Telephoto lens buying guidance needed for Nikon D500?

My wife has the Tamron which she uses with her D5100, almost exclusively for birding. For the price and range, it is a very impressive piece of kit.

I have no experience with the Nikon 200-500. As stated elsewhere, I would expect it to be slightly sharper than the Tamron. Price is comparable.

As far as the range difference, you'd have to take the same photo with both lenses side by side to really see the difference between 500 and 600mm. It is there, but not as much as you'd think. With a D500, you should be able to crop out half the photo without issue, making the 100mm difference really a moot point.

Renting both would be the way to choose, or go ahead and throw the Sigma 150-600 in there too just to make sure.

If you go to opticallimits.com they have reviews of the Tamron and Sigma 150-400mm zoom. Their analysis shows that the Tamron is a bit sharper than the Sigma, but they don't have a review of the Nikon lens. If I were you, I would go to lensrentals.com or some place similar, and rent each lens. The cost of the rental will be less than the amount of depreciation that you'd take if you had to sell the lens because it wasn't the right choice. Because you can't get a feel for what the lens is like to actually use it, let alone it's AF performance, you really do need to rent before you buy because as a wildlife photographer, AF is going to be so incredibly important. My guess, and that's all it is, is that the Nikon version will probably be a bit sharper and a bit faster in focusing, too. The fact that the Nikon version has a constant f/5.6 aperture throughout the zoom range would enough for me to go with it over the Tamron or Sigma.

I would also recommend that you go to 500px.com and do a keyword search for each lens. It's one thing to look at MTF charts, but something all together different when looking at photos taken in real life situations.

I have the Sigma 150-600 Contemporary lens. At the time my choice (limited by budget) was between that and the Tamron. The Sigma won because of the USB dock that you can use to customise various properties of the lens and to update the firmware.

The customisation is VERY useful. One of the properties you can set is the focus distance limits. I shoot birds from several hides and can set the focus limits to suit the particular location to get faster AF.

Last time I was in a hide, 3 people had the same Sigma lens! One using Canon (me) and 2 Nikons. We all agreed that the Sigma was an A1 piece of kit.

You buy the most mm you can find at the lowest f/number you can afford.

You don't buy lenses just to replace them later. You add lenses to your collection!