Question

Why a flying bird appears higher in sky compared to its actual height?

Answer 1

Due to the temperature gradient in the air, light ray gets refracted and bends. Temperature gradient refers to a physical quantity, which illustrates that in which orientation and at what rate the temperature modifies the most briskly around at a specific location.  

The ray bends make the concave sides face towards the surface of the Earth, that is, why the apparent height of the bird appears more than its actual height.