I got a new lens about a month ago. I shoot with a Canon R and also a Canon R7, these are the new mirrorless line of cameras. The lens I got is an 800mm f/11. It's only an f/11 with no option to change the aperture which helps keep the price down as well as the weight, a lower aperture number means bigger lens elements so more weight. Obviously this lens works better when it's bright outside so not a good lens at sunrise or sunset. The moon looked good because the moon is pretty bright. This was shot an hour before the last eclipse started. 

The Canon R is a full frame camera so 800mm is 800mm. The R7 uses an aps-c sensor which is slightly smaller than full frame so an 800mm is multiplied by 1.6 and becomes a 1280mm super telephoto. I did the shot of the moon just outside my back door in the driveway. The birds were all down by the riverbank not far from my house.