There are a few conventions that help illustrators make useful identification drawings for field guides. Real birds do not look like the illustrations in field guides. The field guide illustration is an attempt to average characteristics of many individuals into a “typical bird”. This bird does not really exist. There is tremendous variation between individuals even within a single sub-species. If you can scan study skins in a scientific collection you will see this variation immediately. Start looking for differences within a common species and you will see this variation as well. For the purposes of a field guide illustration it is helpful to average and blend characteristics between individuals. I do not recommend this when field sketching where the goal is to observe accurately and record what you see.
In field guide illustrations, the light source is usually from the top left corner and the bird is often placed facing into the light so that the patterns and colors of its breast will not be in shadow. Shadows are deemphasized so that they are not confused with patterns and colors. Just enough to convey the contours and volume of the bird. Background details are often deemphasized and are arranged so as not to obscure important details on the birds.
Birds are often shown in similar positions, often in profile allowing viewers to compare proportions without the confusion of foreshortening.The illustrator needs to strike a balance between standardizing the positions of the birds to help viewers compare between drawings and showing characteristic and lively postures.