From Wikipedia (sources uncited so this is by no means definitive):
Despite the widespread speculation that Tweety was female, he is and has always been a male character, something that he often has confirmed in The Sylvester and Tweety Mysteries. On the other hand, his species is ambiguous; although originally and often portrayed as a young canary, he is also frequently called a rare and valuable "Tweetybird" as a plot device, and once called "The only living specimen". Moreover, the title song suggests that it is a Canary. His shape more closely suggests that of a baby bird, which in fact is what he was during his early appearances. The yellow feathers were added but otherwise he retained the baby-bird shape.
