Moorland Bird Call

Was listening to moorland birds (Scottish Borders) yeaterday evening,  Lapwings, Curlews, Grouse, Oyster Catches, Sky Lark etc.  But I was also hearing a bird I could not identify,  it was a much deeper sound (but not a Bittern)  at first I wondered if it was a Lamb but it wasn't, I'm sure it was a bird.

Any ideas of what it could have been.