Originally posted by Biff Seems then for a dedicated wildlife/bird set-up the D500 may still be the way to go. Or maybe that's splitting hairs.
I think the issue has more to do with lenses rather than the camera. If you are OK with either the DA 560 or DFA 150-450 or you have an older k mount telephoto that gets you the reach you need, then the K-3 III is probably more than adequate (see the K-3 III photo page for plenty of wildlife photos with it). On the other hand, Pentax does have a relatively small number of lenses in this focal range and third party support is not great. This certainly could push you toward Nikon.
I would say that the D500 is getting long in the tooth and Nikon shows every sign of abandoning the F mount in the future, which means you're probably better going with a Z mount camera -- once again with the caveat that Nikon isn't going to do a lot of APS-C support either and so you'll have to deal with the Z-50.