It occurred to me that people who pop in here only occasionally and haven't been following the thread closely, might think it's daft to take pics of yard birds at 4000mm. So, to qualify my last answer..... I did that just to demonstrate what the digital zoom is like on the P900, since many here (including me) did use DZ on the SX50 and were wondering if it's a useful function on the Nikon.
In my experience, the "photography first" people are always loathe to use digital zoom. My better half is in this camp, and he'll decline to take a shot if he doesn't feel it has the potential to be a good photo. Tiny birds high in the canopy, or those that are far away aren't subjects these folks will try to shoot.
I find the "birding first" people are more inclined to embrace digital zoom because they prioritize "getting" a shot, even if it means the resulting photo won't be stellar. Of course everyone wants to get great pics all of the time, but birds have a way of thwarting that goal, especially when it's one you really want to document. All tools that give the birder the edge are welcomed by this crowd.
Conventional wisdom is that digital zoom is nothing more than a crop, so one is better off taking a pic in optical and cropping later. With superzooms though, things aren't always so cut and dried. Many SX50 operators find the teleconverter function to be superior to "regular" digital zoom, and that using it often results in better pictures than cropping of optical ones does, although no one seems to know exactly why, or how Canon accomplishes this.
I do know that distance from the subject, and busy-ness of the background always factored into my use of digital zoom on the Canon. I found that being able to hone in on the subject with digital zoom, often allowed the camera to focus more effectively than it was able to in optical range. The resulting photo (while perhaps not pixel-peeping-worthy,) was certainly better than an out-of-focus one would have been, cropped or not.
I bought the the P900 hoping to get the reach I was getting with digital zoom on the SX50, but within optical range, and that is indeed what I'm getting. The camera auto-focuses well throughout the optical zoom range (except for BIFs, where the SX50's sports mode is far superior) so I haven't felt the need to use digital zoom. Those who have been using the SX50 though, should know that 2400mm eq. (via teleconverter) doesn't require nearly as steady a hand as 2000mm optical does on the Nikon. Tranquilizers, anyone....?