I would agree with the guess that the pitch of the sirens has changed. Funny occurrence: I live very near some watershed property (deep woods). Every now and then at night you will hear a lone coyote howl a call that it caught something .... and every dog in the area gets hyterical with barking ... like they are all tough guys ... and then.... the pack of 12-14 howl back ... and dead silence ... like you could hear a pin drop ! To me, its funny every single time !!