I keep waiting for the music to start! Very fun capture with great pose, splashes, angle in frame. Nice job on whites, which can be really tricky with all the reflections.
How big of a crop?
I often give myself assignments on outings. It is so easy to get distracted by all the different options, that you don't learn much on a specific subject. Good plan.
Yes, the longer lens is nice. But, I have seen a lot of outstanding images from the 400 f /5.6 lens, so keep working it!
I love the sense of action elicited by this pose, with the outstretched wings and water splashing of the legs. Exposure and sharpness are spot-on. The soft water and reflections add to the image. And the under-wing reflections are quite interesting; nice to get some additional light there.