As such, they are generally preferred over plant material. While we think of avian nutrition as one all-encompassing topic, birds are as varied in their gastrointestinal organization and nutritional needs as are mammals. Most of our knowledge of avian nutrition comes from research on production birds, including chickens and turkeys. However, other species, such as raptors, sea birds, and browsers such as grouse, still require much research just to define their basic nutrient requirements. As with most exotic and marine mammals, it is easiest at this time to provide them with their wild diet while in captivity.
Many birds possess an additional organ anterior to the stomach known as the crop.
The crop can serve several purposes, depending on the species. Because many birds must travel considerable distances to food sources or they must feed rapidly and then return to a safe area secluded from carnivores, the crop can act as a means of carrying larger quantities of food than the stomach can immediately handle. The crop can also serve to help process hard, dry foods, allowing them to become softened by water and secretions prior to digestion. Additionally, in some birds, the crop serves as a transport device for food destined for nestlings that are not yet able to forage on their own. And, at least one bird, the hoatzin from South America, is known to use the crop as a significant foregut fermentative organ.