at 300mm on a wide ranging zoom, image quality isnt going to be stella at best. F8 would be the peak here.
I dont see too much grain here, its rather more not nice bokeh. I think it would be ambitious too, zooming in on the bird, even if it
had been shot on a D800, considering its so tiny in the picture. Your problem here was simple really.
the bird was simply out of range for your lens. The bird is so tiny in the picture I doubt it would fill the screen at 100%.
When shooting things so small, you still need to be fairly close to get a decent pic even with a 300mm.