Polly is a green parrot, originally from Zanzibar. In Swallows and Amazons he belongs to Captain Flint, provides the green feathers for the Amazons' arrow flights and likes to say "Pretty Polly", despite Nancy's attempts to get him to say "Pieces of Eight" like a proper pirate's parrot.
Captain Flint promises parrots all round and one monkey for Roger next year after he has been south to London to see his publisher (SA27), but then he says that would be too long to wait and gives Polly to Titty, saying that He's a young parrot and I’m a dull companion for him (SA29).
Polly eventually learns this piratical skill, along with the rudiments of the multiplication table.
When they are looking at the frozen-in houseboat, the D's hear about the burglary and how Captain Flint gave Titty a green parrot and Roger a monkey because they found his trunk. But Titty says that the parrot is at the zoo. I couldn't very well have him at school and mother couldn't take him to Malta. So he's gone to the zoo (as has Gibber) (WH13).
Nancy says that the animal Captain Flint is sending home from South America is called Timothy. Another monkey? said Roger. Or a parrot?" said Titty. He said he might be getting another (Dick thinks that it is an armadillo) (PP1).