Why can't we see shadow of bird flying up in the sky?

A shadow forms when light is blocked. Consider a light source, an object that might or might not cast a shadow, and a surface on which the shadow might or might not be cast. In order to cast a shadow, the object must block ALL light from the source reaching the surface. Now, the sun is not a pinpoint source of light, but a massive body, about 1.4 billion meters in diameter. And every point on the surface of the sun that is facing us is casting light in our direction. That object, in this case the bird, must block all of that light from reaching some area on the surface in order to cast a shadow. The higher a bird flies, the smaller area it can block sunlight from reaching. When this area reaches zero, no shadow is cast. Think of it this way. In order for an object to cast a shadow, it must "appear", from the surface, to be larger than the light source. It's apparent size must be sufficient to completely hide the light source if placed in front of it (between the light source and a point on the surface). The higher the object is, the smaller it appears to be. It is easy to see that, at say 50 feet, all but the largest birds would not appear big enough to block out the sun, and would therefore cast no shadow.