Didn't have a look here inside for a while...
First of all: Thank you all for your comments
It hopefully helps me to avoid mistakes at my next paintings. After all I'm not happy with the composition, too. Too much background over that bird.
Mostly I'm having problems with taking photos of my paintings. The background of this one isn't really that black. As there was always an ugly reflection in the photo, I actually worked at the background of the photo with photoshop. That was the biggest mistake I made. And I also darkend it too much. The original painting isn't that dark. Except the dark part underneath the beak, it's really black