billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

Really, there is not THAT much difference between the D850/200-500 combo and the A9/100-400 combo. In fact, I have shot both, and I have a vertical grip on my D850 for balance. I like the solid feel of my D850/w vertical grip with the 200-500 lens.
I personally find the A9 with the 100-400 awkward to hold and a little front heavy.
But you should really check out the new 200-600 Sony lens, better reach.
You may also want to check out the newest lens I am using on the D850, the new Nikon 500mm f5.6 E PF ED VR. It is much lighter, I hand hold and find that combo really nice to use, and this option would be a lot lighter than the 200-500 combo.
Below is a shot taken with the D850 with vertical grip and the 500 5.6 E PF ED VR lens hand held. It shows a Wood Stork trying to remove a young Cormorant from it's nest, the shot is entitled, " David vs. Goliath.
If you want to see a good comparison of the two lenses please feel free to come to my show in Youngstown this month.
My photo show will be at the Weller Gallery at the Davis Education and Visitors Center in Fellows Riverside Gardens, Youngstown Ohio. 123 McKinley Ave. Tuesday-Sunday 10-5. Closed Monday. 330-740-7116. The show will run from August 17th to October 13th. Meet the artist date is Sunday, September 8 starting at 11:30 am.

Canisdirus

The bulk weight of any combination is going to be in the lens, not the body.
As with almost any long lens ... if it has heft ... it's going to be a better lens.
I wouldn't worry about losing a .lb by switching to an entirely new set up.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Someone's got some 'splainin' to do, huh?

https://www.nikonusa.com/en/learn-and-explore/a/tips-and-techniques/reach-for-the-sky-a-breakthrough-telephoto-is-a-perfect-fit-for-the-fast-action-of-bird-photography.html

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Couldn't agree more!

_DSC3649-DSC-RX10M4--(17-12-18) by Gene Lugo, on Flickr

_DSC2876-DSC-RX10M4--(19-08-18) by Gene Lugo, on Flickr

_DSC2662-DSC-RX10M4--(22-04-18) by Gene Lugo, on Flickr

_DSC0571 by Gene Lugo, on Flickr

_DSC0061-DSC-RX10M3--(28-11-17) high res by Gene Lugo, on Flickr

mizzee Loc: Boston,Ma

Yes! I have an Oly 4/3s. The 400 is equivalent to 800 on my camera as the two cameras are sort of similar. I looked at the G9 but the Oly felt better to me. What finally sold me on the Oly was the 5-axis internal stabilization. You’ll be amazed at what you can hand hold.

CaptainBobBrown

For birds I'd go with the D500 and the Nikon 500mm pF lens. The D500 is an excellent crop sensor camera with up to 10 fps for fast movers like BIFs or sports action and the 500mm 5.6 pF lens is about the size and weight of the Nikon 70-200 lens but you get the 500 mm reach. Nikon reduced the 500mm's size and weight using a fresnel lens in the design so it cuts out a lot of glass but they overcame the resolution problems Canon had when it tried to do the same a few years ago. It's light enough and the vibration reduction and focus is good enough that I use it in the field for birds and wildlife exclusively on a monopod which makes carrying and traveling so much easier than what I had to do with my Nikon 300mm f2.8 lens.

davyboy Loc: Anoka Mn.

Panasonic G9 has I believe 6 stop stabilization

davyboy Loc: Anoka Mn.

Some many people say it’s hard to tell what camera shot what pic m4/3 or full frame. You say full frame is not only better but far better?

davidcaley Loc: Utah

What do you mean be AI Clear?

JFCoupe Loc: Kent, Washington

I have had the Olympus OMD EM1 MK II for almost three years. The Panasonic 100-400 is my go to wildlife and bird lens. Used this combination on about 95% of my images in Tanzania in April.

This Olympus body is 20 MPs. Also with the 2x crop factor I have the equivalent of 200 -800 mm. The 100-400 is smaller than the Canon 70-200 F2.8 that I had before moving to the MFT format.

Rent the new Olympus OMD M1X and the Panasonic 100-400 and give it a solid trial. Olympus also has its own 150-400 lens coming out later this year.

fishy

Since this is a Nikon question I'll be brief and say that my newly acquired 5D IV with the Canon 400mm f5.6 produces stellar results with amazing feather detail. It comes close in image quality to the 300mm f2.8 with TC 1.4. It was ho-hum with the 7D MK II crop camera. I actually switched to Canon to get this lens! I've gotten very good results with the old Nikon 300mm f4 D lens if the OP is on a budget.

howellb

I love my 500mm AF-S f/5.6E PF ED VR lens - 3.2 lb. which I use with the D500. I have the 200-500mm lens as well but rarely use it anymore because of the weight. I just purchased the D850 and have only used it with the 500mm a few times.

imagemeister Loc: mid east Florida

NO.

If I were you, I would go to SONY and use their Clear Image Zoom with their 100-400 - the A9 or A7RIV being the best options but the A6400 will give more reach.

Here is a shot I did with the A77II and Sigma 100-300 f4 with 1.7X CIZ for 510mm@f4 ..... (on crop frame)
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(Download)

Ichiban365

This was actually taken with a D5500 and the 70-300 AF-P. And cropped slightly. It may not be a great photograph, but I took it during a few hours downtime while on a business trip, and I am happy with it.