It's an eagle...not a chicken folks
If you see a two-headed eagle, then you're looking at the symbol of the Holy Roman Empire. If you're looking at an eagle with only one head, then it's the symbol of the Hohenzollern dynasty, or the royal house that dominated Brandenburg, Prussia, and the later German Empire (also included is Romania and of course Hohenzollern). The Hohenzollerns lead the state of Prussia to dominate the Second German Empire, and lead to Prussia's dominance over all over states. Look here to see the massive symbol painted on the Fortress gate entrance!
