The goal of a scientific illustration is to communicate specific information, such as the parts of a cell or how to identify a bird. In these illustrations, the artist emphasizes some information and deemphasizes other information. You show what you need to show for your specific purposes. In the case of a field guide illustration, a bird is positioned at an angle that will highlight key features and other details such as shadows are minimized or eliminated altogether. Illustrators usually light the subject from the top left. In this Blue Jay study, I rotated the bird so that you can clearly see the bar across the chest. I added a slight shadow to help indicate the three-dimensional form of the bird but not enough that it would distract from the plumage pattern or possibly read as a dark pattern on the belly.
The purpose of this demonstration is to highlight the painting technique, not the drawing process that made the bird (which is only shown in three steps). If you want to follow along, you can download the accompanying worksheet, or redraw the bird yourself. Click on the first illustration to start a step-by-step slideshow.