Hi, I'm no expert in photography or bird photography for that matter however:

To me, the light looks nice and soft (a good thing), but as Tony mentioned the focus isn't quite there - it's either 'mis-focused' or blurred due to camera shake? The composition is good, (I like the square crop too), with great eye contact from the Chaffinch, and I like the inclusion of seeds in this shot - it adds more colour and some context.

Are you cropping massively from the original shot?
Are you shooting at high enough shutter speeds (should be at least 1/300 with that lens / camera combination I think?
Is there anything you could use to stabilise the camera more?

I do hope this helps a bit!