Congratulations on the new camera.
For a critter lens the 400 f5.6L won't disappoint if you learn to shoot in Manual mode and shoot wide open. The 300 f4L might be a good idea and you could couple it with a Tamron 1.4x multiplier for more reach. With a solid rest the 400 f5.6 is great with the Tamron 1.4x for 560mm f8, but again a steady rest and some good light.