Aquatic macroinvertebrates are vitally important food for waterfowl. When many people think of "duck food," grains like corn, wheat, and rice immediately come to mind. Most grains are high in carbohydrates and energy, but low in protein. The seeds of moist-soil plants like smartweed and barnyard grass often contain high to moderate levels of energy, but are much richer in fiber. Hard mast, such as some types of acorns, can be composed of almost 20 percent fat. Invertebrates are a rich source of protein and other key nutrients that ducks need at particular times of year. For example, amphipods (scuds) are approximately one-half protein and are among the most important invertebrate foods for certain species of ducks, such as scaup. Northern shovelers are adapted to sifting small, swimming invertebrates from the water column. Early nesting species such as mallards and pintails consume large quantities of midge larvae and earthworms that are prevalent in ephemeral wetlands. These and many other invertebrates are especially important food sources for rapidly growing ducklings, molting adults, and breeding hens, all of which require large quantities of protein to meet their particular nutritional needs.
Another essential nutrient that waterfowl obtain from invertebrates is calcium. Invertebrates with hard exoskeletons, like insects and other arthropods, are good sources of calcium for waterfowl. However, most female waterfowl depend on snails to supply the calcium they need to produce eggshells. Snails can be susceptible to having their central nervous systems hijacked by other invertebrates, most often parasites. This "brain-jacking," as it's called, makes the snails more vulnerable to feeding waterfowl (see sidebar). A parasitic flatworm called a trematode has been shown to cause its host snail to feed later in the morning, when waterfowl also more actively feed. This helps ensure that the snail is eaten by a duck, transmitting the parasite to its final host.
Brain-jacked! In a real-life story that sounds like something from a science-fiction horror movie, some of the invertebrates eaten by ducks get "brain-jacked" by other invertebrates, most often parasites. For example, spiny-headed worms, which are parasites of waterfowl, begin their life cycle in amphipods (scuds), a common waterfowl food. To avoid being eaten, amphipods usually try to hide under vegetation and out of sunlight. However, when infected by the spiny-headed worm, their central nervous system gets hijacked and starts producing neurochemicals that change their behavior. Instead of staying near cover, they swim toward light and up to the surface. This makes them much more susceptible to being eaten by feeding ducks. Some species of spiny-headed worms have cysts that are bright orange and make infected amphipods even more obvious and likely to be eaten by ducks, the worms' final host. Once inside a duck, the spiny-headed worm lives attached to the gut and produces eggs, which the duck excretes into the water. The eggs are then consumed by amphipods, completing the worm's life cycle.