Some context for puzzled readers:

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).