Both pictures (same bird minutes apart) are cropped from the pictures below and lightly sharpened in Photoshop. Additionally both pictures are taken handheld and not in the brightest of days. So nothing overly scientific about this comparison. Now all I have to do is figure out how to get a little closer to these shy birds so I can get some nice crisp photos of them!
Nikon 18-300 at 300mm
Sigma 120-400 at 400mm