Page "Retroactive continuity" Paragraph 21

Another example of this would be in the character Birdo from the Mario series.

Birdo was originally intended to be a male who pretended to be a female in Super Mario Bros. 2.

However, every game since then has stated her as being a female, the reasoning behind this is likely that the ending of the game revealed that Mario had been dreaming, therefore the game doesn't necessarily play into other elements of the series.