i had a lab that did nearly the same thing, he used to bark at the rabbits pasing by. You should make her learn by a command to be quiet, maybe you should distract her with her favorite toy and when she shuts up give her a yummy doggy treat to keep her happy and make her understand that u want her to be quiet, toys can keep her busy for a while. Maybe she barks because she is bored and is trying to get attention speacilly to you.