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

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.
(P.S. If this is a VNS student reading this for Physics research, then I'm pretty sure you're in my grade and you know who I am)