Bitterns

We have recently returned from a holiday on the Riemore Estate in Perthshire. Every evening we listened to 3 distinct Bitterns booming across the glen. Is this a first for this area? There was a lot of activity on the Loch of Lawes but nobody had known Bitterns in the area. Once heard the sound is never forgotton, it was haunting and we were very lucky to be there.