How Do Bats Avoid Obstacles In Their Way While Flying?

Most bats are active at night or twilight, but their eyes are small and poorly developed.  They do though have a very good sense of vision and an excellent sense of smell and hearing.  

They emit high-pitched sounds and listen to the responses (echolocation).  Dolphins and whales also do this and this is how they locate prey and other nearby objects.

The bat sends out the high pitched squeak and listens for the returning sound that bounces off nearby objects.  The bat can then work out how far away and how big an object is.  All this happens in a split second.  

About 70% of species of bats use this as their primary way of finding their way around.  They are the only flying mammals and although the expression "blind as a bat" is common, they are in fact not blind.  They can in fact see quite well, but as the process of echolocation has developed, so they have had less and less use for their eyesight.