The call of a curlew always works for me, too. And skylarks. Not a fan of herons though, ever since I saw a pair jobbing a moorhen's nest by the canal in Silsden. Yes I know they have to eat, but can't they stick to fish???
I'm sure there must be otters on the Wharfe, I know there's some on the Aire near Bingley, but it's years since I saw one in the wild. Now that would be a real tick on the 'Wildlife to See' list!