IMHO, stacking is the greatest thing since sliced bread for those that wish to avoid any hint of distortion...why tighten that lens down when you can open that lens and stack the images. This was a 300mm at f/2.8, about 20 some image stack.
I do a lot of stuff just to see if I could do it. This dove is 20 images, focus stacked and photo-merged using a tripod mounted 2X TC with a 300 mm 2.8 lens, manual focusing/exposure. At full size, it would be a 20" x 30" print at 300 dpi.
It's really difficult...plug all the holes in the feeder but one, wait a day for the birds to accustom themselves to one hole> using a tripod with a long lens and a remote shutter, pre-focus near the feeder hole. Now the tough part>grab a cuppa and wait, with the remote in your hand...when a bird comes close to your pre-focus area, fire off a burst. You won't have a high keeper rate but, you will have some killer images...like this