Raptorial digestion?

You would think that raptors all digest their prey similarly and that their digestive systems wouldn’t show any difference.

Although, there are in fact differences between the digestive morphology and processes found in some raptors (Ford 2010, Smith & Richmond 1972, Duke, et. al 1975). The differences that are discussed in this blog focuses on digestive morphology, the formation of pellets and gastric pH of the stomach.

Unlike Psittaciformes (parrots) and most other birds, raptorial diet consists highly of protein in the form of small lizards, rodents, birds or fish where the piscivorous raptors diet consists of fish (Fowler, Freedman & Scannella 2009). However raptors when eating, tear fairly large morsels of flesh from its prey, consuming a large amount in a short time, where it is stored in the crop (lacking in Strigiformes) (Ford 2010).

The digestive process in raptors is also more chemical than mechanical, where they have a gland that secrete mucus and another secreting HCl and pepsin (Ford 2010). Allowing the breakdown of protein structures, even so raptors such as hawks and owls form pellets (Fig 1) (Ford 2010, Duke, et. al 1975). 
 Pellets are undigested compressed materials that are egested (regurgitated) from owls and hawks, though it is observed that the amount of undigested materials (mainly bones) in Falconiformes is lower than that of owls (Moon 1940, Duke, et. al 1975). Reasons for this include Falconiformes initially not ingesting as many bones in comparison to owls (swallowing prey whole) and that the gastric pH in the of owls are higher than that of the Falconiformes (Moon 1940, Duke, et. al 1975, Smith & Richmond 1972).

Fig1. Simple illistration depicting the process of pellet formation (sourced by: https://www.tes.com/lessons/PBj-79gS-EQIjQ/science-owl-pellets) 

Reference:

Duke, G.E., Jegers, A.A., Loff, G. & Evanson, O.A. 1975, "Gastric digestion in some raptors", Comparative Biochemistry and Physiology -- Part A: Physiology, vol. 50, no. 4, pp. 649-656.

Ford, S. 2010, "Raptor Gastroenterology", Journal of Exotic Pet Medicine, vol. 19, no. 2, pp. 140-143.

Fowler, D.W., Freedman, E.A. & Scannella, J.B. 2009, "Predatory functional morphology in raptors: Interdigital variation in talon size is related to prey restraint and immobilisation technique", Plos One, vol. 4, no. 11, pp. e7999.

Moon, E.L. 1940, "Notes on Hawk and Owl Pellet Formation and Identification", Transactions of the Kansas Academy of Science (1903-), vol. 43, pp. 458-465.

Smith, C.R. & Richmond, M.E. 1972, "Factors Influencing Pellet Egestion and Gastric pH in the Barn Owl", The Wilson Bulletin,vol. 84, no. 2, pp. 179-186.