The reoson is that when the object is close to ground, the shadow is formed on ground,but when the object high in air, the shadow is formed on air which is invisible.
It is the case related to formation of shadows actually it can be understood by that when an aeroplane and bird flies high in sky they cast a low shadow and when it travels towards ground and as it is too small so it disappears in the air only but if a bird flies low near  the ground we can see their shadow as the distance covered is less.