What did we learn? Among hens in our study, invertebrate consumption increased and seed consumption decreased as the birds moved farther north. The proportion of invertebrates consumed by hens also increased as spring progressed in all species studied—although this varied significantly among species.
Some of the results were surprising. For example, we found that hen scaup consumed equal amounts of seeds and invertebrates on southern and mid-latitude study sites. And while consumption of invertebrates by scaup was higher on northern study sites, the birds consumed more seeds than had been observed in past studies at similar latitudes. Our research also found that gadwalls, considered the most vegetarian of the five species examined, still consumed mostly plant material during spring migration. Nevertheless, invertebrate consumption by hen gadwalls did increase as the birds moved north and spring progressed. Blue-winged teal consumed the greatest proportion of invertebrates of any species at southern latitudes, which was consistent with earlier studies in these locations. However, the proportion of invertebrates consumed by hen bluewings on our northern sites was lower than invertebrate consumption documented in past research in North Dakota, suggesting that hen bluewings continue to increase the proportion of invertebrates in their diet as they migrate north.
Our study also examined waterfowl food abundance in a variety of spring migration habitats. Food availability varied by wetland type and location. Food biomass estimates in these habitats were lower than at other times of the year, suggesting that wetland habitats have a lower capacity to support the energetic demands of ducks in spring than during fall and winter. Among habitat types, emergent wetlands provided the most seed and invertebrate food resources for waterfowl. But surprisingly, invertebrate biomass in flooded agricultural fields rivaled that of some natural wetlands. This result was quite unexpected and is worthy of further investigation by waterfowl managers.
Another important objective of our study was to measure the body condition of female ducks during spring migration. Examination of hens collected on our study sites revealed that levels of fat and protein in hens increased as the birds moved north, again likely because of increased consumption of invertebrates. In general, hens increased their nutrient reserves (protein and fat) as they moved north, suggesting that the birds acquire these reserves specifically to prepare for reproduction later in spring. As expected, hens collected at some locations were in better condition than hens collected at other sites, indicating that some areas may not have enough feeding habitat for the birds to acquire nutrients at optimal rates. The increasing preference for invertebrates among all hens studied suggests that waterfowl populations may suffer if invertebrate availability declines as a result of wetland loss and degradation on spring staging areas. Poor water quality in wetlands, which has a negative effect on invertebrate abundance, is one of the greatest concerns and challenges for waterfowl managers in the Upper Mississippi and Great Lakes watersheds.