Old morphological classification grouped all diurnal raptors together, into one large order named the falconiformes. Within that were five broad families, of which were the New World vultures, the osprey, accipitrids (hawks, eagles, owls, and the Old World vultures), the falcons, and the secretary bird of Australia. With modern technology, and through a process known as DNA-DNA hybridization, taxonomists have been able to see that Old World vultures and New World vultures are completely unrelated. In fact, New World vultures have been found not to be related to any other raptors, they have been placed with the storks in the taxonomic order. Biologists have also noted many physical differences that American vultures share with storks. The vulture lacks the powerful hind toe (hallux) of other raptors and they have "perforate" nostrils, with a hole through the beak from side to side. Most American vultures also have an excellent sense of smell, which is lacking in other raptors and

The similarities in both classes of vultures are not due to being related, but to convergent evolution. Vultures on both sides of the Atlantic have bare heads because they feed on carrion. Feathered heads would become matted with blood from feeding inside large carcasses, while bare skin is much easier to keep clean. Since their prey doesn't require killing, vultures lack specialized feet and talons. Some New World vultures cannot completely close the foot to grab. Unlike Old World vultures, American vultures have a well-developed sense of smell. Their sense of smell is even able to discriminate between meat that is merely spoiled and that which is completely putrefied. The Old World vultures probably haven't evolved a sense of smell because most species live on open plains or in mountains, where carcasses are easily found by sight. Biologists have found these vultures to have up to eight times the amount of receptor cells in the eye than the human, while New World vultures may have only two to three times the amount. The New World vultures, j

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