Very lovely image and composition. I agree with Ramon's suggestion of off centering the warbler and as far as the BG the only thing I find distracting is the one light green leaf to the left of the bird. Many thanks for sharing.
I agree that the image looks dark. I love the straight on glance and the beautiful flowers out of focus on the left. To me the upper right looks just a bit funny like some cloning or a a quick mask was applied...maybe I'm just seeing things.