Effects of dietary fiber and diet diversity on digestive organs of captive Mallards (Anas platyrhynchos)

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Abstract
We studied the effects of dietary fiber, of plant versus animal diets, and of diet diversity on the digestive organ morphology of captive Mallards (Anas platyrhynchos). Four experimental diets were used: whole corn (low fiber, plant), trout chow (low fiber, primarily animal), rabbit chow (high fiber, plant), and a diverse diet consisting of each of the preceding diets fed daily in random order. Every 5 days for 25 days, we sampled birds from each diet group (N = 6-8) and weighed each bird and its abdominal fat, gizzard, ceca, small intestine, and liver, and then measured its ceca and small intestine length. Other than differences explained by their different body sizes, the sex of the birds had no effect on gut measurements. Changes in body weight and abdominal fat weight suggested that Mallards existed equally well on each of the four diets, presumably because changes in their digestive organs allowed them to have similar digestive efficiencies when eating different diets. Birds on the high fiber diet had the largest digestive organs, but birds on all diets showed changes in digestive organs. The diverse diet produced effects on digestive organ morphology similar to those of the two low-fiber diets. Except for a larger gizzard for macerating corn in birds on that diet, there was no difference in the digestive organs between birds eating animals foods and those eating low fiber plant foods. The maximum weight of gizzard, intestine, and ceca of birds on the high fiber diet was reached in 10 days. However, ceca and intestinal lengths of these birds were still increasing after 25 days. This observation suggests that birds can continue to respond to a change in diet after limits to the mass of their digestive organs have been reached.