Why we see shadow of bird flying up in the sky?

The reason we see the shadow of a bird flying up in the sky is due to the position of the sun and the angle at which the bird is flying. When the sun is behind the bird, the light from the sun casts a shadow of the bird onto the ground. The higher the bird flies, the larger the shadow will be as the angle of the light from the sun to the bird becomes shallower. The shadow of the bird will also move along the ground as the bird flies, following the direction of the sun's rays.