Why do dogs howl at sirens?

Most likely because a siren resembles a howl, and howling is a social communication behaviour dogs inherited from wolves.

What howling is for. In wolves, howling serves several purposes: locating separated pack members over long distances, assembling the pack, and advertising territory to neighbouring groups. It carries far better than barking, travelling several kilometres.

Why sirens trigger it. A siren's sustained, wavering, high-pitched tone sits in a similar acoustic range to a wolf howl and shares its key features — long duration, pitch modulation, and carrying power. The most straightforward interpretation is that the dog perceives something howl-like at a distance and responds as the behaviour evolved to work: by howling back, announcing its own position, or signalling to its group.

Competing explanations, none conclusively settled:

Territorial announcement — responding to a perceived distant intruder.

Contact calling — attempting to locate whatever produced the sound.

Discomfort. The idea that sirens hurt dogs' ears is widely repeated but weakly supported. Dogs hear higher frequencies than we do, but a dog in pain would more typically flee, hide or flatten its ears than sit and vocalise at length.

Learned attention. Howling reliably produces a human reaction, which can reinforce it.

Why some dogs do it and others do not. Breed matters considerably. Huskies, malamutes, beagles and hounds howl readily — the latter having been selectively bred to bay while tracking. Many breeds rarely howl at all.

Other howling triggers: music, harmonicas and singing, for the same acoustic reasons; and separation, where a dog left alone howls as a contact call, which is worth distinguishing since separation-related howling with destruction, pacing or toileting suggests distress rather than instinct.