Everyone has an opinion as to which is the best camera format, but as someone already stated, it is next to impossible to find a "bad" camera among the major manufacturers these days. I also have quite a large investment in Nikkor lenses going back several decades, but I finally decided to try mirrorless and I chose Micro Four Thirds. Why? Well, size/weight was one reason, but more importantly, virtually any lens ever made can be easily adapted to MFT, and the image quality is also purely fantastic. So, my wonderful Nikkor lenses are still completely useable (in manual mode), and native Olympus M.Zuiko and certain Panasonic lenses also happen to be fantastic optical instruments.

However, rather than jump head-first into the deep end, I bought a slightly used Olympus PEN E-P5 to test the waters. It's already a five year old model but has essentially all the marvellous technical advancements Olympus has pioneered over the past few years including 5-axis image stabilization. I've attached some of my Nikkors to it, and I'm simply amazed by what this little beastie of a camera is capable of. I believe I'm now fully ready to make the dive into the Olympus OM-D MFT system. I think you would be doing yourself a favor by at least checking out MFT. But you know what? I still miss the old days of shooting with my now ancient yet wonderful Nikon film cameras. If you love photography, the equipment is almost secondary.

Over the last year I have seen growing signs that digital photography is moving toward mirrorless cameras and associated lens systems. I have significant Nikon equipment for my bird and nature photography interests including a 500mm F4 and 300mm F2.8 prime lenses. I have a recently purchased D500 I love and have been saving for a D850 - which I have anticipated buying at during the end of the year sales when I hope to see a modest discount to the long stable initial price.

I have come to the realization that at some point there is likely to be an significant advantage such as size and weight of gear - and even technical innovation leadership - to move to a full mirrorless setup.
I understand that there is an adapter to allow me to continue to use my current lenses with the new Nikon mirrorless cameras...but I am not sure that I would necessarily stay with Nikon if I go mirrorless. Steve Perry's evaluation of the new Nikon mirrorless suggested that they are not yet as capable (insufficient burst mode buffering) as current DSLR offerings for action bird photography. I understand that some of the Sony are of comparable capabilities. A friend has a Sony mirrorless and set of lenses that are significantly smaller and more manageable that my gear.

I know from regularly reading the Forum that there are many very thoughtful and knowledgeable members - and maybe some that have already made this transition. So I would like to ask for your thoughts on which companies currently appear to be the true leaders in mirrorless cameras and lens systems - especially as applicable to bird photography. And more specifically, knowing that the technology will continue to evolve, your thoughts on timing - is it the right time to convert or to move forward with my D850 purchase knowing that there will likely come a clear time to make the switch?

Thanks in advance for your thoughts!

Unless you are going Olympus/Fuji/Sony there is no compelling reason to go mirrorless with your Nikon equipment unless you just want to. The Z7 and Z6 are a first foray into real mirrorless for Nikon and most likely will be followed by more D850 type mirrorless in the future.
I would/am waiting for this reason. The Canon and Nikon mount diameters will allow for some pretty incredible lenses that Sony will never be able to produce because of their small diameter mount they chose to keep things fashionably small. Canon is getting ready to put out another lens, the 14-21mm f1.4L, https://www.the-digital-picture.com/Reviews/Canon-RF-24-105mm-f-4L-IS-USM-Lens.aspx. This is possible with the larger diameter mount. Nikon with a similar diameter will likely do the same as time goes on.
If you have the latest Nikon lenses then they should work reasonably well with the Nikon mirrorless system.
But you need to make the call.

Over the last year I have seen growing signs that digital photography is moving toward mirrorless cameras and associated lens systems. I have significant Nikon equipment for my bird and nature photography interests including a 500mm F4 and 300mm F2.8 prime lenses. I have a recently purchased D500 I love and have been saving for a D850 - which I have anticipated buying at during the end of the year sales when I hope to see a modest discount to the long stable initial price.

I have come to the realization that at some point there is likely to be an significant advantage such as size and weight of gear - and even technical innovation leadership - to move to a full mirrorless setup.
I understand that there is an adapter to allow me to continue to use my current lenses with the new Nikon mirrorless cameras...but I am not sure that I would necessarily stay with Nikon if I go mirrorless. Steve Perry's evaluation of the new Nikon mirrorless suggested that they are not yet as capable (insufficient burst mode buffering) as current DSLR offerings for action bird photography. I understand that some of the Sony are of comparable capabilities. A friend has a Sony mirrorless and set of lenses that are significantly smaller and more manageable that my gear.

I know from regularly reading the Forum that there are many very thoughtful and knowledgeable members - and maybe some that have already made this transition. So I would like to ask for your thoughts on which companies currently appear to be the true leaders in mirrorless cameras and lens systems - especially as applicable to bird photography. And more specifically, knowing that the technology will continue to evolve, your thoughts on timing - is it the right time to convert or to move forward with my D850 purchase knowing that there will likely come a clear time to make the switch?

Thanks in advance for your thoughts!