They store bacteria which are essential in the breakdown of cellulose. Like mammals, birds do not produce the enzymes that allow them to digest cellulose so they can only do so by housing these bacteria. The large intestine is greatly reduced in birds when compared with mammals and plays practically no role in bird digestion. Finally the bird's digestive tract ends in the cloaca where faecal material is collected before it is expelled through the vent or anus. The digestive tract of birds varies in length depending on diet, with carnivores having the shortest intestinal length and herbivores the longest.
Omnivores are somewhere in between.
In some species such as the common Starling, Sturna vulgaris , the intestinal length varies through the seasons being shorter in summer while the birds are feeding mainly on highly proteinaceous insect foods and larger in winter when they are feeding mainly on carbohydrates - seeds. These pages are here free for you to use, I would like to keep them that way, however making and running this site, and the other sites I am currently making or have made, on Birds, Mammals, Insects and Fish and Bacteria costs money. There are simple ways you can help. By using the search engines here you make me 2 or 3 cents a time, by buying goods from Discovery Store you also help. Why not use the special Earthlife Web search page for all your future searches.