a pit bull terrier, a griffon terrier mix and a Chihuahua. The larger two are fantastic with her flock and are very protective of them...the Chihuahua on the other hand, chases them relentlessly any chance he gets and tries to attack them even though they're three times his size. She and her family do their best to make sure he doesn't get out of the house unless it's straight to the dogs' fenced area but once in a while he'll slip out the front door and he's so little and quick, he's not easy to catch.