How do birds see when their eyes are in the side?

Not all birds have their eyes on the side (eg owls, eagles etc) don't.

Those birds which hunt for their food have eyes that are front facing to give them stereo (3D) vision.

However those birds which are hunted (prey) (eg Ducks, Songbirds etc) have eyes on the side of their heads so that they can see all round themselves to spot hunters sneaking up on them. They do this at the expense of stereo vision.

The same is true of mammals, hunters have stereo vision and prey (eg horses) have eyes more on the side.