There are several species of hornbills found only in the Old World. They range in size from the smallest, the red billed drawfed hornbill that is about 15 inches, to the largest, the southern ground hornbill which is about the size of a turkey. These birds have large down-curved bills and many have a large growth on the upper bill called a a casque. These bills come in many striking shapes and colors. They also have what appears to be eyelashes, but they are not made of hair, they are small feathers that serve the same function. Hornbills have an unusual nesting behavior. They nest in a tree cavity. The entrance to the nest is sealed after the female enters. Depending on the species either the female constructs the seal with feces or the male helps in the construction by bringing in clay or dirt. While the female is sealed inside incubating the eggs, the male provides food through a small hole in the seal. When the young hatch the female may stay with the nestlings for several weeks and then break out of the nest to assist in feeding the young. The young will assist in resealing the nest. In some hornbill species, the female remains in the sealed nest until the young are fledged. Ground hornbills do not use this nesting behavior.