Except for a very few birds like penguins, who swim in VERY cold water, most birds don' t need to grow feathers over their whole bodies. One individual feather is a lot wider than one hair, and can cover a bigger space on a bird's body than one individual hair can cover on a mammal's body. But feathers take a lot of energy to produce. To save energy, birds produce just enough outer feathers to completely cover their bodies when the feathers are groomed properly. An adult hummingbird has only about 940 feathers on its whole body.
Human eyeballs can turn and roll to see up and down and to both sides.
Our eyelids are shaped as they are so that our eyes can still see when they move side to side. Also, when birds fly, the rush of air has a drying effect. To protect their eyes from drying out, they cover as much of the eye as possible behind skin and feathers. And bird eyes can't roll like our eyes do. Birds can't open their eyes any wider than their pupil and iris.