Digestion in Owls
Like other birds, Owls cannot chew their food - small prey
items are swallowed whole, while larger prey are torn into smaller pieces before being
swallowed. Some Owl species will partially pluck bird and larger mammal prey.
Unlike
other birds, Owls have no Crop. A crop is a loose sac in the throat that serves as storage
for food for later consumption. Since an Owl lacks this, food is passed directly into
their digestive system.
Now, a bird's stomach has two parts:
The first part is the glandular stomach or proventriculus, which produces enzymes,
acids, and mucus that
begin the process of digestion.
The second part is the muscular stomach, called the Ventriculus, or gizzard. There are no digestive glands in
the gizzard, and in birds of prey, it serves as a filter, holding back insoluble items such as bones, fur, teeth and
feathers (more about this below).
The soluble, or soft
parts of the food are ground by muscular contractions, and allowed to pass
through to the rest of the digestive system, which includes the small and
large intestine. The liver and pancreas secrete digestive enzymes into the small intestine where the food is absorbed into body. At the end of
the digestive tract (after the large intestine) is the cloaca,
a holding area for wastes and products from the digestive and urinary
systems. The cloaca opens to the outside by means of the vent. It
is interesting to note that birds (apart from the Ostrich) do not have a
bladder. The excretion from the vent is largely made up of an acid which
is the white part of a healthy dropping.
Several hours after eating, the indigestible parts (fur, bones, teeth
& feathers that are still in the gizzard) are compressed into a pellet the
same shape as the gizzard. This pellet travels up from the gizzard back to the
proventriculus. It will remain there for up to 10 hours before being regurgitated. Because
the stored pellet partially blocks the Owl's digestive system, new prey cannot be
swallowed until the pellet is ejected. Regurgitation often signifies that an 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.
The pellet cycle is regular, regurgitating the remains when the digestive system has
finished extracting the nutrition from the food. This is often done at a favourite roost.
When an Owl is about to produce a pellet, it will take on a pained expression - the eyes
are closed, the facial disc narrow, and the bird will be reluctant to fly. At the moment
of expulsion, the neck is stretched up and forward, the beak is opened, and the pellet
simply drops out without any retching or spitting movements.
Owl pellets differ from other birds of prey in that they contain a greater proportion
of food residue. This is because an Owl's digestive juices are less acidic than in other
birds of prey. Also, other raptors tend to pluck their prey to a much larger extent than
Owls.
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References: Campbell, Wayne. 1994. "Know Your Owls (CD-ROM)". Axia Wildlife
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