Why Does My Dog Howl?

This is one of my dogs. Her name is Curlie. She's a black lab mix rescued from Georgia by Strays and Others of New Canaan (thank you Claudia Weber) and adopted by me. She is perfect in every way, but she has a habit that perplexes me.

Curlie howls every time she hears a siren. It might not be worth mentioning, if not for the fact that we live two blocks from both the police and fire station in Greenwich.  The stream of sirens is constant. 

Growing up in Darien, my family had a small Poodle, followed by a Doberman, a Great Dane, two Dachshunds, two Standard Poodles and a Weimaraner. I've owned a CRAZY Basenji-mix named Chip, followed by Buddy who is a Pit Bull mix rescued from Greenwich Animal Control (he was even a therapy dog in Greenwich for a few years), and, now, in addition to Buddy, we have Curlie.

Of all those dogs over, I never knew one who howled, so I really just wonder. Is it fear based? Is she communicating with what she thinks is another dog? Do her ears hurt? 

Does anyone else have a dog who howls? Is it a trait particular to black labs? What sets them off? Sirens?  I am just curious. Thanks.