Dave, your shot of the bird's nest looks admirable. It would be safe to say your effort was well rewarded.
I found my sample is not particularly sharp at the closer range. The plum fruits were shot with non-IF ED 400/5.6 at almost the closest focusing distance of 5m wide open. The first one is the entire frame, and the second one, the central part which is not resized. You see a slight purple fringing. Shot with the 24MP SIGMA fp with no AA filter. No sharpening is applied.
I would figure the lens doesn't shine at closer ranges as much as at distance.