Interesting.

Seems there would have been some level of redundancy considering you dont know if your pigeon made it, like this guy (or gal) who somehow ended up in a chimney.

I would guess you may send off multiple pigeons with the same message.

And once the first message was received, subsequent messages were responded with a simple "Q".

Typically yes, as conditions allowed. The supplies of carrier pigeons weren't necessarily unlimited.

There's also the somewhat difficult logistics -- typically carrier pigeons go one way -- from wherever they are to their home coop. So once the bird is back in England (from France, say), then you have to transport the bird BACK to France manually. You can't just say "ok, say XYZ and send the bird back".

All that said, they're great, and were used by the military for thousands of years.