THE DIGESTIVE SYSTEMOverviewBirds have a number of features of the digestive system that distinguish them from mammals. These include the lack of teeth and a soft palate and a feeding strategy that allows for maximum ingestion of food in a short time. As birds need to keep down weight to fly most cannot afford the luxury of prolonged food storage or digestion. In those that do gorge and store food in a crop for a time (eg pigeons) they may be temporarily incapacitated and vulnerable to predators.Some carnivorous birds can digest a meat meal in a matter of hours, and many frugivorous birds pass seeds through in 20-30 minutes. As seed-dispersers birds play an important role in the ecosystem, in fact many seed types will only germinate after having passed through a bird's system. The mechanisms of digestion thus have some unique features. Oral cavity and pharynxThe sharp-edged horny beak evolved by the fusion of several separate keratinous plates. In some birds (eg Fulmars and Shearwaters) the plates remain separate while in others (eg Ostriches) the plates can be distinguished. Generally, the bill grows continuously to compensate for wear and tear. It is extremely sensitive, particularly in birds that forage by probing for food (eg Ducks and Curlews).There is enormous variation in bill shape and surrounding appendages such as bristle feathers. This is not surprising as being the major food-gathering organ it is subject to intense selection pressure as birds compete for nutrients. Likewise the tongue may show specialisations for food-gathering such as the spikes seen on fishing birds (eg Cormorants) and the feathery filtration devices seen in dabbling birds like ducks. While taste per se does not seem very important for birds, the rich innervation of the tongue seems to be central for food selection. Unlike mammals, birds do not possess a soft palate and therefore have to throw their heads back to swallow. This is assisted by a very flexible neck. Internally the oral cavity and pharynx form a single unit, the oropharynx, connected to the nasal cavity by a triangular choanal slit. The Eustachian tubes open by a common slit-like infundibular opening caudal to the choana: this remains open continuously to permit pressure equalisation in flight. Ventrally the glottis opens to the larynx via a slit flanked by the laryngeal mounds. Figure 4-1. Scheme of the avian digestive system (redrawn from Freethy. 1983) There are fewer taste buds than in mammals, even though taste is thought to play a role (albeit minor) in food selection. The salivary glands are predominantly mucus-secreting and have a multitude of openings both dorsally and ventrally. Unlike mammals, saliva in birds is primarily a lubricant or (as in woodpeckers) a sticky coat to the tongue to trap insects. OesophagusAs birds don't possess teeth food is swallowed in large boluses, in a complicated series of movements of the tongue assisted by caudally directed papillae with reflex closure of the glottis and choana. The oesophagus is wide and muscular with longitudinal folds to accommodate food passage. It is lined with stratified squamous epithelium with subepithelial mucous glands.Many birds store food in the oesophagus if the stomach is full: this storage role may be elaborated by the formation of a distensible crop just cranial to the thoracic inlet. In addition, this storage zone may be used to regurgitate food for nestlings. While in the crop the food softens and swells before it is passed to the stomach. In pigeons the crop of both sexes produces a nutritious fatty crop milkthat is regurgitated to the nestlings: interestingly the production of the milk is controlled by the protein hormone prolactin. StomachGenerally the stomach consists of a cranial glandular proventriculus and a caudal muscular ventriculus or gizzard. Food may be passed backwards and forwards between these compartments.As in mammals the glandular stomach has a high surface area with simple columnar epithelium. Tall papillae project into the lumen, each containing a multilobar gland discharging at the tip. The wall of the ducts are lined with tall mucus-secreting cells. The glands themselves have only one kind of cell that secretes both acid (HCl) and protease (pepsin). The gizzard is a highly muscular structure lined with a tough, abrasive keratin-like layer of koilin, known as the cutica gastrica. It shows great variation, and is most elaborate in granivorous and herbivorous birds. The sandpaper-like grinding action may be assisted by grit and stones deliberately ingested. This action is enormously powerful and can even deal with metal particles. In carnivorous birds most digestion occurs by prolonging the stay of food in the proventriculus. In these the gizzard serves to filter out fur, skin and bones that get regurgitated periodically as a pellet. IntestinesFinely macerated food passes by peristalsis out of the gizzard into the intestines. These are much more firmly fixed in the body cavity that those of mammals. Their function is similar: absorption of nutrients and resorption of water.While the intestines are conventionally divided into various regions they show much less morphological variation along their length than do mammals. Generally speaking, carnivorous birds have shorter and less complex intestines, while herbivorous birds have longer small intestines and well developed paired blind-ending intestinal caecae containing microorganisms involved in cellulose breakdown. The liver and pancreas connect to the duodenum and secrete bile and pancreatic juices. As in mammals, these act to emulsify fat, digest carbohydrates and proteins and neutralise the acid secretions of the stomach. The liver has two large lobes surrounding the lower pericardium and these occupy most of the ventral and cranial space of the abdomen. There may or may not be a gall bladder. The liver acts to store carbohydrates and fats. It's relatively larger than in mammals and is most elaborate in carnivorous as compared with herbivorous birds. As in mammals, nutrients are resorbed over the intestinal wall and pass into the portal hepatic venous system. The large intestine in birds is short and relatively featureless except for the intestinal caecae, leading in to the cloaca. As in mammals, this is a site of water resorption. There may be a small spleen in the mesentery between the gizzard and proventriculus, but this is absent in many birds (eg Pigeons). QuestionsWhat's strange about the digestive system of the S American Hoatzin?How do birds prepare nutritionally for the rigours of migration? Do all birds need a powerful grinding gizzard? What adaptations does the avian digestive system demonstrate for the requirements of homoeothermy and flight?
Adapted from: The Digestive System |
