Combinatory logic is a way to express logic without using variables and quantifiers (e.g., no "x", no "forall x").
In combinatory logic, formulae are strings of combinators: (e.g., "SKK").
To help introduce readers to combinatory logic, some people think it is helpful to have analogies where a combinator (like S) is explained as a type of bird.
1) using colored shapes instead of variable names.
2) (initially) presenting combinators as having 3 arguments, where they only need 1 or 2. So instead of the usual M x = x x, you see M x y z = x x y z.
One (improper) bird I find very interesting is the Starling's cousin S* x y z = x z (y (K z)), from which all other birds can be reconstructed.
This site uses combinatory logic (https://en.wikipedia.org/wiki/Combinatory_logic).
Combinatory logic is a way to express logic without using variables and quantifiers (e.g., no "x", no "forall x").
In combinatory logic, formulae are strings of combinators: (e.g., "SKK").
To help introduce readers to combinatory logic, some people think it is helpful to have analogies where a combinator (like S) is explained as a type of bird.
To my knowledge, this was first suggested by Raymond Smullyan (https://en.wikipedia.org/wiki/To_Mock_a_Mockingbird).