Prussia indeed had an eagle.
I know it says that Gilbird portrays the Prussian Eagle, but let's think about this.
Gilbird is too small for it. Also Gilbird is a regular bird, there's no way Gilbird could live throughout all those centuries.
Back to Prussia's eagle.
His eagle was black and always flew above Prussia when he was outside, mostly battling.
The eagle died when Prussia was first dissolved 1936 if I remind it correctly.
Prussia was really upset about it, he liked his eagle and cared for him really well.
Since Prussia is in some way a micronation, Gilbird could be like the "new" Prussian eagle.
But Prussia will never forget his previous bird, as much as Gilbird tries to hold him up, he will never forget it.