I’ve shot with a 30D, a7D, a 7Dii, a 5Diii and now a 5Div and a 1DXII. All this talk of crop factor cameras being better than full frame when it comes to Birds and Wildlife is just inaccurate. Just take a picture with a 1DX and the picture lights up. I can’t justify a 1DXiii but my 1DXII is a amazing. A canon 100-400 mkii with or without a 1.4 mkiii extender is superb for walk around photography and put a 500 or 600 on and you’re in another world. Everyone wants their setup to be the best there is and if we can put aside the old rant of it’s not the camera it’s the photographer when it comes to image quality you really do have to accept you get what you pay for.

Yes.

For most photographers I think the 90D would be the best choice among Canon DSLRs.

And I think the 7DII.... with its somewhat different feature set... is a close second place. It might be first choice for some users.

Again, I'm not knocking full frame at all.... If a bird/wildlife photog has the budget for the cameras and lenses, and is willing to lug around the hefty lenses necessary for full frame....Hey, go for it! 5DIV or 1DXIII. Both are excellent and good choices.

Mirrorless versus DSLR is a whole different discussion. In my opinion, for a number of reasons DSLR are still the better choice for sports and most action photography (including some birds & wildlife). But, mirrorless just keep getting better and better... and that's where most of the manufacturers are concentrating their R&D efforts.

Can't comment about the EOS R's AF system. Don't have one and am not familiar with it. I do recall when the camera was announced that Canon bragged the AF system was the fastest of any digital camera, to date. It's also rated to be able to focus in much lower light conditions than any DSLR.

I just recently got my first mirrorless... a Canon M5 (APS-C, 24MP).... and am learning it. But I'm only using it with a small suite of manual focus lenses, primarily as a "walk-around", street photography, portrait camera. It's fun, but in a lot of ways not very comparable to the full frame Canon R-series.

If the focus is bird & wildlife photography... My answer for your friend would be "none".

With 32.5MP the Canon 90D is an APS-C camera that actually has higher resolution than any of the Canon full frame cameras (except the 50MP 5DS series, which is a "no go" for wildlife for other reasons) and would be a better choice for the vast majority of bird & wildlife shooters. An APS-C camera "leverages the reach" of telephoto lenses wonderfully... and there's a saying among bird/wildlife photographers that "you never have a 'long enough' lens". There are always subjects you'd like to shoot who are just a little too far away, no matter how much $ you've sunk into big telephotos for the purpose. In some ways... such as it's AF system & controls and a more robust, better sealed build... the APS-C format 20MP 7D Mark II is also a better Canon DSLR choice for most people, but will not allow as much cropping or enlargement for printing as the higher resolution 90D.

Just for example, say your friend wants a lens around 500mm full frame equivalent. With an APS-C camera they have choice of Canon EF 300mm f/4L IS USM... under 3 lb., about 12" long dwith hood, 3" diameter, costing under $1400, and quite easily hand held for long periods of time. To accomplish the same thing with a full frame camera, your friend would need a Canon EF 500mm f/4L IS USM... roughly 8 lb., about 18" long with lens hood, approx. 6" diameter, around $9000, and only hand-holdable for very short periods of time, so figure another $1200 to $1500 for a sturdy tripod, or at least a very good monopod for shorter shooting sessions. You can go down a list of telephotos where similar comparisons can be made... 70-200/2.8 (~3 lb., $1800) versus 300mm f/2.8 (~ 6 lb., $6000)... 100-400mm (~3.5 lb., $2000) versus 200-400mm with built in 1.4X (~ 8 lb., $12,000).... even 135mm f/2 (~1.65 lb., $1000) versus 200mm f/2 (~5.5 lb., $5600).

How about just shooting with full frame using the same lens you would on APS-C, then cropping the image? That will almost always mean lower quality images. The Canon APS-C sensor has 38% of the image area of one of their full frame sensors.... as a result, a full frame, 30MP 5D Mark IV's image will be reduced to around 11.5MP when it's cropped to match the APS-C format. Even the upcoming EOS R5, rumored to be around 45MP, would be reduced to around 17MP if it's images were cropped to APS-C. Heck, even the 50MP 5DS models only end up with around the same 20MP as the 7D Mark II, if their full frame images are cropped to match the APS-C camera's format (and the 5DS have other possible short-comings for this purpose, such as quite limited high ISO capabilities and a less high performance AF system).

