I think that sometimes with über sharp photos, a modern camera picks up bits that the eye does not normally see. The light is often critical in that it highlights different aspects at different angles and intensity. What appears at a quick glance, on feather pattern alone, to be Song Thrush, turns out to be Mistle Thrush when other aspects as described above are taken into account. Another point as raised by Alexander, is it is a split second image that you have all day to look at. Modern DSLRs can take upwards of 10 exposures a second at the highest rate. Compared to an artist's impression found in field guides, that depicts an average bird of the species, that may take from several hours to several days to get right.