This is quite intereseting.
It's no coincidence that the author Raymond Smullyan used birds to model combinators. Combinators were invented by Haskell B. Curry and he loved watching birds.
Here's an article published in Reading Eagle, a newspaper in Reading, Pennsylvania, on Dec 2, 1967 that interviews Haskell Curry about his hobby. It's quite funny. Read it.
Until next time!