I agree that the image could be sharper. I'm not sure a "better" camera would do better. There are 2 things I would do different.

First, the f/4 is probably wide open or close to it. That is the worst setting for depth of field. I would try again with a smaller aperture.
Second, I would also try a fasted shutter speed to minimize blur from any motion, camera or subject.

Both of those would require higher ISO, but I'm sure you could go up on it without getting into distracting noise.

I'm in the market for a new camera - the attached was recently taken with a D7000 using a 16-80mm AF-S Nikkor DX zoom at 80mm - from roughly 50 or so feet away. This particular picture was one of 40 or 50 exposures of the subject at various f-stops, etc. - and IMHO was the best I could do. But, setting aside the intended depth-of-field blur in the foreground and background, the eagle itself has some room for improvement in resolution.

I'm considering upgrading to either the second generation version of the Z6 or Z7, but I just don't have enough experience to have a "gut-feel" on whether the lower cost Z6 II (with an FX lens at 80mm) would provide the improvement I'm looking for, loosely defined as "holy cow, I had no idea!" - versus the more expensive Z7 II. I've no need to chase resolution for its own sake - nor do I print poster-size photographs, but if the Z7 II is the right answer, so be it. I'm just your typical enthusiast who hikes around Maine and occasionally runs across subjects or landscapes that deserve a photograph and would like to see some demonstrable improvement in resolution over my current equipment.

I've got some older Nikon FX glass - so I'd like to stick with that brand. I also realize I'm asking a subjective question, but with most comparisons of the two cameras mired in technical cha-cha that I'm not experienced enough to understand, I need it dumbed-down to my level. And because I don't have access to these cameras for real-world comparative analysis, I'm left with getting opinions from folks with more experience. Thanks much.

"...I don't have access to these cameras for real-world comparative analysis..." Actually you do Geosailor... Rent before your even think of buying... Even then go with "Previously Owned" from a vendor who will allow you to return it if it doesn't meet your expectations... i.e. KEH; mbp; B&H ect...

"...I'm in the market for a new camera..." Why Geosailor, your D7000 is more than capable (in skilled hands) of turning this hat trick without even breathing hard. It's your optic that is limiting... as Paul mentioned "Bird" photography starts at 400mm (minimum).

"...I'm left with getting opinions from folks with more experience..." Something that is typically in short supply... Unless you pony up the fee for a "Hands On" seminar from the likes of a highly skilled "Bird" photographer such as Steve Perry https://www.uglyhedgehog.com/user-profile?usernum=60547 Get on Steve's web site if you are serious about bird photography.

I would also suggest you join flickr and become a member of many of the groups devoted to "Bird" photography... Study their masterful work... The best thing about flickr is there is very little BS since flickr posts the actual EXIF data from the photographs... So you get a valid window into what it takes.

Food for thought: Wildlife photography is for those who are independently wealthy... It (along with Sports photography) demands Pro-Level kit. Maybe rethink going down the bottomless rabbit hole... Just saying...

Final thoughts? Your D7000 can easily handle this mundane shot of a "Sitting Duck" or eagle in your case... You seriously need to capture your Bird In Fight otherwise referred to "BIF" to gain any real traction or recognition from your photographer "peers" As others have said, you actually may need considerably more training, NOT a new camera, albeit consider a decent optic (that can approach reaching an "angle of view" of over 400mm.)

Below is a BIF capture with a D7000... Your issue isn't with the D7000 Geosailor... It's with the optic you decided to mate to it... and very possibly your skill-set: Oft said "It's a poor craftsman who blames this tools for want of success..."

Hope this helps... and please give flickr a try, there you'll find some breathtaking imagery and talent...
One of the many flickr "birding" groups https://www.flickr.com/groups/bird_photography_excellence/
Cheers!
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Resolution has to do with pixel density - whether it be a crop sensor or full frame. Your D7000 has MORE pixel density than most Full frame sensors ! The other parts of resolution are focus, shutter speed/stabilized camera, software and proper application, and lens optical quality. In your case, there are many optically "better" lenses - mostly non-zoom prime lenses.
Regarding Z series cameras, they would be an upgrade for YOU only if they had a Full frame sensor with 46+ MP sensor and/or using a Z native lens. The Z50 crop frame pixel density WOULD be an upgrade and further, with Z lens to match.
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