If a dog is going to be attracted to a chicken the chances are increased I believe when they are in the chick stage. Maybe because they are so small and fast the dog just reacts to their natural instincts. Both of my labs are soft mouth but if they were to catch a chick the chances of survival would be slim to none. The longer they are exposed to chickens the more they accept them as part of the family yard or farm animals. I doubt if the dog meant harm but just reacted, now knowing the consequences you must assure it doesnt happen again it is just a learning process for all, it happens it part of life live and learn. Good luck