Birds flying very_______ do not cast their shadows, as ________ finishes in air.

1. The source of light (here sun ) is very large as compared to the opaque body of the bird.
2. The umbra formed is so small that it finishes in the air.
3. The penumbra formed by the bird is very large and very faint as the distance of earth is very large from the object(bird). Thus it is hardly visible.
4. Hence no shadow is formed

Hence, Birds flying very high do not cast their shadows, as umbra finishes in the air.