Understanding Waterfowl: Acorns and Aquatic Invertebrates

By Richard M. Kaminski, PhD, and J. Brian Davis, PhD

The Mississippi Alluvial Valley (MAV), or Delta, is a continentally important wintering area for North America’s waterfowl. This region was historically forested with red oaks and other lowland-adapted trees. The acorns produced by these trees are important food for ducks, especially mallards and wood ducks. Acorns provide metabolizable energy, similar to levels of raw soybeans, and fatty acids, both of which are used by migrating and breeding ducks to build fat reserves. When bottomland hardwood forests are flooded, they also support a diversity of aquatic invertebrates, which are consumed by ducks as a natural source of protein.

Biologists and conservation planners in the MAV need a reliable understanding of waterfowl foods produced by different habitat types to guide conservation delivery priorities and projects. Despite the importance of acorns and invertebrates to wintering ducks, estimates of the abundance and natural variation in these food resources across the MAV were unavailable until recently. In a recent study by Mississippi State University researchers, the scientists calculated the foraging carrying capacity of bottomland hardwood forests across the MAV, based on biomass of acorns and aquatic invertebrates—the two principal foods eaten by ducks in these habitats. This study also examined the relationship between red oak acorn abundance on the ground and the percentage of red oak canopy coverage. Red oak species were the focus of this study, because they are the species that produce acorns that prior research has shown to be of greatest food value for wintering ducks. Biomass of acorns on the ground was assumed to be the best measure of food available to ducks, because the birds are known to eat acorns in both dry and flooded bottomland forests.

An important part of science is ensuring that the data collected are representative of the larger study area of interest, in this case the MAV. The researchers sampled six areas across four states (Missouri, Tennessee, Mississippi, and Louisiana), including four national wildlife refuges, one national forest, and one state conservation area, all of which were considered representative of bottomland forests in the MAV. Acorn sampling was conducted during fall and winter across three years, 2009–2011, ensuring that the researchers captured the natural ups and downs of acorn abundance.

Acorn biomass was estimated from sample plots on the ground or in shallow water areas that were considered accessible to feeding ducks. In flooded areas, acorns and aquatic invertebrates were collected using a standard “sweep net” approach and were later sorted and weighed to estimate their respective biomass. Each year was classified as either a “good” or “poor” acorn-production year to quantify differences in acorn biomass in a manner consistent with how land managers typically describe mast crops in the field.

The study revealed that in both good and poor production years, acorn abundance was related to the percentage of the forest canopy made up of red oaks, but the relationship was not directly correlated as expected. Surprisingly, acorn abundance was greatest in forests with about 50 percent canopy coverage of red oaks. As for timing of when acorns were available to ducks, peak acorn abundance occurred in February, when other food resources in croplands and moist-soil wetlands have often been depleted by ducks or decomposed.

Differences in acorn abundance in this study were likely related in part to seed-maturation rates, which differ among red oak species. Acorns began dropping for most sampled trees in November and kept dropping into January, such that by February oaks had dropped most of their acorns. Interestingly, Nuttall oaks had two peak acorn drops—one in December and a second in February. This was unique among the oaks sampled because they drop more acorns in February than other species. Thus, Nuttall oaks may be an especially important late-winter food source for ducks and other wildlife.

Acorn consumption by other wildlife species was another potential explanation for the unexpected relationship between red oak canopy coverage and acorn abundance on the ground. Acorn predators such as squirrels, deer, turkeys, and grackles might be more concentrated in areas with greater than 50 percent red oak canopy coverage, as these areas may have more acorns to support these wildlife species, resulting in higher rates of acorn consumption. Conversely, acorn predators may be less abundant in forests with lower red oak canopy coverage, resulting in an greater abundance of acorns for ducks.

Acorn and invertebrate biomass data were also used to estimate duck energy-days (DEDs) for bottomland hardwood forests of the MAV. A DED equals the amount of energy provided by available food resources to support a duck’s energetic needs for one day. The study showed that in years of good acorn production, areas of the MAV with red oak canopy coverage of 45 percent provided 920 DEDs per acre, while in years of poor acorn production DEDs in these same habitats fell to 65 per acre. Clearly, flooded bottomland hardwood forests provide valuable foraging habitat for migrating and wintering ducks, especially during years of good acorn production and when seasonal flooding makes acorns and invertebrates widely accessible to ducks.

An enhanced understanding of acorn production across the MAV will also enable conservation planners to refine regional assessments of carrying capacity and conservation goals. The Lower Mississippi Valley Joint Venture of the North American Waterfowl Management Plan currently uses an estimate of 156–203 DEDs per acre for bottomland forests with 40–50 percent red oak canopy coverage. The study found that these forest stands provide a median of 492 DEDs per acre across good and poor acorn production years, including energy available from aquatic invertebrates. This estimate was three times greater than the Joint Venture’s current estimate and should be considered for updated conservation planning.

Seasonal availability of food resources is an important consideration when estimating food abundance and habitat carrying capacity for wintering ducks. Because acorns and aquatic invertebrates persist through winter and reach peak abundance later in the season, bottomland hardwoods may have been undervalued for their potential to provide energy for wintering ducks and other wildlife. Conservation programs that enhance or protect remaining tracts of mature hardwoods, while seeking opportunities to restore historical bottomland forests, are vital for ducks and other wildlife.

Sizing Up Acorns

Acorns vary in size among red oak species, which in the Mississippi Alluvial Valley include cherrybark, Nuttall, pin, water, and willow oaks. Nuttall and pin oaks produce the largest acorns, followed closely in size by water and cherrybark oaks. The smallest acorns are produced by willow oaks. Wood ducks and mallards obtain similar amounts of energy from acorns produced by these species, despite the variations in size and husk thickness among them.

Dr. Richard M. Kaminski is a professor emeritus, and Dr. J. Brian Davis is the James C. Kennedy Waterfowl and Wetlands Endowed Chair with the Department of Wildlife, Fisheries, and Aquaculture at Mississippi State University. The authors acknowledge and sincerely thank the graduate students who contributed their graduate findings to this article.