Non-Human Anatomy

A pencil sketch of a Mallard. Immediately one can see that it is resting which is shown by the attention given to the head and breast. It is possible to get a feeling of implied line, travelling right to left, which is met with nothing which finishes the image at the wrong focal point.

Better observation of the markings would improve this images

Figure

Another pastel drawing this time showing form.

More care has been taken with the arms, shoulder and head area keeping it well proportioned.

It is obvious in some places where observation wasn't focused, this is especially obvious with the left leg, I believe was caused by inexperience with that particular pose/leg angle.

Better observation on the shadows helps the image quite a lot in the upper third.

Figure 

This pastel drawing is showing the major differences between shadow and light.
The general proportions of the model appear correct and the S/L seem quite well observed. 
On further inspection, the left arm looks rushed and the lack of shadow on the head gives the model an un-human feel.

Figure 

This skeleton was drawn quite quickly, due to this, the time spent observing has suffered which takes a lot away from the image. The limbs are out of proportion and the curvature of the spine is less than perfect. Paying closer attention to the reference pays off.