See for yourself.
Looks to me like someone has some explaining to do.
enSue has pointed out that the bird also shows up in a WordPress theme by Randa Clay.
This is leading us to believe that the bird is a stock graphic available for anyone to use. This also means that there is no Twitter bird, just a bird that shows up on Twitter’s homepage.
If you head over to iStockPhoto, helloyiying is more than likely who created the original graphics. This artist has many Twitter like settings and even a graphic of the now famous, fail whale. Fail whale, and the bird above, no longer seem to be available though. Maybe someone bought them outright.