Answer:
This is because of birds perceive the idea of refraction of sunlight instinctively.
In general, the additional optically dense the medium, the slower the sunlight can move Light changes direction . however wherever the incident beam is perpendicular to the boundary (that is, up to the normal), there's no modification within the direction of the sunshine because it enters the second medium.
The bird flies vertically over the position of the fish, so dives into the water at a ninety Degree angle. The idea here is that the commonly incident rays don't bear refraction, thence the fish lies specifically wherever it seems to be. At the other angle, the apparent location of the fish would diverge from its real location.