I've proven this beyond doubt to myself.... some years ago... comparing images shot with an 18MP APS-C camera versus a 21MP full frame camera... both with the same lens, both shooting the same subject from the same distance using all the same settings. By the time the full frame shot was cropped down to match the APS-C image, it showed very obvious loss of image quality. The crop sensor image was always superior. And that was comparing an APS-C camera that used an unusually strong anti-alias filter against a full frame camera with a rather weak one. Years later, both full frame and APS-C have seen a number of improvements. It's arguable that the APS-C have seen the most improvement.

There also can be other factors... For example, in many cases an APS-C camera has a faster maximum continuous frame rate. Both 90D and 7DII can shoot at 10 frames per second... In comparison, full frame 6D Mark II maxes out at 6.5 fps, 5D Mark IV at 7 fps and 5DS models at 5 fps. Those FF cameras also have slower flash sync, 1/200 or less... versus 1/250 that the APS-C offer. All the above models, both FF and APS-C have a top shutter speed of 1/8000, too... except the 6DII (FF) and Rebel models mentioned below (APS-C), which max out at 1/4000. Only the big, heavy and most expensive, top-of-the-line 1DX-series full frame models are faster. Those are "sports oriented"... like the APS-C 7DII... and, similarly, are right around 20MP resolution, too.

It makes more sense in most cases, to get the 32.5MP possible with 90D... Or, if sufficient, the 20MP possible with 7D Mark II. Or perhaps the 24MP of a Rebel T8i, 77D or T7i (or, if considering used, the 24MP of an 80D). For most photographers, any of these APS-C cameras would be better choice than any full frame for "long lens work".

Plus, these APS-C cameras also typically cost less, so might leave more money in the bank that might be put to better use for a lens upgrade or other purpose.

I am not knocking full frame cameras. They're excellent and very useful for a lot of things. Ideal for wide angle work, in particular, a full frame is a great choice (and probably better than APS-C) for landscape, architectural and similar photography. They also can be a top choice for portraiture (depth of field control) and low light shooting (high ISO capabilities).

BUT, unless you have a huge lens budget and someone to help you lug around heavy gear and tripods to sit it upon, they are NOT the best choice for birds and most other wildlife photography. I am not saying it can't be done... just that for most people, most of the time, an APS-C camera would be a better choice.

P.S. Personally I use both FF and crop cameras, usually each for their own best purposes.

The knock on the initial EOS R bodies is the AF system for action. If time is not pressing, wait for the EOS R5 to be announced over the next few weeks as well as the hands-on reviews over the next few months. The industry expects the EOS R5 to change how we think about mirrorless, how we think about photography, how we think about life.

The 1DX options are a $2000 to $6000, depending on model and new vs used. The body is large, with a built in grip. The 5DIV has been spotted at $2000 now twice on sale, new, from Canon, with the 5D line also being a large full-frame camera, but still nothing like the size of the 1D line.

In the meantime, as asked by another post, what lenses are being considered as technique and focal length matter far more than fps.

Full-frame: the 5DIV mixes the advanced EOS AF system of the pro-level 1DXII with the larger megapixel sensor for stunning results, including more opportunities to crop into the frame, retaining plenty of pixels to fill a 2-page sized magazine print. The 1DXIII is currently Canon's premier camera, with the rugged build for everyday, all day, all weather, heavy use for all photography needs. Canon explained in the white paper that accompanied this release that the 20MP (ish) resolution of the 1DX line is intended for magazine-sized (2-page) images. Any of the 1DX models, as well as the older 5DIII, feature Canon's most advanced auto focus systems.

The 1DXII and 1DXIII models features a second onboard processor (computer) to drive the AF system separate to the camera operation. The cropped 7DII also features this design feature. Of course, 20MP and upward will print stunning images at larger than a 2-page magazine spread.

Understand concern about mold, etc. and storing lenses for an extended time. Especially when you mentioned 'astro.' Safest is to put each lens in a separate heavy duty (freezer) ziplock bag with a recently baked bag or two of silica gel. The silica gel will absorb any moisture from the lens and the air in the bag when you sealed it.

I know to do it, but, rarely do. And I live in a high humidity area of the USA. Humidity in ATL is rarely below 50%. - But, I've seen the results of mold and separation with older lenses. It is not pretty. And, no, you can't afford it. - What was that old motor oil ad? 'You can pay me now or you can pay me later.' What can you afford?

I choose safety and caution over carelessness and risk. (Nikon 500 F8 mirror, Nikon 500 F5 mirror, Nikon 1000 mm F11 mirror and others deserve extra caution to protect their current 'near pristine' vintage condition.)