We bird photographers love to capture as much feather detail as possible. Having a high resolution camera and sharp lenses certainly helps, but I've found that getting close to the birds is by far the most important factor, particularly when photographing tiny birds like Lesser Goldfinches and hummingbirds (the Lesser Goldfinch above is no more than 4" tall, about the size of your thumb.)
Over the past few months I've been working on acclimating the local goldfinch flock to my presence. I try to spend a little time each day quietly sitting near my Finch feeders. It's at a point now where they will actually land closer to me than the minimum focus distance of my telephoto lens.
This photo was captured this evening with my Fuji X-T2 and the Fujinon XF100-400 f/4.5-5.6 at 400mm (600mm equivalent). The minimum focus distance on the lens is around 6 feet, this was captured at around 8-9 feet at 600mm. I did a tiny bit of cropping just to balance the composition, otherwise this is close to out of camera. Please click on the image to see the detail.