Original of a blue grosbeak - fairly far away and simple crop yielded low resolution and not much detail: Some examples (I have resized all of the shots for posting online, so the originals were all in the 6-8MP I mentioned - I resized them to no more than 1800 pixels on the long side for this thread):

In all these cases, I was working with JPG originals, not RAW. I also had a few shots where the final crop might have been around 6MP, but were a little soft or lacking in fine detail.I used the same process, enlarging to 4x and resizing back down.

I used Gigapixel to enlarge them 4x, then resized back down to the 6-8MP range. My typical workflow was to take shots that involved heavy crops - where the bird was a small part of the frame, and see if I could get a photo that had a reasonable resolution for display or print - at least in the realm of 6-8MP.many of these after the original crop would have been down to 3-4MP. The newer uprez programs like Gigapixel claim to do a more intelligent job at predicting and calculating those additional pixels in the enlargement so that you can get more resolution that's close to the original pixels - and can be resized back down to reasonable sizes for nice detail and sharpness improvements, removing noise, and even processing as if it were an original photo - brighten, white balance changes, etc. Normal enlargement doesn't create new pixels to build up a shot - they just enlarge the pixels there, or duplicate them. I recently picked up Gigapixel AI to see if it could do what I hoped, which is take shots that were fairly heavy crops, therefore low resolution and/or missing fine details, and try up-rezzing them with an intelligent enlargement tool. I have been using Topaz's Denoise and Sharpen for a while, and both are good tools for cleaning up shots especially for birders, when we have to go to very high ISOs or deal with a little motion blur.

Gigapixel review