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Mystery, Superstition and Imagination Ashton's Owls - part 3 of 1 2 3 4 5 6

by Jon Ashton Published 01/02/2012

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. A bird's stomach has two parts, the first part is the glandular stomach, where enzymes, acids, and mucus begin the process of digestion. The second part is the 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. 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. Several hours after eating, the indigestible fur, bones, teeth and 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 (glandular part of the stomach). It will remain there for up to 10 hours before being regurgitated and because the stored pellet partially blocks the owl's digestive system, new prey cannot be swallowed until the pellet is ejected. When an owl is about to produce a pellet, it makes for an interesting sight - the eyes are closed, the facial discs narrow, the neck is stretched up and forward, the beak is opened, and without any retching or spitting movements the pellet drops out. Owl pellets differ from other birds of prey; they contain a greater proportion of food residue, 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.

One of an owl's most striking features is its eyes. Large and forward facing, they may account for one to five per cent of the owl's body weight, depending on species. The forward facing position of the eyes that give an owl its 'wise' appearance, also give it a wide range of binocular vision. This means the owl can see objects in three dimensions (height, width, and depth), and can judge distances in a similar way to humans. The field of view for an owl is about 110 degrees, with about 70 degrees being binocular vision. By comparison, humans have a field of view that covers 180 degrees, with 140 degrees being binocular. An owl's eyes are large in order to improve their efficiency, especially under low light conditions, the eyes are so well developed, that they are not eye-balls as such, but elongated tubes. They are held in place by bony structures in the skull called sclerotic rings. For this reason, an owl cannot 'roll' or move its eyes - it can only look straight ahead, hence the need for owls to keep moving their heads. They are able to achieve this by having a long and very flexible neck, which is not always apparent, as it is hidden by feathers and the owl's posture. An owl's neck has 14 vertebrae, which is twice as many as humans. This allows it to turn its head through a range of 270 degrees, measured from a forward facing position.

• Mystery, Superstition and Imagination Ashton's Owls page 1
  
• Mystery, Superstition and Imagination Ashton's Owls page 2
  
• Mystery, Superstition and Imagination Ashton's Owls page 3
  
• Mystery, Superstition and Imagination Ashton's Owls page 4
  
• Mystery, Superstition and Imagination Ashton's Owls page 5
  
• Mystery, Superstition and Imagination Ashton's Owls page 6
  

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