5Dlll Rental and Canon 70-200 2.8 IS

Did you rent the 70-200 f/2.8 IS II or the original IS version? The original IS is suppose to be a bit softer at f/2.8 than the IS II.

Using this lens at f/2.8 requires very good focusing technique. Focus and recompose does not work well with this lens at f/2.8 (no problem at f/8). You really need to get the focus point right on one of the eyes. The 5D III has enough focus points that you should be able to do that, but it takes some practice to develop good technique.

In the pictures of the pelican, it is hard to tell what the issue is. What shutter speed and f/stop where you using? What focusing settings? Where you using AI Servo?