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Why do ducks eat that?
by Sarah McClain and Heath Hagy, Forbes Biological Station, Illinois Natural History Survey

Illinois is an important stopover for waterfowl during migration through the Mississippi Flyway. Waterfowl stop to rest and refuel in floodplain wetlands and backwater lakes that naturally and historically exist along rivers. Unfortunately, within the past century, most of the natural floodplain wetlands along the Mississippi and Illinois Rivers have been lost or degraded. In Illinois, wetlands have been destroyed because of the installation of locks and dams for commercial navigation along rivers, stabilization and elevation of water levels for recreation, intensive urbanization and other development, and drainage for agriculture.

One consequence of river floodplain wetland degradation is the loss of habitat and food for migrating waterfowl. Wetland managers question if the remaining wetlands provide enough food to support healthy populations of migrating waterfowl. One way to answer this question is by using energetic carrying capacity models. Model inputs include the energy requirements of birds, the number of waterfowl that use a region (population size), the length of time these waterfowl stay in the region during migration (stopover duration), the amount of food (seeds, invertebrates, vegetation) available in wetlands, and the energetic value of available foods.

It takes much time, effort, and financial support to estimate each of these parameters for energetic carrying capacity models, but the energetic value of food is an important topic for conservation planners and wetland managers. The true metabolizable energy (TME) of a food is an estimate of the amount of energy from a particular food available to a particular organism. TME values are determined with captive feeding trials, in which the researcher provides a food of interest to captive animals and then collects what comes out of the animal after feeding (Yes, the poop!). It is a glamorous job! Previous waterfowl TME studies have been largely interested in moist-soil seeds, such as smartweed and grasses, which compose a large portion of many duck diets.

 Eurasian watermilfoil
 Eurasian watermilfoil

We were interested in submersed aquatic vegetation (SAV), which was historically widespread throughout the backwater lakes and wetlands of the Illinois River. Because of wetland destruction and degradation, SAV has been mostly eliminated from large portions of the Illinois and Mississippi Rivers. Wetland restoration projects, such as The Nature Conservancy’s Emiquon Preserve in Fulton County, have replaced a portion of historical SAV communities on the landscape, and waterfowl are using it. The INHS Forbes Biological Station found that the Emiquon Preserve made up 30 percent of waterfowl-usage days during autumn migration (2007–2013), despite accounting for only 5 percent of the flooded area in the Illinois River Valley.

SAV, such as coontail and invasive species such as Eurasian watermilfoil, grow quickly and accumulate large amounts of biomass in a single growing season. There is a potential for these species to provide a significant amount of energy for migrating waterbirds, but first we need to know how much energy birds actually digest from SAV. We caught wild mallards from wetlands in the Illinois River Valley and kept them in captivity at the Forbes Biological Station for the duration of the study. We fed six species of SAV that are common throughout the Midwest and have been documented in duck diets. We fasted each bird for 48 hours, fed a species of SAV, and collected the bird’s excreta for the following 48 hours. The results of this process, plugged into a fancy equation, give us an estimate of the amount of energy that birds assimilated from the vegetation.

Interestingly, we found that energy available to mallards within SAV species varied widely. The average across all six species was 0.68 kcal/g, compared with the average moist-soil seed which has a TME value of 1.78 kcal/g. SAV species such as Canadian waterweed (Elodea canadensis; 1.69 kcal/g) and southern naiad (Najas guadalupensis; 1.40 kcal/g) rival moist soil seed energy values. Other species, such as the Eurasian watermilfoil (Myriophyllum spicatum; -0.53 kcal/g) require more energy to digest than the bird receives from it and, therefore, may be less energetically beneficial for the bird.

Wetland restoration projects that promote SAV provide many resources for waterfowl and other water-loving creatures. SAV communities not only provide food through foliage and seeds, but also support greater populations of aquatic macroinvertebrates and fish than open water communities support. Active management, including removal of less beneficial species such as Eurasian watermilfoil, may result in greater overall wetland health and energetic value for the waterbirds that stop in for a snack on their way further south.

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