Your background gives a lovely feeling of depth to this drawing. It makes me feel as though I'm there on the bird's territory - rather than the bird being on mine.Robins are quite a dark brown and I'd like to see a little more strength of tone here.
As much as I like the twig I think it's competing with the bird for attention. A little darkening of tone to the head and back might overcome this. But I appreciate that this really is a difficult subject as the red breast is almost the same tone as the brown - you've defined the lower edge of the red very well - and I can see that darkening the head and body might compound your problems 
This is difficult to explain but my eye tends to want to read this bird as being green or blue or even grey but not dark brown.
Apart from that minor criticism - I think you've done a splendid job.
MIKE
www.SibleyFineArt.com
www.Starving-Artists.net