If you turn your head to the right, as far as it will go, it is stopped by a bump of bone called an occipital condyle. Another stops your head when you turn it left. Because you have two of them, you are limited on how far you can turn your head.

Birds only have one occipital condyle, so they can turn their heads almost facing directly behind them. This is true for all birds, but most birds have their eyes on the side, so they don't need to turn that far. Owls have their eyes in the front, requiring them to turn around to see behind them.