I'm not sure really. Could be camera shake? Looks like you shot at 1/60s, which is quite low, unless you had support? Having said that I also shot at 1/60s, handheld, at twice your focal length (400mm), but I have IS to help me out a bit. I suppose it's how steady your hand is.
BTW, my shot is full frame, apart from the square crop. Robins usually let you get reasonably close to them, if you're careful. My technique is to take a few shots, take a step forward, take a few more and repeat until he flies off!! There are far more experienced bird photographers here. Maybe one of them can tell you where you are going wrong?