In the IDW comics series, Jimmy is a pigeon who is Pete's non-mutant cousin, and friend to Pete and Slash. As a non-mutant animal, Jimmy cannot speak as humans or humanoid mutants do, but he can understand their language, and communicate information from Pete.
Jimmy was introduced in TMNT #65, where he informed Pete that the Turtles were planning a Christmas party. He re-appears in issue 67 where it was revealed that he has an appreciation for fine literature when Slash was reading The Grapes of Wrath by John Steinbeck and urged Slash to continue reading, with Slash understanding his coos as if they were in perfect English. It is very clear from the outset that Jimmy is far smarter than Pete.
When Slash was captured by Bishop's forces, a panicked Jimmy immediately flew to the Mutanimals and attempted to warn Pete of what had happened.