"one dog is left behind..."
It's a matter of what you decide gives moral value. Unlike chickens dogs do have emotional relations with humans, you can read and react to their emotions and they can read and react to the less subtle human emotions. Also dogs, if property trained, understand the concept of duty (most understand it much better than humans) which is the keystone of the traditional pre-romantic value system. On top of that dogs are pets so we make a commitment to care for them when we adopt them into our families.
Chickens have sensation but that's about it and a moral value system based on the capacity for sensation is demonstrably absurd.