We do not see the shadow of a bird flying high up in air because in their shadow, the umbra is absent and the penumbra is too faint that is not visible as the distance of screen (i.e. earth) is very large from the object (i.e. bird).

That’s an interesting question! The answer depends on your specific location and circumstances, but here are the most common reasons why you might not see any airplanes or birds in the sky at a given moment:

For Airplanes:

1. You’re not under a flight path. Major air routes (air highways) are specific corridors. If you’re not near an airport or a common route between airports, you might see very few or no planes.
2. Altitude and Visibility. At cruising altitude (30,000+ feet), jets are tiny, faint specks and easy to miss unless they have a visible contrail (condensation trail). You often need to actively look for them.
3. Weather Conditions. Overcast skies can completely hide high-flying aircraft.
4. Time of Day. In some regions, especially near smaller airports, there is much less air traffic at night.
5. Airspace Restrictions. You might be in controlled airspace (near a military base, government building, etc.) where general flight is restricted.

For Birds:

1. Time of Day and Season. Birds are most active at dawn and dusk when feeding. You’ll see fewer during midday heat or at night (except for nocturnal birds like owls). Many species migrate, so they may be absent in certain seasons.
2. Weather. Birds often take shelter during strong winds, heavy rain, or storms.
3. Habitat. If you’re in a city center with few trees, parks, or water sources, there will be far fewer birds than in a suburban, rural, or wetland area.
4. Predator Presence. A nearby hawk, cat, or other