Question

why birds flying high up in he sky do no cast their shawdows

Answer 1

because they make negligible or extremely small obstruction in the path of light

Answer 2

Birds flying up in the sky do not cast their shadow because the source of light (sun) is too large compared to the object (bird) and the umbra of the shadow formed on the screen (ground) is very small, negligible.So it is tough to see its shadow on the ground.