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

Nikon D610.

ELNikkor

Wanting a FF for BIF can get expensive and heavy. That D3100 with a 55-300 DX(under $200) might be the most efficient way for him to "reach out" to those birds.

rmalarz Loc: Tempe, Arizona

The D700, D750, D800e, and D850 are all excellent cameras. The 800e is probably the best deal for the money. Purchasing used is a good way to go but be careful when you select the dealer. B and H and Adorama are probably your best bet.
--Bob

BebuLamar

Nikon entry level for FF DSLR is the D610 but it's $1600 and the higher level D750 is $1500 so there is really no reason to buy the D610.
For mirrorless it's the Z5 which is $1400 and with the FTZ adapter to use F mount lenses is $1450 as a bundle.

Leon S Loc: Minnesota

I agree with Thomas and Dandi. The D700 is a wise choice for an entry level camera into full frame. It is fast enough for sports, tough, dependable, great battery life, very good low light capable, good color rendition, weather resistant, and still used professionally. Not long ago it was, along with the D3, Nikons flag professional camera. I still use mine along with my D810 and have never had a problem with it. Couple the D700 with a 50mm 1.8 and wall hangers are around the corner. You can get the same pixel count with the D700 (12 mega pixel camera) as the D610 or D750 (24 mega pixel) of your subject by just moving up closer by half the distance. A D700 with a 50mm 1.8 can be found for about $400.

imagemeister Loc: mid east Florida

Anyone on a budget or physically challenged doing nature/wildlife SHOULD be looking at a crop frame system !! For Nikon mirror less this would be a Z50 - about $800 body only new. in SLR, the 71/7200 would be a good candidate. But, if he goes Canon he could maybe borrow your lenses ?? - maybe T7i new or used..
.

imagemeister Loc: mid east Florida

BebuLamar

The OP asked for full frame and none of the cameras you suggested are full frame.

imagemeister Loc: mid east Florida

Yes, I know .....

imagemeister Loc: mid east Florida

Drip Dry McFleye

If wildlife/birds is the target but money is an issue, crop sensor cameras and the Tamaron 100-400 or Sigma 150-600 zooms should probably a big part of the conversation for a number of reasons. Take a look at backcountrygalleryforums. It is a forum that is primarily aimed at wildlife photography.

MadMikeOne Loc: So. NJ Shore - a bit west of Atlantic City

imagemeister Loc: mid east Florida

Robert1 Loc: Davie, FL

without knowing the budget is difficult to ascertain, but for BIF with a FF camera you need a long telephoto lens, such as a zoom 200-500mm, 150-600, or a fix 300mm f2.8 or f4 with a 1.4X or 2.0X tele extender in good light. This entails a lot of money, but he/his father can go the used route.

Just be aware that a 300mm lens is not close enough for BIF photography, unless you are shooting in your backyard.

I think that the best bang for the money with a Nikon FF would be the D750, rather than 610 for an entry level camera. You can get a used one with low actuations under $1000.

For used camera gear merchants they can peruse B&H, Adorama, KEH, Roberts camera, MPB as a starting point, also don't forget to tell them that here at the UHH buy & sell section some good deals can be had.

larryepage Loc: North Texas area

Good morning.

As Nikon has clearly stated for their strategy, they do not make an "entry level" full frame camera, nor would I expect one in the future. So the suggestions for a used D610 or D750 are on target. Either will be a huge advance from the D3100.

If he decides to stay with a crop frame camera, the minimum consideration for upgrade would be a camera with two command dials (front and back), allowing faster adjustment of exposure parameters. This implies, at minimum, a D7xxx of some nature. As he is able to move up to these models and beyond, he will increasingly be able to make adjustments to the camera without entering the menu system nearly as frequently.

Long lenses can be a challenge. My "long lens" for a long time was a 300mm f/4 AF-D on my crop frame cameras. You don't ever hear much about that lens here, but it worked fine for me for a long time. I don't have a teleconverter for mine, and can't remember whether this lens is designed to work with one, but would suspect it would do fine.

I don't do birds in flight, so you will have to help resolve my suggestions around what will work acceptably and what is likely to be problematic.

Good luck as you work through this search with him.