from your metadata, it looks as if you are using a zoom wide open and full stretch which will give you minimal depth of field, a iso setting that subject to your particular camera may be a quality compromise and it looks as if this has been heavily cropped which is also a shortcut to loss of quality.
there is no real need for the bird to be this large in frame and the closer you get the less depth of field you will achieve. better to back off slightly, and attempt to isolate the bird against a bg that will easily throw into out of focus to lose detail and distractions.
i do not like taking my d300s Nikon past 500 iso for birds and like to keep any zoom just short of its extremes and at least 1 to 2 stops down from wide open as most lens have a sweet area and its pretty much always not wide open at full reach.
your speed was ok at 400 and if supported you may have got away with a robin at slower but most small birds are pretty twitchy and its hard to freeze them. usually some quality light helps give some contrast in the feather detail and brings the picture more to life.
its no good pretending its easy to get sharp birds particularly all over but in general the beak and eye must be pin sharp. its no secret that serious birders generally invest in prime lenses as they perform much better and often have wider apertures than zooms that areactually usable.
also dont forget that direct from camera your picture may need some levels adjustment and sharpening to achieve a satisfactory upload.