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Understanding Waterfowl: Duck Digestion

An inside look at the dynamic digestive system of waterfowl 
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By J. Dale James, Ph.D.

Just like the things we eat, waterfowl foods vary in nutritional content and quality, and this in turn determines how efficiently these foods are digested. Waterfowl management guidelines often describe foods that are beneficial to ducks and those that are not. Understanding why and when certain foods are good for waterfowl and how they are digested is an interesting and somewhat complex subject. In fact, digestion is the function that links the environment (habitat) to the well-being of waterfowl.

Waterfowl digestion involves many organs, beginning with the bill. Each species has a bill designed specifically for acquiring certain types of food. Waterfowl bills vary considerably in size and shape, from the broad, sieve-like bill of the northern shoveler, which is designed for straining tiny aquatic invertebrates from the water column, to the shorter and narrower bill of the wood duck, which is designed for grabbing acorns and seeds of moist-soil plants. 

Like nearly all animals, waterfowl have a tongue, which is used to move food through the oral cavity into the esophagus. Because waterfowl often feed in areas where they are threatened by predators, it's often advantageous for the birds to "eat and run." Consequently, waterfowl have extra storage capacity in their esophagus, which enables the birds to carry considerably more food from foraging areas than they could otherwise eat. Unlike doves and pigeons, however, waterfowl do not possess a true "crop," or widened portion of the esophagus. Instead, their esophagus is capable of expanding to accommodate substantial amounts of food.  

The next stop after the esophagus is the glandular stomach—known as the proventriculus—which secretes digestive enzymes that soften food and make it easier to digest. The esophagus and proventriculus collectively form the upper digestive tract of waterfowl, and this is where biologists typically obtain food samples from individual birds when studying waterfowl diets. 

Once the food moves through the upper digestive tract, it enters the ventriculus, more commonly known as the gizzard. This thickly muscled organ essentially functions as a duck's "teeth." The ventriculus often contains grit (sand or small stones), which aids in mechanically breaking down large food items. Protein digestion is also initiated in the gizzard. 
The Link between Food and Habitat Knowledge of waterfowl foods and feeding behaviors is fundamental to effective long-term management of waterfowl populations. Species differ considerably in their dietary requirements, with some consuming both plant and animal foods and others having an exclusively vegetarian diet. Plant foods such as smartweeds, pondweeds, and widgeon grass often occur naturally in wetlands, while agricultural foods like corn, soybeans, and rice must be cultivated. The management strategies for these habitats differ, and timing—particularly of water depth and duration of flooding—is key to maximizing the production and availability of waterfowl foods. Consequently, knowing the dietary requirements of waterfowl greatly improves our ability to provide sufficient foraging resources for desired populations.
It's also important to understand changes in waterfowl feeding habits throughout the birds' annual cycle. Just as we enjoy hot, rich foods in cold weather, waterfowl often favor foods that are high in carbohydrates during winter, when energy demands are high. But like people, waterfowl also must eat a varied diet to meet all their nutritional needs. During late winter and spring, ducks consume more protein-rich foods (largely invertebrates) to prepare for the rigors of the upcoming breeding season. Continued monitoring and evaluation of waterfowl diets through research provides Ducks Unlimited and its partners vital information to guide its conservation programs and benefit a variety of waterfowl species.

One of the most interesting things about the gizzard is its ability to adapt to changes in a bird's diet by increasing or decreasing in size. Waterfowl species that consistently feed on harder food items like mollusks and hard-cased seeds will have a larger gizzard than species that consistently forage on softer food items. But the gizzard also grows larger when individual birds shift to a diet rich in hard foods. Think of the gizzard as being like your bicep muscles. If you use them more, by lifting heavy weights for example, they will grow larger in size. Similarly, the muscular gizzard of waterfowl will grow with increased use caused by frequent consumption of foods that are hard to break down. 

After being pulverized in the gizzard, food particles pass into the small intestine, where they are digested and the nutrients are absorbed. The length and structure of the small intestine varies among species. Ducks that eat more animal matter, such as insects, snails, and crustaceans, tend to have shorter, less complex small intestines. Waterfowl that eat more plant material—among them wigeon and geese—have longer, more developed small intestines. This difference in intestinal length is due largely to the difficulty of breaking down cellulose in plant fibers and the birds' need to consume larger amounts of vegetation to acquire adequate nutrition. Just as in humans, enzymes produced in the pancreas break down proteins and fats in the small intestine of waterfowl. The liver also secretes bile, which emulsifies fats. Nutrients are then absorbed through the intestinal membranes and into the bird's bloodstream. 

Before entering the large intestine, the partially digested food moves into the caeca. The caeca, which often look like two wormlike structures, extend from the large intestine. The size of these structures also varies among species according to their diets. The caeca's primary functions are to aid in the absorption of water and proteins and to digest plant cellulose, or fiber. Once all available nutrients have been absorbed, the undigested waste passes into the large intestine, where water is again reabsorbed and the waste passes. The undigested waste, along with urine, is then excreted through the cloacal opening.

The dynamic digestive system of waterfowl often responds to changes in the birds' behavior. This response usually takes the form of changes in organ size (such as the size of the gizzard), which allows ducks to more efficiently obtain and store required nutrients. These changes often occur at times when ducks and geese need additional energy or specific nutrients to keep warm, molt, reproduce, or migrate. Understanding how the waterfowl digestive tract functions in relation to diet and nutritional requirements is essential for Ducks Unlimited and its partners to identify and conserve key habitats for the birds throughout their annual cycle. 
AMAZING FOOD FACTS
  • Mallards have about 200 taste buds, whereas humans have around 9,000.
  • Mallards can carry up to a quarter pound of grain in their esophagus.
  • In 1911, a gold rush was spurred in western Nebraska after hunters found small gold nuggets in the gizzards of ducks they had shot. The source of these gold nuggets, however, was never discovered.
  • Female wood ducks must ingest 75 grams (2.6 ounces) of invertebrates to obtain enough protein and minerals to produce one egg. To acquire these nutrients, the birds must consume more than 300 invertebrates an hour for eight hours.
  • The specialized structure of an American wigeon's bill enables it to exert great force at the tip of the bill, enhancing the bird's ability to pluck vegetation.


Dr. Dale James is manager of conservation planning at DU's Southern Regional Office in Ridgeland, Mississippi.
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