It's probably one of the oldest things she owned.
I don't know for sure who shot it, but I do know it was a family member on her mother's side. I'd therefore guess it was either her great-grandfather, John Wighton (1828-1873) or her great-great-grandfather, John Wighton (1800-1873) who did the deed. I'd go with the latter, as he was a farm labourer whereas his son was a stonemason.
My niece Louise is due to inherit this piece of taxidermy, as she's the youngest of my mother's grandchildren and all those older than her rapidly passed on it. Unfortunately for her, she doesn't get to pass on it. That said, the case hasn't lasted as well as the bird inside, so if she doesn't want it I expect landfill will.
I don't know where Amber Gate is, but expect it's on an estate somewhere in Perthshire.