My previous description for this image was wrong. Down below you can find the old version.
As namu-the-orca ([link]
) pointed out, this bird is an fact a Goldcrest (Regulus regulus) and a real european bird in contrast to my previous claim (the two species are looking nearly exactly the same). Thanks for the correction!
It was a little bit difficult to figure out that this little fellow is a Golden-crowned Kinglet (Regulus satrapa), because according to Wikipedia and other sources this bird only lives in north and central America.
Regarding the fact, that this photo is a "urban wildlife" shot made in Germany it isn't very hard to imagine that bird identification websites for german species weren't very helpful in this matter.
But because I found no information about this kind of bird living in Germany at all, this photo fascinates me even more.