Dogs howl together to show the strength of the pack. I asume sirens sound like a pack and the dogs are responding the way they know how.
I would not agree on hurting the dogs ears I watch this same responce every time sirens come by my place the dogs do not look in any discomfort they actually get excited and start howling all together
Same at my place, i have a rail road 500yds from my front door and fire station 750yds and mine ALL howl at the sound of either. I dont think it hurts their ears personally, i may be wrong tho
