Jo, count yourself lucky that you don't live in a busy part of Phoenix where the sirens go off frequently, day and night. My dogs bark at anything they see or hear but when the sirens go off, so do they. It is a chorus of howls to see who sounds most like the siren. They didn't always do this and I think they must have gotten started by neighbor dogs' siren songs. It can be annoying as all get out, but sometimes I just howl along with 'em. You should see how embarrassed they are for me when I do that!
