At the very least you should know which field marks to look for, but don't stop there. Discuss what other species should be considered as candidates for the ID, and which field marks you used to eliminate them. The length and amount of detail should fit the rarity of the species and the difficulty of identifying it in the field. For example, if you saw a female Red-breasted Merganser on a lake where they have often occurred in the past, it is probably sufficient to mention the lack of contrast between throat and breast colors, although it wouldn't hurt to discuss bill size and shape as well. For a first state record or something almost that rare, though, several pages of details are not excessive.
Few birds are really that close to the pictures in the field guide.
Many species have a lot of individual variation. The pictures often exaggerate differences, or they are schematic to some degree. Sometimes the artists just don't get that good a likeness. Descriptions of the observer's reputation. We want to know what the observer saw, not how confident they were or how much others admire their abilities.