[davidmanze]

So between Nikon's two lens formats, not to mention Canon's and Sony's autofocus improvements, there will be lot of ways to get great photos of birds.  

Best,
Vic

"It sounds like for birding you want a large body with a crop sensor.  That would give you more resolution with a "longer" telephoto reach and a larger grip for holding big lenses? "

 
  Yaaay  ..... there are many that are calling for this, me included !! ...(Mac also btw) ....... I like the APSc sensor for reach reasons ....... plus large aperture long lenses for shallower DOF and less noise ..... 
  ......... however, the Z50 is no sports camera ...... AF is no great shakes nor is the Fps rates or buffer ..... so Nikon need to design completely anew.
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"If you put a DX lens on a Z mount, it automatically crops the image.  With that, I could see a full frame camera that would work with a faster frame rate and larger buffer when you put a DX lens on it (or maybe even set a setting)."
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  It must be said that there are very few Nikon DX long telephoto lenses ...... all the tele-zooms are for the most part FX (tpm) as are the fixed focals (oem) ..... whether there will be future DX lenses we will see ...... but with Nikon on the back foot, it may take some years to achieve ........
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"Usually you get a bit higher resolution on crop sensors because there are more photo-receptors in a smaller area, and they can be built more cheaply, but part of cheap manufacturing is setting up the factories."

  Higher pixel density .......... yes! 
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  There's never been a better time to be shooting birds ....... for sure !!