The images below are from a test of our two birding lenses, the Canon 400mm f/5.6L USM prime lens and the Canon 100-400mm f/4.5-5.6 IS USM (Mk I) zoom lens. The tests were shot from positions 2ft apart of a stationary target 20ft away. The log target has fine details similar to a bird.
Test images were taken with the same Canon 7D II camera set to fine resolution center AF and exposed at f/5.6, 1600sec, ISO-800. Both shots were taken on a tripod with a cable release but mirror lockup was disabled. IS was turned off on the zoom lens. The CR2 images were imported into Photoshop CS6 with the default conversion parameters. Same 100% crop was applied to both images but no further processing performed. Images were saved as a single highest resolution 8bit JPG at original resolution. Both contrast and resolution are clearly superior in the prime lens. Though this difference is not critical for frame-filling subjects, it is noticable on tight crops of small birds.