Digestive anatomy and physiology

The digestive system of these piscivorous birds is designed specifically for the digestion of proteins and lipids, as this bird does need to digest carbohydrates [5,13].

(Stan Osolinski)
Osprey using its sharp, curved beak to tear the flesh from its prey

The Osprey’s digestive tract begins with its hooked beak, which is specifically designed for tearing the flesh from its prey [5,12,13]. It is also equipped with a muscular tongue, which can be used to manipulate the food inside its mouth [5]. When the bird swallows its fleshy catch it is moved down the elastic esophagus [12,13]. Continuing down the digestive tract, the food moves on to either the crop or the ‘stomach’, which is composed of the proventriculus and ventriculus [5,12], depending on when the last meal was taken. The crop is used as a storage area when food was recently consumed and is still being digested [13]. However, if the ‘stomach’ is empty then food will enter immediately and begin digestion [13]. The proventriculus is the first compartment that the meal will enter, and here it will undergo chemical digestion by pepsin and hydrochloric acid [5,13]. Moving on to the ventriculus, the food will undergo mechanical digestion through weak muscular contractions of smooth muscle [5,12,13]. The proventriculus of piscivores is larger than the ventriculus, since digestion of proteins of animal origin does not require much mechanical manipulation [5,12]. Enzymatic digestion of the food will continue in the relatively short small intestine [12,13]. The enzymes contributing to digestion here can be of small intestinal or pancreatic origin [13], and consist of: trypsin, chymotrypsin, elastase and carboxypeptidases [5,13]. Absorption also begins in the small intestine, before moving on to the large intestine [12]. The large intestine is quite short, and its main role is to absorb water from waste products [5]. After passing through the intestine, the food will continue on to the rectum, cloaca, and vent. The wastes will then be excreted through the vent [5,12].

(Stevens & Hume, 1995 – CD ch. 5)
The Red-tailed hawk (Buteo jamaicensis) is a carnivorous bird of prey with a very similar digestive anatomy to the Osprey

The overall rate of digestion in piscivores is relatively slow, taking approximately 6-13 hours; to allow enzymatic digestion and absorption of nutrients to occur since the vestigial ceca do not contribute to enzymatic digestion [13]. Another technique for increasing digestion is retroperistaltic action, which is a muscular movement that moves the partially digested food from the intestine back to the ‘stomach’ for further digestion [5,13]. This action is required because the digestive tract of the bird is so short [5].

Because the ceca are vestigial, all enzymatic digestion is classified as autoenzymatic rather than alloenzymatic. Autoenzymatic digestion means that only enzymes from the bird will act in digesting the food. Whereas alloenzymatic digestion involves enzymes from microbes which could be found in birds that have functioning ceca. [13]

Piscivores possess a quite acidic proventriculus, with a pH around 1.7 [5,13].

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