The end of the day out here is a subtraction. Colour goes first, then detail, until all you have left are shapes against whatever glow is still in the sky. A bare tree does the job: branches fanned wide, everything below it already gone to black.
The bird was the last frame of the drive, holding its perch while the light gave out around it, more a silhouette than a subject by then. There is a point where the camera is working on faith and the animal is just a dark mark you hope resolves into something.
Both on the 135, and you can watch the light go in the EXIF: ISO 1600 on the tree, then 12800 by the time the bird held on. After that there was nothing left to shoot, which is the right note to end a Serengeti day on.