In The Story of Edgar Sawtelle, dogs are main characters in the plot of the book, particularly one named Almondine. Almondine may be a work of fiction, but one that is very dear to my heart, as is the entire story.
In the book, the author makes it clear that the dogs are able to make a choice at pivotal points in the book.
That's how I feel about introducing new chickens. When they feel ready to join the flock, I want them to be able to do so.