Question

Why do the birds flying high in the sky not cast their shadows on the earth?

Answer 1

Shadows are formed when the light gets obstructed by an opaque object. The formation of shadow also depends on the distance between the opaque object and the screen. Here the bird is the opaque object and earth is the screen. When the bird is flying high in the sky the distance between the bird and the surface of earth is too large.
Hence, birds flying high in the sky do not cast shadows.