Question

why is the shadow of a bird flying high up in the sky not seen on the ground

Answer 1

For the shadow to fall near you, the bird or plane would have to be close to the line directly from you to the sun.  Most of the time, the object isn't close to that line and the shadow falls a few blocks or a few miles away. Also, the higher an object is in the sky, the larger its shadow will be on the ground.  And, because of geometric optics, the shadow will be more diffuse when the object is higher in the sky. 
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Answer 2

Birds flying in the sky do cast their shadow but because they are shading on area that is very tiny, the shadow is not visible. The higher the birds fly, the smaller the shadow it casts.