Differentiate between tallness and bulk. If there is no metre-stick for comparison, say so. Describe the shape using the same method. Describe what the bird did. If it flew, describe the flight pattern.
Vocalizations are important, even the smallest chip note.
Try to be as precise as possible, although admittedly describing a call or song is very difficult. This is not considered valid evidence. Use phonetics if you are familiar with them. However, I have rarely heard two people describe the same song with the same phonetics. In this section you should explain why you believe it to be the stated species.