Disadvantage of a fixed lens

Vulturine guinea fowl in Africa are bizarre looking birds with stunning blue feathers and a head like a vulture. They are very hard to photograph because as soon as they come into view, they turn their backs on the camera and run away. I shot this with a 500mm fixed focal length telephoto, and because the bird was relatively close to the camera, I couldn't include the feet. If I switched to a zoom such as a 100-400mm, I felt the guinea fowl would be too small in the frame. This is one of the disadvantages of a fixed focal length lens. I still like the picture, but the feet should have been included. Having said that, I was lucky to get a frame-filling sharp picture of such a unique bird. My settings were 1/320, f/4.5, and 200 ISO. I took this several years ago with a Canon 5D Mark II at a time when higher ISO settings meant too much noise that couldn't be repaired. Today, I would shoot this with a higher ISO and a faster shutter with my 1Dx Mark II.