Why do the dogs bark when they listen sirens

REASON
But when the howling occurs as a result of the sound of a siren, the origin is different and is linked to its behavior as a herd animal that it is.

Most experts agree that this howl could be a kind of response to the sound of the siren, which resemble the call of the herd. Therefore, it is nothing more than a behavior that responds to your own canine instinct, a way of communicating with others of your species.
Given this, we can not do anything, it is something innate in our dog. If we see that he gets very nervous, we can try to distract him with toys or with some cookie so that he stops concentrating on the sound. Of course, it is good that we observe if the howl is accompanied by a strange behavior that may indicate a problem.