I have both zoom and prime 600mm lenses. There is no question at all but that the prime consistently produces slightly sharper pictures of birds.

The Ameristep pop-up blinds are great, and they are not expensive.

I just took my D810 in to be serviced because, through no fault of the pilot, the fn-mode button at the bottom of the front right of the camera just ceased to work. So, stuff does happen.

I don't have the Sigma Sport Model. From everything I have read, it is markedly better than the Contemporary, which is what I have. I recently purchased the Nikon 200-500, and noticed a marked increase in the focus acquisition speed, when paired with the D500. I don't know whether I would notice it so much if paired with the D810. However, when the Nikon 200-500 is paired with the D500, it acquires and retains focus much more quickly than when paired with the Sigma Contemporary. The Nikon lens is night and day better. As for sharpness, I cannot tell that there is much difference.

I think both are, or can be, U.S. lenses. As far as I can tell, if you are using PocketWizard, there is no difference other than price. The L-478 DR only supports PocketWizard. The L-478 DR-U allows you to pick one of several remote flash units, including PocketWizard. So, if you are going to use it with PocketWizard, I decided it was better to get the L-478 DR and save $50. Am I wrong?

The way this has been advertised on the B&H Website is very misleading, and arguably just plain inaccurate to boot. It made me think that you really need to watch them, that they are not being above-board on this one. You think you are getting the L-478 DR-U at $339, when you are really only getting the discontinued brand. My conclusion was that it didn't make any difference for PocketWizard users, because the discontinued item was the same as the newer, more expensive, light meter, IF you are going to pair it with PocketWizard. Still, I thought it was shady the way it was being advertised.

I absolutely love mine for bird photography. It acquires and maintains focus faster than my D810. It is a pleasure to handle, which is to say great ergonomics. Even at 10 frames per second there is no longer a buffer issue. It is almost identical to the much more expensive D5, but for the fact that it is not full-frame, which is not a bad thing, some say it is even desirable. What's not to like.

I have read half a dozen books on Lightroom, and can say unequivocally that Tony Northrup's "Adobe Lightroom 6," especially the Kindle version, is head and shoulders above the rest for starting out.
https://www.amazon.com/Adobe-Lightroom-Video-Book-Photographers-ebook/dp/B00WZS916E?ie=UTF8&qid=1465320035&ref_=tmm_kin_swatch_0&sr=8-1

Well, that's good to know. And all this time I thought some lenses were sharper than others. What a relief.