**6. Digestive system**

Owls have evolved to eat their smaller prey whole and unlike other birds, they do not have a crop (**Figure 9**). This system reduces the owl's need to drink water, as much of its liquid intake comes directly from the body fluids of its prey. The whole prey is passed head first straight down the oesophagus and into the proventriculus (glandular stomach).

Digestion begins in the proventriculus, which produces digestive enzymes and stomach acid. The food mass, along with the digestive enzymes, then passes into the second part of the stomach, the ventriculous or gizzard (muscular stomach) where the chemical digestion started in the proventriculus continues and manual digestion begins. The gizzard uses strong muscular contractions to aid in digestion. The soft and digestible parts of the food are allowed to continue along with the digestive system into the small intestine [27]. The indigestible parts (fur, feathers, claws, bones etc.) are retained in the gizzard and compacted into an oval-shaped pellet (oval due to the gizzard's shape). The digestion process up to this point takes several hours (**Figure 10**). The pellet is then passed back into the proventriculus where it will remain for several hours more before finally being regurgitated. Additional digestive enzymes are likely digesting any remaining digestible

**Figure 9.** *Juvenile tawny owl (*Strix aluco*) swallowing rodent prey. Photo: Alan Sieradzki.*

#### **Figure 10.**

*The digestive system of an owl. Image: Alan Sieradzki.*

#### **Figure 11.**

*Owl pellet species comparison. L-R: Tawny owl (*Strix aluco*) barn owl (*Tyto alba*), short-eared owl (*Asio flammeus*). Photo: Alan Sieradzki.*

material during this time [28]. Because the stored pellet partially blocks the owl's digestive system, new prey cannot be swallowed until this pellet is ejected.

Regurgitation often signifies that the owl is ready to eat again. When the owl eats more than one prey item within several hours, the various remains are consolidated into one pellet. When the digestive process is finished, the owl will regurgitate the pellet by the process of reverse peristalsis, where smooth muscular contractions push the pellet up the oesophagus and back into the mouth. This process is different from coughing or retching

and can prove to be quite strenuous for the owl, especially with larger pellets—this is why an owl will often take on a pained expression when producing a pellet and the reason why owls cannot produce pellets in flight. At the moment of expulsion, the neck is stretched up and forward, the beak is opened, and the pellet simply drops out (**Figure 11**).
