Different bird species have differently shaped beaks because each species has evolved a beak design that suits its diet and lifestyle. Beaks function somewhat as human tools do, and they help the birds to access food. While some birds have beaks suited for a variety of foods, most possess beaks that display some level of specialization.
For example, many birds have evolved short, stout beaks for cracking open nuts and seeds. Sparrows, finches and cardinals have beaks of this type, matching their natural diet. Parrots, macaws and their relatives take this to the extreme, and have strong beaks that are capable of cracking open the toughest nuts in the forest.
Hummingbirds have long, thin, tubular beaks that work like drinking straws. This enables the birds to sip nectar from inside flowers. Killdeer are worm-eating birds that probe holes in the ground and have long, thin beaks that resemble tweezers, which facilitate the practice. Robin beaks are not as elongate as those of killdeer are, as they subsist on surface dwelling worms, and do not need such long and thin beaks.
Hawks and owls have scissor-like beaks that they use to remove the flesh of animals. The colorful toucans of the rain forest use their large, lightweight bills to collect fruit from difficult to reach places.