The Bird is a great little institution in Berlin and I always recommend friends to come here when visiting Berlin. The place is always packed and often quite loud, perfect for a steak and burger place with cheap beer!! Always make a reservation unless you are happy to sit in the bar.
As pointed out by other reviewers before me the fries are ok but not that great. But who cares when, in my opinion, the burgers are the best in the world?! These burgers taste really BBQ'd and are best when served medium rare to rare. Your biggest problem if just passing through Berlin will be to chose which one of the various burgers you are going to order! My tip - The Fat Stingy Gonzales with guacamole, this burger is to die for!!! Other ones that stick out are the The Filthy Harry and the BBQ burger.
The Bird also serves some of the best steaks in Berlin but compared to the burgers the steaks are fairly pricy. Having said that, good quality meat does not come cheap. Simple as that. Personally I always go for the NY Strip when having steak here but have also tried the filet and this is also very nice.