ICSE Class 6 Answered

Shadows are formed when the light gets obstructed by an opaque object. The formation of shadows also depends upon the distance between the opaque object and the screen. In this case the bird is opaque body and the Earth's surface is a screen. When the bird is flying high in the sky the distance between the body and the surface of the Earth is too large and hence even though the light is obstructed by the bird's body it cannot cast its shadow due to the large distance. If the bird is flying near to the Earth's surface then the shadow is formed as the distance between the bird and the earth is less. Hence, birds flying high in the sky do not cast shadow.