Birds have the best colour vision of all vertebrates. Some species even see ultra-violet light, which means they are able to pick up highlights and tones in plumage unseen by the human eye.
Raptors, whose survival depends on hunting, have eyesight two to three times better than humans. Each image they see is magnified because of the internal structure of the eye, which is built like a telescope. Frontally placed eyes give them depth perception essential to a successful hunt.
Songbirds, on the other hand, have eyes placed on the side of their head. This allows them better peripheral vision so they can watch for predators while feeding.
Some species are even more specialized. The American woodcock has eyes placed on the top of their head, so they can watch for danger while feeding, which is done by probing into the ground with their bill. American bitterns have eyes placed very close to the base of their bill, which allows them to see straight ahead when they are in their defensive freezing posture with the head pointing straight up.
From Canadian Feathers: A Loon-atics Guide to Anting, Mimicry and Dump-nesting, by Pat Bumstead