Fueling the Engines

Feeding and digestion in waterfowl

By Keith McKnight, Ph.D.

Natural selection dictates that the fittest individuals (i.e., those best at survival and reproduction) dominate the population, while those less adept at making a living contribute less to the gene pool, or are eventually eliminated altogether. Fundamental to this concept is the notion that the physical and behavioral attributes of the animals we see today are well matched to the environmental challenges they face.

For wildlife, the basic challenges are simple-survive and reproduce. However, the intricacies of survival and reproduction in birds are nothing short of amazing. Nowhere is this more evident than in the interplay among foraging, flight and reproduction in waterfowl.

Many aspects of a duck's physical and physiological machinery are adapted to flight. Wings and feathers are clearly adaptations for flight, but other features may not be so obvious. For example, the energy requirements of flight are high. As a result, ducks and geese must have ready access to habitats with sufficient food sources, and must consume hefty volumes of food while there.

Eating is only part of the battle, though. To extract nutrients from food items, ducks and geese must be able to efficiently and effectively digest them. Just as with many other vertebrates, the digestion process begins with physical grinding. To accomplish this, waterfowl (in fact all birds) use their gizzard, not teeth. This, believe it or not, is an adaptation for flight. The jawbone structure and teeth required to grind food in the mouth result in a substantial amount of weight located in the front end of the body. This causes some problems with in-flight balance. Maneuverability in flight requires an even distribution of body weight.

This is achieved in birds by centralizing heavy structures, such as the gizzard, in the body cavity. So in essence the gizzard serves as a centrally located set of teeth. Because there is no sense in carrying around unnecessary weight, we find that gizzard size in waterfowl is closely related to the nature and/or volume of food eaten. This is most notable among species where consumers of relatively fibrous foods such as seeds and leafy vegetation (mallards, gadwalls, pintails) possess larger gizzards than do fish eaters, such as mergansers.

Other diet-related physical characteristics in waterfowl are more conspicuous. The most recognizable of these is bill shape and size. Most waterfowl observers have noticed the substantial (almost comical) difference between the bill of a merganser and a northern shoveler. The serrated pincer-like bill of mergansers is an effective fish grabber, whereas the shoveler's bill functions like a great scoop and sieve. Snow and Ross's goose bills are short and stout; ideal for digging and muscling out below-ground plant parts like rhizomes and tubers. The significantly longer and more slender Canada goose bill, on the other hand, is better suited for grasping and snipping above-ground plant parts such as blades of grass.

Because habitats offering the greatest opportunities for feeding often do not provide the greatest refuge from predators and/or the elements, it is advantageous to grab lunch and go. In the absence of a grocery bag, waterfowl rely on the storage space provided by the esophagus. Unlike doves and pigeons, waterfowl do not possess a crop (outpocketing of the esophagus). However, the esophagus in waterfowl provides substantial storage space. Mallards, for example, can carry as much as a quarter pound of grain in a packed esophagus. For those who have witnessed mallards leaving a rice field, or wood ducks after gorging on acorns, this capacity is not hard to believe.

Once the food has been grabbed by the bill, stored in the esophagus, and ground in the gizzard, digestion is much the same as in other vertebrates. The small intestine is responsible for further breakdown of food particles and removal of nutrients. The dominant role of the large intestine is to recover water before waste is expelled. Between these two organs are a pair of elongated sacks called ceca. Although scientists still debate the precise function of this pair of organs, it is clear that herbivorous waterfowl such as gadwall and snow geese possess larger ceca than do the carnivorous northern shoveler, eiders and mergansers.

So, even with a body function as seemingly uninteresting as digestion, we find examples of the ability of waterfowl to adapt and adjust to the environmental conditions they face. DU supporters understand better than anyone else, however, that we must continue to ensure that there is something out there for the birds to digest.