The apparent meaning of the author’s words is that there is no difference whether the bird is startled or not. That is, there is no difference whether he startles the bird when he opens the cage or does not do so; nor is there any difference whether, when the cage is opened, the bird moves to one side of it and then flies away after the one who opened it has left, or whether it flies away while he is present. For some birds, when the door is opened, retreat to one side of the cage out of fear of the opening door. Thus, the apparent meaning of the author’s words is that whenever he opens the cage and what is inside flies away, he is liable for compensation.
The modern equivalent of a cage is a wire enclosure. Thus, if there is an enclosure containing birds and someone opens its door, after which the birds fly away, he must compensate for them, because he is an indirect cause. But if a direct agent joins him, liability rests upon the direct agent. For example, if one person opens the door and then another comes and startles the bird, causing it to fly away, the latter—the direct agent who startled the bird—is liable for compensation.
His statement, “or a door,” means that he opens a door and what is inside escapes. Thus, if he opens a door for, for example, a sheep, and when he opens it the sheep goes out and perishes, the one who opened the door must provide compensation, because he is an indirect cause, and the indirect cause is liable for compensation. But if a direct agent joins him, liability rests upon the direct agent. That is, if he opens the door and then someone comes and takes the sheep out, after which it perishes, liability rests upon the second person, because he is the direct agent. No liability rests upon the indirect cause alongside the direct agent, unless the direct action is founded upon the indirect cause.
His statement, “or unties a stopper,” means that if there is oil or honey in a container and he unties the stopper of that container, causing the oil or honey to spill out, he must provide compensation, because he is an indirect cause.