The S and K combinators are simply referred to here as S and K, but the pictures of them resemble a Starling and a Kestrel respectively. This is surely a refer

The S and K combinators are simply referred to here as S and K, but the pictures of them resemble a Starling and a Kestrel respectively. This is surely a reference to Raymond Smullyan's To Mock A Mockingbird, a ... unique ... book in which combinators are personified (avified?) as birds.